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                     Deep-Sea Research I 51 (2004) 1685–1708
                                                   www.elsevier.com/locate/dsr




  Community structure and feeding ecology of mesopelagic
    fishes in the slope waters of King George Island
       (South Shetland Islands, Antarctica)
                 C. Puscha,Ã, P.A. Hulleyb, K.-H. Kockc
  a
    Alfred Wegener Institute for Polar and Marine Research, P.O. Box 120161, Columbusstrasse, 27568 Bremerhaven, Germany
            b
             Iziko Museums of Cape Town, P.O. Box 61, 8000 Cape Town, Republic of South Africa
                 c
                 Sea Fisheries Research Institute, Palmaille 9, 22767 Hamburg, Germany
           Received 17 September 2003; received in revised form 18 June 2004; accepted 22 June 2004
                         Available online 24 August 2004



Abstract

  The role of mesopelagic fishes in the Southern Ocean ecosystem and more particular their trophic effect on the
standing stock of mesozooplankton is at present poorly understood. To get a deeper insight in the Antarctic mid-water
ecosystem the mesopelagic fish community of the King George Island slope (South Shetland Islands) was sampled with
a pelagic trawl in 1996. The community structure was analysed and the feeding ecology was studied of the five most
abundant species. A total of 18 mesopelagic fish species in 10 families was identified. Of these, the Myctophidae was the
most important family by species number (9 species), individual number (98.5% of all individuals) and fish wet weight
(87.3% of the total weight). The assemblage was numerically dominated by four myctophids (Electrona antarctica,
Gymnoscopelus braueri, Gymnoscopelus nicholsi, Protomyctophum bolini) and one gempilyd (Paradiplospinus gracilis).
Multivariate statistical analysis of the mesopelagic fish data reveals two major groups of stations according to the
sampled depth: a shallow group of stations (295–450 m depth) and a deeper group of stations (440–825 m depth). The
change in relative abundance of mesopelagic fish species at 440–450 m coincides with the presence of warmer and denser
Circumpolar Deep Water at and below these depths. Deeper stations were characterized by a higher density and
increased diversity of mesopelagic fish species. The community patterns identified correlated well with the vertical depth
distribution of the most abundant species. Dietary analysis reveals that myctophids are mostly zooplanktivorous, while
the gempilyd P. gracilis is classified as a piscivorous predator. The small P. bolini feed mainly on copepods of the species
Metridia gerlachei, while the most important prey item of the larger myctophids E. antarctica, G. braueri, and G. nicholsi
were various species of euphausiids. Investigation of feeding chronology showed that G. nicholsi and P. bolini were
feeding day and night. Daily ration estimates for myctophid species ranged from 0.28% to 3.3% of dry body weight
(0.5–5.94% of wet body weight). Krill (Euphausia superba) were the most important food of E. antarctica and G.
nicholsi, accounting for 53.1% and 58.3% of the total food weight, respectively. The annual removal from the krill


 ÃCorresponding author. Tel.: +49-471-4831-1652; fax:+49-471-4831-1425.
  E-mail address: cpusch@awi-bremerhaven.de (C. Pusch).

0967-0637/$ - see front matter r 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dsr.2004.06.008
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                 C. Pusch et al. / Deep-Sea Research I 51 (2004) 1685–1708
1686

stock by both species was estimated to amount to 11.1–26.7% in the South Shetland Islands region. This estimate
emphasizes the important role of mesopelagic fish in the Antarctic ecosystem as a prevalent consumer of krill.
r 2004 Elsevier Ltd. All rights reserved.

Keywords: Marine fish; Myctophidae; Mesopelagic zone; Community structure; Stomach content; Predation; Daily ration




                                 world oceans, they occupy the third level and are
1. Introduction
                                 consumers of the second order. They are an
  One-quarter of all known fish species in the          important food source for the predators of higher
Southern Ocean live in the mesopelagic and            trophic levels like benthopelagic fish (Bulman et
bathypelagic zones (Kock, 1992). Myctophids are         al., 2002), seabirds (Guinet et al., 1996), fur seals
the dominant fish family in these zones, as far as        (Cherel et al., 1997) and squid (Rodhouse et al.,
diversity, biomass and abundance are concerned.         1992; Phillips et al., 2001). Any estimation of
Thirty-three myctophid species are known from          energy transport within the pelagic system must
the Southern Ocean, of which 11 have a circum-          include analyses of the individual diet composition
polar distribution. Although their geographical         of the mesopelagic fish and their rates of food
distribution and taxonomy have been described          consumption.
(Hulley, 1981; McGinnis, 1982), comparatively            Although the diet composition of the most
few studies have examined the vertical distribution       abundant myctophid species is documented (Ro-
of oceanic micronekton by intensive depth strati-        wedder, 1979a; Ascenio and Moreno, 1984;
fied sampling (Torres and Somero, 1988; Lancraft         Williams, 1985; Kozlov and Tarverdiyeva, 1989;
et al., 1989; Piatkowski et al., 1994; Duhamel,         Lancraft et al., 1991; Hoddell et al., 2000) only a
1998). All these studies indicate that the common        few studies have investigated feeding chronology
Antarctic myctophids are diel vertical migrators.        (Rowedder, 1979a) and estimated daily rations
  King George Island is located in the southern         (Gerasimova, 1990; Pakhomov et al., 1996).
                                  Krill (Euphausia superba) plays an important
part of the Drake Passage and is strongly
influenced by the Antarctic Circumpolar Current          role as the key prey item of a number of top
(ACC). The ACC is the major oceanographic            predators, especially in the Atlantic Sector of the
feature of the Southern Ocean; it is an extensive        Southern Ocean (Barlow et al., 2002). Because of
eastward flowing circumpolar current (Hofmann           their high biomass (the total stock of the Southern
et al., 1996). The upper waters of the ACC in the        Ocean was estimated by Lancraft et al. (1989) to
study area comprised Antarctic Surface Water           account for 133–191 million tonnes), mesopelagic
(ASW) and the associated Circumpolar Deep            fish could be one of the most important predators
Water (CDW), which flows from the Bellingshau-          of oceanic zooplankton (Lancraft et al., 1989;
sen Sea into southern Drake Passage (Stein and          Pakhomov et al., 1996). Numerous studies have
Heywood, 1994). The study site on the slope of          shown that myctophids play a significant role in
King George Island is characterized by a shelf-         the consumption of juvenile and adult krill
break front resulting in enhanced production and         (Rembiszewski et al., 1978; Rowedder, 1979a;
a higher krill abundance compared to oceanic           Williams, 1985). This conclusion has more recently
waters. For this reason the area north of King          been challenged by a suggestion that a substantial
George Island is one of the most important krill         consumption of krill occurs only during certain
fishing regions of the Southern Ocean (Ichii et al.,       periods and within specific regions (Pakhomov et
1996).                              al., 1996).
  Myctophids play a significant role as consumers          Finally, it should be noted that a preliminary
of zooplankton in the food web of the Southern          analysis of community structure of the mesopela-
Ocean (Lancraft et al., 1989). As in the other          gic fish assemblage from cruise ANT XIV/2 of RV
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‘‘Polarstern’’ has already been presented by Hulley                  The sampling program consisted of 16 hauls,
et al. (1998), and the results used for a cladistic                 with the objective to sample three different depth
analysis of the myctophid tribe Electronini (Hul-                  horizons: 200–300 m, 400–500 m and near-bottom
ley, 1998). In the present study, however, commu-                  (2–28 m above seafloor) at soundings of 400, 600
nity structure of the mesopelagic community over                  and 800 m (Table 1) (Kock et al., 1998). Mesope-
the slope of King George Island is analysed in                   lagic fish were sampled with a pelagic trawl PT-
                                          1088 with an estimated mouth opening of 200 m2
greater detail. The community pattern and the
vertical distribution are related to the feeding                  (width 20 m and a height of 10–12 m). The mesh
ecology of the most abundant mesopelagic fish                    size was 12 mm in the cod end. It was expected that
species. Four myctophids and one gempylid are                    juvenile myctophids (SLo30 mm) would not be
analysed with respect to diet, feeding chronology                  sampled adequately with this net configuration
and daily rations. In conclusion, an estimation is                 (Gartner et al., 1988). An SCANMAR depth
made of the predation impact of the mesopelagic                   sensor controlled the sample depth and net open-
fish community on the krill stock in the region of                  ing during trawling. Towing time varied between
King George Island.                                 30 and 60 min; trawl speeds ranged from 3.5 to
                                          4.0 knots. Ship speed was increased during net
                                          deployment and decreased during retrieval. This
                                          procedure minimizes the effects of net contamina-
2. Material and methods
                                          tion by fish resident in water layers above the
                                          fishing depth. Station 73 was excluded from
2.1. Sampling
                                          community analysis as the net snagged on the
                                          bottom during trawling (Hulley et al., 1998). All
  Data were collected during the cruise ANT
                                          fishes were identified to species according to the
XIV/2 of RV ‘‘Polarstern’’ in November/Decem-
                                          most recent keys (Gon and Heemstra, 1990). Fish
ber 1996. The study area was located over the
                                          from the entire sample or a sub-sample of each
slope northwest of King George Island (South
                                          species from each station were counted and
Shetland Islands), in southern Drake Passage
                                          weighed, and standard lengths (SL) were taken
(Fig. 1).



          ˚S                                           1500 m

                                                       1000 m

         61.5                                            500 m
                                        51

                                        73
                                    70
                                 57
                                 52
                             53      68
                       55        69
                          59
                                58
                           63
                        64
                  65       54
                     60
                                            200 m



                                                  orge Is.
         62.0                                   King Ge




                                                               °W
                60.0        59.5         59.0     58.5      58.0        57.5

        Fig. 1. Sampling localities in 1996. Line indicates hydrographic section through the study area.
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Table 1
Station data for 16 PT-1088 trawl samples

ST     Sampling Locality    Date       Time     Day    Global      Sample   Bottom   Ship
                                  Timea
      Longitude Latitude    1996       (Local)         Radiation (wm2)  Depth (m)  Depth (m)  Speed (km)

      581430 W   611360 S
51                 30 November   16:49–17:49  Day    197.8       400–450   786–875   4.0
      591170 W   611450 S
52                 1 December    08:00–08:30  Day    75.0       415–515   660–730   4.0
      591190 W   611450 S
53                 1 December    10:57–11:27  Day    153.0       273–302   635–738   4.0
      591310 W   611480 S
54                 1 December    14:50–15:20  Day    230.3       283–295   475–652   4.0
      591340 W   611470 S
55                 1 December    17:35–18:05  Day    145.8       400–450   680–995   4.0
      591160 W   611430 S
57                 3 December    08:19–08:50  Day    179.7       397–465   730–790   4.0
      591130 W   611470 S
58                 3 December    20:26–21:26  Day    12.4       283–325   380–387   3.5
      591320 W   611480 S
59                 3 December    23:20–00:20  Night    0.0       431–495   850–1058  3.7
      591440 W   611480 S
60                 4 December    02:15–03:18  Night    0.0       380–440   681–690   3.5
      591320 W   611480 S
63                 4 December    20:30–21:11  Day    98.5       520–580   555–608   4.0
      591350 W   611470 S
64                 5 December    01:06–01:41  Night    0.0       610–640   731–792   4.0
      591510 W   611490 S
65                 5 December    04:26–04:56  Day    80.6       750–800   1287–1468  4.0
      591090 W   611440 S
68                 5 December    21:10–21:41  Day    46.0       340–360   360–367   3.5
      591170 W   611460 S
69                 6 December    01:01–01:32  Night    0.0       560–597   584–599   3.7
      591020 W   611380 S
70                 6 December    04:50–05:20  Day    23.0       790–825   810–833   3.7
      581440 W   611360 S
73                 6 December    21:07–21:10  Day    20.0       550–575   570–580   3.5
   a
     Defined by sunset 22:06 and sunrise 03:23.


to the nearest millimetre with sliding callipers.             (DWi ) were calculated (George and Hadley, 1979).
Hydrographic data were collected by conductivity,             By the following equation, all three indices were
temperature and depth casts (CTD, 22 stations).              combined to describe the prey utilization by the
The CTD was deployed at each station in advance              ‘Relative Importance Index’ (RI) for each prey
                                      category i (George and Hadley, 1979; Hyslop,
of the trawl tows.
                                      1980):
2.2. Diet analysis                                ðFi þ Ni þ DWiÞ Â 100
                                      RI i ¼ Ps           ;          ð1Þ
                                           i¼1 ðFi þ Ni þ DWiÞ
  Diet analysis was performed on the five most
abundant mesopelagic fish species. A maximum of
                                      where s is the number of prey categories.
20 individuals of these species was selected from
                                       Feeding chronology was analysed by the Sto-
each sample. In samples containing 420 speci-
                                      mach Content Index (SCI):
mens individuals were chosen haphazardly. Fish
                                           dry weight of stomach content
were weighed wet, measured (SL, mm below) and               SCIð%Þ ¼                 Â 100:
                                               body dry weight
the whole stomach removed. The dry weight of fish
specimens was determined by oven-drying speci-                                      ð2Þ
mens at 80 1C until constant weight was reached.
  Prey organisms were identified to the lowest                In addition, the stage of digestion of each prey
possible taxon and measured under a binocular               item was determined by the modified method of
microscope. Dry weight of the prey organisms was              Pearcy et al. (1979): Stage 1=undigested prey,
reconstructed by length–weight regressions taken              Stage 2=slightly digested with some appendages
from the literature (Mizdalski, 1988; Groeger               damaged, but body shape still preserved, Stage 3:
et al., 2000).                               body shape of prey deformed. The ratio of
  Three indices, the frequency of occurrence (F i )            digestion stages was calculated for each time
of each prey item in non-empty stomachs, the                interval based on these criteria.
                                       Daily ration (mean daily food consumption, C w )
percentage of each food item by number (N i ) to
the total number, and the percentage by dry weight             of the four myctophids was investigated by the
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method of Eggers (1977):                   intensity were log transformed to validate the use
                               of normalized Euclidean distance for the calcula-
C w ¼ I Â 24=T;                   ð3Þ
                               tion of the abiotic similarity matrix. We applied
where I is the daily average SCI (%) and T the gut      the Spearman Rank correlation to relate the biotic
passage time (h).                       matrix based on mesopelagic fish abundances with
  In this study, no gut passage time data were        the abiotic similarity matrix (Clarke and Ains-
collected. Therefore, data from the literature were      worth, 1993).
used. Two estimates of egestion times for Antarc-        Two different water masses were discernible
tic myctophids are available: the first estimate by      over the slope of King George Island, separated at
Rowedder (1979a) for Electrona antarctica and the       a depth of 450 m (see Section 3). Mesopelagic fish
second by Gerasimova (1990) for E. carlsbergi.        samples above and below this depth were com-
These studies estimated the egestion time to be 8       pared with different sub-routines of the Primer
and 8.5 h, respectively. In our study, we used the      computer program.
8.5 h suggested by Gerasimova. For the calcula-         One-way analysis of similarity, ANOSIM
tion of daily ration some authors (Pakhomov et        (Clarke and Warwick, 1994), was employed to test
al., 1996) have recommended the substitution of 24      the hypothesis of no differences in mesopelagic fish
 10 in Eq. (3) for species that have an active        assemblage above and below 450 m. This subrou-
feeding period of 10 h. Other authors argue that       tine compares the average rank similarities within
this substitution introduces a significant conserva-      the predefined groups of samples with the average
tive bias (Williams et al., 2001). We calculated two     similarity between groups. Values close to 1 indicate
alternative daily rations using both 10 and 24.        a strong separation between groups, while a value
                               of 0 indicates no differences between groups.
2.3. Data analysis                        The similarity percentage routine (SIMPER)
                               (Clarke and Warwick, 1994), which was applied to
  Density and biomass data for mesopelagic fish        square-root transformed mesopelagic fish abun-
were calculated as individuals per filtered water       dances, identified the contribution from individual
volume. The filtered volume was calculated by         species to the dissimilarities between (the deep and
multiplying the trawled distance of the vessel with      the shallow) sample groups.
the estimated mouth opening (200 m2) of the PT-         Various univariate indices were calculated in
1088.                             order to characterize the species assemblages of the
  Community structure was investigated with the       deep and shallow group of samples: species
                               number, Shannon’s diversity index (H0 ) (Shannon
Primer-E5 Software package (Clarke and War-
wick, 2001). To reduce the weighting of dominant       and Weaver, 1949) and Pielou’s Evenness Index (J)
species, the densities were square-root transformed      (Pielou, 1975). As these indices are known to be
prior to the computation of the triangular          influenced by sample size, we also calculated the
similarity matrices based on Bray-Curtis simila-       taxonomic diversity D and taxonomic distinctness
                               DÃ ; which consider the taxonomic relatedness of
rities (Field et al., 1982). The results of the latter
were classified by hierarchical agglomerative clus-      species (Warwick and Clarke, 1995). Taxonomic
                               diversity D is empirically related to H 0 but
ter analysis using the group average linking
method, and ordinated by a non-metric, multi-         contains, in addition, information on the taxo-
dimensional scaling technique (MDS).             nomic separation of the species in a sample, i.e.
  The BIOENV sub-routine was used to relate the       besides the distribution of individuals among
community patterns of the mesopelagic fish           species it also takes into account the distribution
assemblage to six environmental variables: light       of species in the taxonomic system by weighting
intensity (W/m2) (indicating time of day), mini-       the co-occurrences of species according to the
mum and maximum values of sample depth,            degree of separation in the hierarchical classifica-
bottom depth, temperature and salinity at the         tion (1=different species, 2=different genera,
                               3=different families, 4=different orders). DÃ is
sampled depth horizon. Values of average light
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derived from D but measures solely the taxonomic          Sea and Scotia Sea mingle. Analysis of vertical
distinctness of species in a sample, without the          temperature and salinity sections through the
contribution from species diversity.                study site revealed the existence of two different
 Oceanographic data were obtained from the             water masses in the slope region of King
program Ocean Data View (Schlitzer, 2003). The           George Island (Fig. 2A–B). The water mass in
results are presented as a section through the study        the upper 200 m was composed of ASW, char-
area.                                acterized by a temperature minimum in 50–150 m
                                  depth (To0:5 1CÞ and a low salinity (o 34.4). The
                                  origin of this water body is due to the cooling of
                                  the surface water during winter. The properties of
3. Results
                                  this so-called winter water are stable year round
                                  (Hofmann et al., 1996). During our study the
3.1. Oceanographic conditions
                                  surface   water  temperature   was  slightly
                                  increased by enhanced solar radiation. The
 The study area is situated in the Weddell-Scotia-
                                  dominating water mass below 400 m depth,
Confluence, where water masses of the Weddell




Fig. 2. (A–B) RV ‘‘Polarstern’’ cruise ANT XIV/2, east–west hydrographic section through the study area as indicated in Fig. 1, (A)
temperature (1C) and (B) salinity.
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Circumpolar Deep Water, was characterized by            other families represented by just one species.
                                  Three myctophids, Gymnoscopelus fraseri, G.
higher temperature (T40.5 1C) and salinity
                                  hintonoides and Protomyctophum choriodon, were
(S434:5). A transition zone between these water
masses occupies the depth range 200–400 m, in            newly recorded for the South Shetland Island
which the gradients of temperature and salinity           region.
were strongest.                            Myctophids dominated the mesopelagic fish
                                  community by number and wet weight accounting
                                  for 98.5% of all sampled individuals and 87.3% of
3.2. Species composition
                                  the total weight. E. antarctica was the most
                                  abundant species by number and weight. This
 The total catch of mesopelagic fish taken on 15
                                  myctophid accounted for 60.7% of all sampled
pelagic trawl stations included 16 343 specimens
                                  individuals, followed by P. bolini (19.6%), G.
with a wet weight of 122.5 kg (Table 2). Eighteen
                                  braueri (13.8%) and G. nicholsi (4.0%). All other
mesopelagic species belonging to 10 families and
                                  mesopelagic fish species were of numerically minor
13 genera were identified. The family Myctophidae
                                  importance (o2% in total).
was by far the most speciose (9 species), with all


Table 2
Number of individual fish, density as number of individuals per 105 m3 filtered volume, wet weight in kg and biomass as g wet weight
per filtered volume in 15 PT-1088 samples

Species                Abundance                     Biomass

                                   Ind/105 m3                   g ww/105 m3
                   No. Inds.    (%)                kg      (%)

Astroneshtidae
 Borostomias antarcticus         1      (0.01)     0.00        0.01     (0.01)     0.05
Bathylagidae
 Bathylagus antarcticus          4      (0.02)     0.03        0.05     (0.04)     0.43
Chiasmodontidae
 Chiasmodon niger             1      (0.01)     0.00        0.02     (0.02)     0.09
Gempylidae
 Paradiplospinus gracilis        182      (1.11)     1.14        14.96    (12.21)     93.29
Gonostomatidae
 Cyclothone sp.              2      (0.01)     0.02        0.01     (0.01)     0.09
Melamphaidae
 Poromitra crassiceps           2      (0.01)     0.01        0.02     (0.02)     0.09
Microstomatidae
 Nansenia antarctica           3      (0.02)     0.02        0.09     (0.07)     0.66
Myctophidae
 Electrona antarctica         9931      (60.71)    59.48        67.93    (55.46)    407.98
 Gymnoscopelus braueri         2253      (13.77)    20.23        13.71    (11.19)    122.72
 Gymnoscopelus fraseri          2      (0.01)     0.01        0.02     (0.02)     0.14
 Gymnoscopelus hintonoides        15      (0.09)     0.11        0.13     (0.11)     0.99
 Gymnoscopelus nicholsi         647      (3.96)     4.2        21.15    (17.27)    136.81
 Gymnoscopelus opisthopterus       27      (0.17)     0.13        0.96     (0.78)     4.52
 Krefftichthys anderssoni        23      (0.14)     0.18        0.07     (0.06)     0.55
 Protomyctophum bolini         3212      (19.63)    20.1         2.97     (2.42)     18.65
 Protomyctophum choriodon         1      (0.01)     0.00        0.01     (0.01)     0.04
Notosudidae
 Scopelosaurus hamiltoni         3      (0.02)     0.02        0.27     (0.22)     1.48
Paralepididae
 Notolepis coatsi            34      (0.21)     0.22        0.11     (0.09)     0.71
Total                 16 343                       122.5
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 In wet weight, E. antarctica accounted for                       made. Nevertheless, some assumptions about the
55.6% of total mesopelagic fish biomass, far                       vertical distribution patterns of the more abundant
exceeding G. nicholsi (17.3%), G. braueri (11.2)                     species are possible. All four myctophids were
and the gempilyd, Paradiplosinus gracilis (12.2%).                    more abundant, in the 273–825 m depth range, in
                                             daytime samples than in nighttime samples (Fig.
                                             3A–D). This observation supports the results of
3.3. Patterns of vertical distribution
                                             earlier studies that the bulk of Antarctic mycto-
                                             phids migrate to epipelagic layers at night (Torres
 Because of the sampling strategy (no samples
                                             and Somero, 1988; Lancraft et al., 1989).
above 273 m depth and only four night tows) no
                                              P. gracilis was concentrated in depths below
detailed conclusions about the vertical migration
                                             400 m during daytime (Fig. 3E). The distribution
behaviour of the mesopelagic community can be


                                                  (B)           G. braueri
                                               0
              0    (A)           E. antarctica

                                              200
             200

                                              400
             400

                                              600
             600

                                              800
             800
                                               300   200   100    0  100  200  300
              300    200   100    0   100    200    300
                                                  (D)
              0                                 0
                  (C)
                              G. nicholsi                         P. bolini
             200                               200
       Depth (m)




             400                               400


             600                               600


             800                               800

                20      10           10       20    80     40     0    40    80
                            0

                  (E)
              0
                              P. gracilis

             200


             400


             600


             800
                                        4
                4     2     0      2

                                        Density
Fig. 3. (A–E) Vertical day/night distribution: (A) E. antarctica, (B) G. braueri, (C) G. nicholsi, (D) P. bolini, (E) P. gracilis, Density as
individuals per 105 m3; open bars=day tows; filled bars=night tows.
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               C. Pusch et al. / Deep-Sea Research I 51 (2004) 1685–1708                 1693

data show an increased density in this species
between 400 and 500 m, suggesting vertical migra-
tion behaviour at least in mesopelagic depths. The
four myctophids showed different daytime depth
distributions: The bulk of E. antarctica individuals
were found below 400 m depth (Fig. 3A) and the
minimum depth of occurrence of G. braueri was
750 m (Fig. 3B). In contrast, P. bolini occurred
over a depth range of 273–825 m, with the bulk of
the population in the shallow 273–450 m interval
(Fig. 3D). G. nicholsi occurred over the whole        Fig. 4. Dendrogram representing the classification of 15 PT-
                               1088 stations according to mesopelagic fish assemblage; Station
sampled depth range (273–825 m) with a centre of
                               number and maximum sampled depth are indicated. Hatching
distribution between 400 and 597 m (Fig. 3C).
                               indicates shallow stations (depth 302–450 m).


3.4. Community analysis
                                                  Deep
                                         Shallow
  Cluster analysis based on densities of mesope-
lagic fish showed a clear separation of stations at                            68
an arbitrary level of 40% (Fig. 4). The first cluster
                                      65
comprises stations sampled at and above 450 m          70
depth. The second cluster is exclusively composed                                 58  54
                                         60 59
                                      69
of stations taken in depth4450 m, with the                         52
                                                     53 55
                                               57
exception of station 60, where sampling depth             63
was 380–440 m.                                  64
  The separation of stations at 450 m depth is
confirmed by the ordination with MDS of the
                                                  51
same assemblage data (Fig. 5). Again, station 60
was more closely associated with the deeper group
of stations.                         Fig. 5. MDS plot for 15 PT-1088 stations according to the
                               mesopelagic fish assemblage; shallow and deep refer to the
  An exploratory analysis, BIOENV, was applied
                               sampled depth; shallow=depth 302–450 m; deep=depth
to examine which abiotic variables could best
                               465–825 m; stress (=goodness of fit)=0.07.
explain the observed patterns in the mesopelagic
fish assemblage. The maximum matching coeffi-
cient was achieved by the abiotic variable max-         An analysis of similarity (ANOSIM) was
imum sample depth (51.7%) and supported           performed to test for statistical differences in
therefore the result from the MDS and cluster        species composition between shallow stations
analyses. Minimum sample depth (46.4%) and          (295–450 m   depth)   and   deep   stations
                               (465–825 m). A value of R ¼ 0:561 supported the
minimum and maximum salinity (34.9% and
28.6%, respectively) were further useful abiotic       results of the classification and ordination of the
parameters to explain the observed pattern in the      data and indicated significant differences in species
mesopelagic fish community. Other abiotic vari-        composition between shallow and deep stations
ables showed a low correlation with the biotic        (P ¼ 0:003).
similarity matrix and yielded matching coefficients       The similarity percentage procedure (SIMPER)
below 25%. Sample depth is thus the best           was applied to identify those species that con-
environmental variable to explain the grouping        tribute most to the observed differences between
of the samples in a manner consistent with the        shallow and deep samples (Table 3). Only three
biotic pattern.                       species were more abundant in the shallow group
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                  C. Pusch et al. / Deep-Sea Research I 51 (2004) 1685–1708
1694

Table 3
SIMPER analysis; discriminating species between ‘‘deeper’’ and ‘‘shallower’’ station groups as revealed by cluster analysis

                      Avg. density (Ind/105 m3)
Species                                                       Cumulative

                      Shallow        Deep         Contribution (%)       Contribution (%)

Electrona antarctica            19.01         94.89        43.33             43.33
Protomyctophum bolini           33.6          8.3         18.8              62.13
Gymnoscopelus braueri            0.01         37.93        16.35             78.48
Gymnoscopelus nicholsi           4.13         4.26         5.89             84.36
Paradiplospinus gracilis          0.3          1.88         5.22             89.59
Notolepis coatsi              0.05         0.37         2.29             91.88
Krefftichthys anderssoni          0           0.34         2.28             94.16
Gymnoscopelus hintonoides          0           0.2         1.67             95.82
Gymnoscopelus opisthopterus         0           0.24         1.19             97.02
Bathylagus antarcticus           0           0.06         0.5              97.52
Nansenia antarctica             0.01         0.03         0.47             97.99
Scopelosaurus hamiltoni           0           0.03         0.46             98.45
Gymnoscopelus fraseri            0           0.03         0.41             98.85
Protomyctophum choriodon          0           0.02         0.28             99.13
Poromitra crassiceps            0.01         0          0.25             99.38
Cyclothone sp.               0           0.04         0.25             99.62
Chiasmodon niger              0           0.01         0.2              99.82
Borostomias antarcticus           0.01         0          0.18             100

Species are ordered in decreasing contribution (%) to the total dissimilarity.



                                     Table 4
of samples, while 15 species were more abundant
                                     Univariate indices of the deeper and shallower groups of
in the deeper group. The myctophid P. bolini was
                                     stations over the slope of King–George Island
the best indicator species for samples taken above
450 m depth and accounted for 18.8% of the                                 Shallow      Deep
observed differences. Deeper stations were char-
                                     No. of species           9         16
acterized by relatively higher abundances of E.
                                     Density (Ind./105 m3)       57.13       148.61
antarctica and G. braueri and P. gracilis , which            Pielou’s evenness (J)        0.35        0.41
together accounted for 65% of the observed                Shannon’s diversity (H0 )      0.51        0.79
differences between the two groups of samples.              Taxonomic diversity (D)      10.03       14.64
                                     Taxonomic distinctness (DÃ )    35.87       37.02
  The calculation of several univariate indices for
the species densities above and below 450 m depth
reflected the observed differences between the
                                     not significant. The taxonomic distinctness DÃ
mesopelagic fish assemblages (Table 4). The
number of species (9) in depth between 273 and              showed a high similarity between shallow and
450 m was lower, compared to 16 species in depths            deep samples.
below 450 m (two individuals of Cyclothone sp.
                                     3.5. Feeding ecology
were not identified to species level). Also the
average density of mesopelagic fish was 2.5 times
                                     3.5.1. Food composition
higher in the deeper group samples. Shannon’s
diversity (H0 ), Pielou’s evenness index (J) and the            The diet of E. antarctica was dominated by
taxonomic diversity D were higher in samples               euphausiids in our samples. By number, euphau-
taken below 450 m depth. However, the observed              siids (47.8%) are followed by ostracods (15.6%)
differences in diversity indices between the deeper           and then by copepods (26.7%) (Fig. 6A; Table 5).
and shallower group of samples were statistically            By dry weight (DW), euphausiids account for
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                                                            G. braueri (n = 21)
                    E. antarctica (n = 61)
                                                         DW (%)               A (%)
                 DW (%)               N (%)

                                               100  80   60   40   20   0     20   40  60    80  100
         100  80   60   40   20  0    20   40   60  80  100




                             20                                      20
               Eu (RI = 61.1)                                 Eu (RI = 52.9)
                             40                                      40

                             60                                      60

                                      Cop (14.9)
                             80                                      80     Os (19.0)
                                      Os (12.0)
                             100                                      100
                                                                         Co (13.9)
                                      Am (6.0)
                                      Ch (4.3)
                                                                          Am (9.3)
                                                                          Ch (4.9)
                          F (%)                   (B)
         (A)                                                       F (%)


                      G. nicholsi (n = 135)                            P. bolini (n = 101)
                 DW (%)               N (%)                 DW (%)                N (%)

         100  80   60   40   20  0    20   40   60  80  100  100  80  60   40   20   0    20   40  60    80  100




                             20                                     20
               Eu (RI = 44.0)
                             40                                     40     Co (RI = 82.0)
                             60                                     60

                             80                                      80
                                Co (37.9)
                             100                                     100     Os (10.0)

                                                                         Eu (6.2)

                                Os (7.3)
                                                                F (%)
                                                (D)
         (C)
                                Ch (5.7)
                                Pi (4.7)

                          F (%)

                     P. gracilis (n = 28)
                 DW (%)                N (%)
        100   80   60   40   20  0    20   40  60  80  100
                                                     Am:      Amphipods
                                                     Ceph: Cephalopds
                             20
                                                     Ch:      Chaetognaths
               Pi (RI = 76.8)       40
                                                     Co :     Copepods
                             60
                                                     Eu :     Euphausiids
                             80
                                                     Os :     Ostracods
                                  Eu (15.1)
                             100
                                                     Pi :     Pisces
                                  Ceph (8.4)
         (E)                F (%)

Fig. 6. (A–E) Percentage composition of the main prey items of (A) E. antarctica (B) G. braueri (C) G. nicholsi (D) P. bolini (E) P.
gracilis, by percent dry weight (DW), percentage number (N) and frequency of occurrence (F), The relative importance (RI) Index is
presented by the size of the respective rectangles. n=number of stomachs containing food.
                                                                                       1696
Table 5
Diet composition of E. antarctica, G. braueri, G. nicholsi, P. bolini and P. gracilis, showing number and reconstructed dry weight of each food item; the respective
percentage of the total number and the total dry weight is given in parenthesis; ‘‘-’’: absent.

             Electrona antarctica     Gymnoscopelus braueri    Gymnoscopelus nicholsi   Protomyctophum bolini     Paradiplospinus gracilis
SL range (mm)      57–113            69–121           123–172           33–53             284–469
No. of fish examined   145              67             186             122              100
No. of stomach empty (%) 84   (57.9)         46   (68.7)        51   (27.4)       21  (17,2)         72   (72)




                                                                                       C. Pusch et al. / Deep-Sea Research I 51 (2004) 1685–1708
             No.  (%) Weight (%)     No   (%) Weight (%)    No   (%) Weight (%)   No. (%) Weight (%)      No.   (%) Weight %

Amphipoda          2  (2.2) 210.86 (13.9) 1   (3.2) 84.08 (23.5) 1     (0.1) 50.75  (0.8) —    —   —     —   —   —   —     —
Cephalopoda         —  —   —   —   —   —         —     —   —    —   —    —   —     —   3   (8.3) 9900    (7.4)
 Psychroteuthis glacialis                                                          3   (8.3) 9900    (7.4)




                                                                                                                     ARTICLE IN PRESS
Chaetognatha         5  (5.6) 3.9   (0.3) 2   (6.5) 1.56   (0.4) 21   (2.6) 241.78 (3.8) 4     (0.3) 3.12   (0.6) —   —   —     —
Copepoda           24  (26.7) 5.42  (0.4) 7   (22.6) 1.97   (0.6) 517  (63.8) 235.67 (3.7) 1515   (96.6) 354.93  (84.2) —   —   —     —
 Calanus propinquus     1  (1.1) 0.8   (0.1) —   —   —    —   23  (2.8) 21.84 (0.3) 8     (0.5) 6.21   (1.5) —   —   —     —
 Gaidius spp.        1  (1.1) 0.37   (0.0) 3   (9.7) 1.11   (0.3) 10   (1.2) 3.7   (0.1) 11   (0.7) 4.07   (1.0) —   —   —     —
 Metridia gerlachei     11  (12.2) 1.62  (0.1) —   —   —    —   341  (42.1) 68.58 (1.1) 1438   (91.7) 319.83  (75.9) —   —   —     —
 Pareuchaeta spp.      1  (1.1) 1.35   (0.1) —   —   —    —   67  (8.3) 95.82 (1.5) 13     (0.8) 15.87   (3.8) —   —   —     —
 Rhincalanus gigas     —  —   —    —   1  (3.2) 0.43   (0.1) 46   (5.7) 29.69 (0.5) 5     (0.3) 2.49   (0.6) —   —   —     —
 Unidentified calanoids   10  (11.1) 1.28  (0.1) 3   (9.7) 0.43   (0.1) 30   (3.7) 16.04 (0.3) 40     (2.6) 6.46   (1.5) —   —   —     —
Euphausiacea         43  (47.8) 1292.07 (85.0) 13  (41.9) 268.66  (75.0) 226  (27.9) 4950.82 (77.9) 8   (0.5) 54.79   (13.0) 8   (22.2) 280.69  (0.2)
 Euphausia cristallorophias —  —   —    —   —  —   —    —   1   (0.1) 7.55   (0.1) —    —   —    —   —   —   —     —
 E. frigida         1  (1.1) 14.63 (1.0) —    —   —    —   2   (0.2) 78.12 (1.2) —     —   —    —   —   —   —     —
 E. superba         14  (15.6) 807.23 (53.1) —   —   —    —   67  (8.3) 3707.89 (58.3) —    —   —    —   6   (16.7) 223.19  (0.2)
 E. triacantha       1  (1.1) 19.96 (1.3) 1    (3.2) 36.82   (10.3) 4   (0.5) 163.74 (2.6) —     —   —    —   1   (2.7) 45.4    (0.0)
 Euphausia spp.       22  (24.4) 425.04 (28.0) 12  (38.7) 231.84  (64.8) 20  (2.5) 386.4 (6.1) —     —   —    —   —   —   —     —
 Thysanoessa macrura    5  (5.6) 25.21 (1.7) —    —   —    —   132  (16.3) 607.12 (9.6) 8    (0.5) 54.79   (13.0) 1   (2.7) 12.1    (0.0)
Gastropoda          1  (1.1) 1.56   (0.1) —   —   —    —   —   —   —    —   1   (0.1) 1.56   (0.4) —   —   —     —
Ostracoda          14  (15.6) 4.78  (0.3) 8   (25.8) 1.74   (0.5) 39   (4.8) 8.76   (0.1) 41   (2.6) 7.13   (1.7) —   —   —     —
Pisces            —  —   —    —   —  —   —    —   6   (0.7) 866.97 (13.6) —    —   —    —   25  (69.4) 123868.28 (92.4)
 Electrona antarctica    —  —   —    —   —  —   —    —   —   —   —    —   —   —   —    —   2   (5.6) 6435.36 (4.8)
 Gymnoscopelus nicholsi —    —   —    —   —  —   —    —   —   —   —    —   —   —   —    —   1   (2.7) 10162.92 (7.6)
 Gymnoscopelus braueri —     —   —    —   —  —   —    —   —   —   —    —   —   —   —    —   1   (2.7) 2270    (1.7)
 Neopageotopsis ionah    —  —   —    —   —  —   —    —   2   (0.3) 408.25 (6.4) —     —   —    —   —   —   —     —
 Protomyctophum bolini —     —   —    —   —  —   —    —   4   (0.5) 458.72 (7.2) —     —   —    —   —   —   —     —
Polychaeta          1  (1.1) 1.84   (0.1) —   —   —    —   —              —   —   —    —   —   —   —     —
Total            90     1520.43    31     358.01     810     1742.7     1569     7.13      36      134048.97
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85.0% of the total stomach content, far exceeding      the identifiable food items respectively, while other
amphipods (13.9%) and the other prey categories       food items accounted for less than 5% each. In dry
(o 2% in total). Euphausiids occurred in 65.6% of      weight (DW) euphausiids made up 77.9% of the
all filled stomachs, while ostracods and copepods       total prey, followed by fish (13.6%), chaetognaths
occurred less frequently (21.1% and 23.0%,          (3.8%) and copepods (3.7%). The frequencies of
respectively). The RI values were 61.1% for         occurrence for copepods, euphausiids, ostracods
euphausiids, 14.9% for copepods and 12.0% for        and chaetognaths were 77.7%, 62.9%, 22.9% and
ostracods, stressing the importance of euphausiids      15.5%, respectively, while the remaining prey
in the diet of E. antarctica.                categories accounted for less than 4.4% in total.
  The size distribution of the 145 individuals       The RI values were 44.0% for euphausiids and
examined for food composition was unimodal and        37.9% for copepods, while the remaining prey
ranged from 57 to 113 mm (Fig. 7A). To investi-       categories accounted foro10%.
gate an ontogenetical shift in the prey size of E.       The size distribution of the 186 individuals of G.
antarctica we compared the ratio of euphausiids       nicholsi examined for stomach content was uni-
(largest prey item) to the total dry weight of        modal and ranged from 123 to 172 mm (Fig. 7C).
stomach content in individuals smaller and larger      The ratio of euphausiids (largest prey item) to the
than 80 mm SL. The ratio of euphausiids to the        total dry weight of the stomach content was
                               54.7764.0% (avg. 7SD) for 123–144 mm SL
total dry weight of stomach contents was
60.7749.0% (avg.7SD) for 57–79 mm sized           individuals and 58.3745.1% for 145–172 mm SL
individuals, and 68.6746.2% for 80–113 mm SL         fish. The difference was not statistically significant
                               (Mann–Whitney U test, p40:05) and we therefore
fish. The difference was not significant (Man-
n–Whitney U-test, p40:05). Therefore, no signifi-       suggest that no substantial ontogenetic shift in
cant shift in prey size was detectable over the size     prey size occurs over the size range examined.
                                Prey species composition showed that G. nicholsi
range examined.
  In this study, feeding intensity of E. antarctica     fed on all five euphausiids species occurring in the
was low, with 57.9% of stomachs empty (Table 5).       Southern Ocean (Table 5). The most important
                               euphausiid by food weight was E. superba (58.3%
More then half of all euphausiids were not
                               of total food weight), while Metridia gerlachei
identifiable to species. Nevertheless, krill (E.
superba) was the most important euphausiid in        dominated numerically (42.1%). G. nicholsi was
the diet of this myctophid, accounting for more       the only myctophid in our study that preyed on
                               other mesopelagic fish (Neopageotopsis ionah and
than one-half (53.1%) of the overall food weight.
  The prey composition of G. braueri was          Protomyctophum bolini).
                                The food spectrum of P. bolini, a relatively
similarly dominated by euphausiids (Fig. 6B,
Table 5). Euphausiids were most important by         small-sized myctophid (SL 33–53 mm, Fig. 7D),
number (N ¼ 41:9%), DW (75.0% of total weight)        was dominated by copepods (Fig. 6D, Table 5).
                               The N, DW and F values for copepods were
and frequency of occurrence (61.9%). The RI
reflected the high dietary value of euphausiids        96.6%, 84.2% and 91.1%, respectively. The RI
(52.9%), while ostracods, copepods and amphi-        value of copepods was 82.0% followed by
pods were of minor importance with RI values of       ostracods (10.0%) and euphausiids (6.2%).
19.0%, 13.9% and 9.3%, respectively. Despite the        The prey species composition showed that
                               copepods of the genus M. gerlachei were by far
high significance of euphausiids in the diet, the
most abundant euphausiid, E. superba, was lack-       the most important prey item, accounting for
ing (Table 5). The feeding intensity of G. braueri      91.7% of all identified food items (Table 5).
                                The diet of P. gracilis, the only non-myctophid
was noticeably low, with 68.7% of stomachs
containing no identifiable food.               in our analysis, consisted almost exclusively of fish
  The diet of G. nicholsi, was numerically domi-      (Fig. 6E, Table 5). The three trophic parameters N,
                               DW and F for this prey category were 69.4%,
nated by copepods and euphausiids (Fig. 6C,
Table 5), accounting for 63.8% and 27.9% (N) of       92.4% and 82.1%, resulting in an RI of 76.8%.
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1698

                                                 (B)
                      (A)
               100                            100
                             E. antarctica                      G. braueri
                             n = 582                         n = 187
               80                             80


               60                             60


               40                             40


               20                             20


                 0                            0
                   0 20 40 60 80 100 120140 160180 200           0 20 40 60 80 100120140 160180 200

                                                 (D)
                                            100
              100      (C)
                               G.nicholsi                       P. bolini
                               n = 299                        n = 189
                                             80
               80
      Number of Fish




                                             60
               60


                                             40
               40


                                             20
               20


                                             0
               0
                                                0 20 40 60 80 100120140 160180 200
                   0 20 40 60 80 100120 140160180 200


                                      Length (SL, mm)
               100      (E)
                              P. gracilis
                              n = 105
                 80


                 60


                 40


                 20


                   0
                     0 20 40 60 80 100120 140160180 200

                         Length (SL, cm)

Fig. 7. (A–E) Length-frequency plots of (A) E. antarctica (B) G. braueri (C) G. nicholsi (D) P. bolini (E) P. gracilis showing size classes
of fish measured (open bars) and sampled for diet analysis (filled bars); n total number of fish measured.
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                                The average and range of SCI for G. nicholsi
  Most fishes found in the stomachs were not
identifiable because of the advanced stage of        were calculated to be 0.29% and 0.12% to 0.55%
digestion. Nevertheless, the common myctophids       (Fig. 9A). There was no statistical difference in
E. antarctica, G. nicholsi and G. braueri could be     feeding intensity between samples taken at day and
                                              U-test, p ¼ 0:178).
identified. Additional euphausiids and cephalo-       night   (Mann–Whitney
pods (Psychroteuthis glacialis) were found in the      However, feeding intensity was increased during
stomachs of P. gracilis, accounting for RI values      evening times at 2100–2200 h (average SCI 0.38%)
of 15.1% and 8.4%, respectively.              and reached a maximum at 2300–2400 h (SCI
                              0.55%). The observation of intensified feeding at
3.5.2. Feeding chronology                  these time periods is supported by a higher
  The influence of sample depth on feeding         proportion of undigested food items. We observed
intensity was investigated by the calculation of      a decrease in feeding intensity after midnight and
the Stomach Content Index (SCI) for E. antarc-       lasting until the early morning hours. The sto-
tica, G. nicholsi and P. bolini. We compared the      machs of G. nicholsi contained fresh food items
SCIs of individuals taken in shallow (sample depth     during day and night (stage 1) (Fig. 9C). For this
295–440) and deep tows (465–825 m) and found no       reason we suggest that feeding activity was taking
differences for E. antarctica, G. nicholsi and P.      place continuously.
bolini (Mann–Whitney U-Test; p40:05). There-          In the small P. bolini average SCI (1.1%) was
                              higher than in G. nicholsi (Fig. 9B). We excluded
fore, it was concluded that sample depth had no
major influence on the SCI of these species. G.       one station (58) with extraordinarily high
braueri was taken in too low numbers in the upper      stomach filling (SCI 10.6%), and which we
depth horizon to be able to draw conclusions        interpret as an outlier. No difference in median
about the influence of sample depth on feeding        SCI was found between daytime and nighttime
                              samples (Mann–Whitney U test p ¼ 0:127). Never-
behaviour.
  The SCI of E. antarctica fluctuated between        theless, feeding activity was increased during hours
0.0% and 0.9% (avg. 0.47) over a 24 h time period      of darkness when SCI values were above the
(Fig. 8A). Highest feeding activity was observed at     average. A second peak in feeding intensity was
night (0200–0300 h). Minimum values in feeding       observed in the morning hours from 0800 to
intensity were found for the intervals 0800–0900 h     0900 h (SCI 1.7%). During the day, SCI reached a
and 1100–1200 h. Nevertheless, no difference in       minimum value of 0.21 at 1400–1500 h. The
feeding intensity was found between day and night      highest proportions of undigested food items
samples (Mann–Whitney U-test, p ¼ 0:081), be-        were recorded in the evening and before midnight
cause of the high variability of the SCI values       (Fig. 9D).
during day. The proportion of undigested food
                              3.5.3. Daily ration and the impact of predation on
items (Stage 1) was reduced during daytime except
                              the krill stock
at one station during the time interval 1100–1200 h
where results are based on a low number of fish (5)       The daily ration estimate was highest for the
                              small myctophid P. bolini, accounting for 2.48%
and food items (2) (Fig. 8C).
  The average SCI of G. braueri was 0.22%, and       of fish wet weight assuming a 10 h feeding period
values ranged from 0.04% to 0.4% (Fig. 8B).         and 5.94% assuming that the species feeds 24 h a
It is difficult to draw conclusions about the        day (Table 6). The lowest daily ration was
                              calculated for G. braueri, accounting for 0.5%
feeding chronology of this species because of the
low overall feeding intensity. We found no         wet body weight under the conservative assump-
statistical differences in SCI between day and       tion and 1.19% with a 24 h feeding period. The
night samples (Mann–Whitney U-test, p ¼ 0:867).       estimated daily rations of G. nicholsi are calculated
                              to be only slightly higher than in G. braueri at
The proportion of fresh prey items was highest
                              0.65% and 1.57%. E. antarctica took an inter-
in the early evening (time interval 2000–2100 h)
(Fig. 8D).                         mediate position with daily rations of 1.06% and
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Fig. 8. (A–D) Diel changes in stomach content of E. antarctica and G. braueri: (A–B) Average Stomach Content Index (SCI); vertical
bars=standard error; n=number of individuals examined. (C–D) Proportion of digestion stage (stage 1=fresh food, stage 2=slightly
digested, stage 3=only indigestible remains). Sunset (SS) and sunrise (SR) are indicted on the x-axis. Filled bars represent hours of
darkness.



2.54% of wet body weight by the 10 and 24 h             region in the years 1996/1997 was estimated to be
                                   92 g/1000 m3 (Siegel et al., 1998b). We estimated
feeding period assumption.
  Two myctophids in our study, E. antarctica and          the biomass of E. antarctica and G. nicholsi to be
                                   4.08 and 1.36 g/1000 m3, respectively, in slope
G. nicholsi, fed on krill (E. superba). Based on the
food composition data and the daily rations, we           waters off King George Island. The daily intake
                                   of krill by E. antarctica was assessed to be 1.06%
assessed the predation impact of these myctophids
on the krill stock in the study area.                of its body wet weight during a 10 h feeding period
  E. superba was the most important prey item in          and 2.54% in 24 h.
the diet of E. antarctica and G. nicholsi and              The predation impact of G. nicholsi on krill was
accounted for 53.1% and 58.3% of the total food           estimated to be 0.65% of body wet weight
weight in these species, respectively. The standing         under the assumption of a 10 h feeding period
stock of krill biomass in the Elephant Island            and 1.57% assuming a 24 h feeding period.
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        Fig. 9. (A–D) Feeding periodicity of G. nicholsi and P. bolini, same abbreviations as in Fig. 8.



Neglecting seasonal variability in consumption,          micronekton community in the Ice-free Zone of
our data suggest an annual removal of 9.0–21.8%          the Scotia Sea (Lancraft et al., 1989; Piatkowski
of the krill stock by E. antarctica and 2.1–5.0% by        et al., 1994).
G. nicholsi.                             It was expected that the mesopelagic fish
                                  assemblage at King George Island, which is
                                  localized in the Seasonal Pack-ice Zone, would
                                  be characterized by a lower species number and
4. Discussion
                                  diversity compared to the Ice-free Zone (Kock,
4.1. Species composition                      1992). The relatively high number of species over
                                  the slope of King George Island could be due to
  Eighteen mesopelagic fish species were identified         increased productivity over the slope of King-
in samples in the slope waters of King George           George Island, itself due to a shelf-break front as
Island, situated in the seasonal pack ice zone. This        described by Ichii et al. (1998). Unpublished
number agrees with previous studies of the             LIDAR measurements performed on cruise ANT
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Table 6
Daily ration and predation impact on the krill stock of four myctophid species

                              E. antarctica     G. braueri   G. nicholsi   P. bolini

Avg. SCI (% body dry weight)                0.47         0.22      0.29       1.10
Feeding period (h)                     10/24         10/24      10/24      10/24
Daily ration (% body dry weight)              0.59/1.41       0.28/0.66    0.36/0.87    1.38/3.3
Daily ration (% body wet weight)a             1.06/2.54       0.50/1.19    0.65/1.57    2.48/5.94
Proportion of E. superba in food weight (%)        53.1         0.00      58.30      0.00
Daily ration of E. superba (% body wet weight)       0.56/1.35       0.00      0.38/0.92    0.00
Predator density (g/1000 m3)                4.08         1.20      1.36       0.19
Daily consumption of krill (mg/1000 m3)          22.85/55.08      0.00      5.17/12.51    0.00
Annual consumption of krill (g/1000 m3)          8.32/20.05      0.00      1.88/4.55    0.00
Percentage of krill stock removed (%)b           9.04/21.79      0        2.05/4.95    0

Daily rations were calculated for both 10 and 24 h periods of feeding.
a
  Conversion factor daily ration dw to ww 1.8.
b
  Taking in account a krill biomass of 92 g/1000 m3 (Siegel et al., 1998b).




XIV/2 indicated an increased chlorophyll-a con-            Front (Rowedder, 1979b; Hulley, 1981; McGinnis,
centration in the vicinity of the Scotia-Weddell           1982; Lancraft et al., 1989; Hoddell et al., 2000).
                                   The size spectrum of E. antarctica suggested the
Confluence, which passed through the study site.
A higher diversity of mesopelagic fish, resulting           presence of the age classes two and three during
from an ‘island effect’, was described by Piat-            our study, according to the results of Greely et al.
kowski et al. (1994) for an assemblage above the           (1999).
                                     The centre of distribution of P. choriodon, G.
slope of South Georgia when compared to an
                                   fraseri and G. hintonoides are known to be in the
oceanic station in the northern Scotia Sea. The
snake mackerel P. gracilis is a characteristic            area of the South Polar Front and north of it
species of slope habitats with benthopelagic             (Hulley, 1981). The first records of these species in
life style (Nakamura, 1990) which was also              the South Shetland Islands region give support to
recorded over the slope of South Georgia               the idea that frontal zones, such as the South Polar
(Piatkowski et al., 1994).                      Front Zone, are not impervious barriers but may
  However, our study supports the results of             be penetrated by mesopelagic fish that are trapped
previous studies that characterize the Antarctic           in eddies (Kock, 1992).
micronekton community as a low diversity oceanic
faunal assemblage, compared to subtropical and
                                   4.2. Community structure
tropical oceans, where species numbers are four to
five times higher (Hopkins and Lancraft, 1984;
Lancraft et al., 1989).                         Our data showed a strong separation between
  The dominant position of E. antarctica, G.             samples taken above and below 450 m depth.
braueri, P. bolini in the Antarctic micronekton            These finding could be well correlated with the
assemblage have also been shown for the Scotia            presence of warmer and denser CDW at and below
Sea and slope waters off South Georgia (Piat-             this depth. Samples from the CDW showed a
kowski et al., 1994). These species are assumed to          higher species diversity based on Shannon’s
be the most common myctophids of the Southern             diversity and Taxonomic diversity (D). In contrast
                                   the Taxonomic distinctness (DÃ ) showed a high
Ocean (Hulley, 1981). The high densities of E.
antarctica over the slope of King George Island            degree of similarity between the deeper and lower
confirmed the importance of this species in the            mesopelagic fish community, due to the dom-
pelagic ecosystem south of the Antartic Polar             inance of the family Myctophidae and the high
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number of congeneric species in the lower meso-        et al., 1998). In our study, no influence of the shelf-
pelagic assemblage.                      break on the mesopelagic fish assemblage was
  The higher density of mesopelagic fish samples        discernable. This could be due to the relatively
taken in the CDW compared to ASW are in            small horizontal extent of our study site. There-
accordance with the results of Piatkowski et al.        fore, further studies of community structure
(1994), who found a high similarity in species         should cover a wider geographical area to fully
composition in lower mesopelagic depths            encompass hydrographic features like frontal
(400–1000 m) sampled at two different stations in       zones.
the Scotia Sea, while samples taken in the upper
                                4.3. Feeding ecology
depth horizons were clearly separated geographi-
cally. They found the highest biomass and
                                4.3.1. Food composition
abundance in the upper layers of the CDW
(400–800 m).                           Our data showed that Antarctic myctophids,
  The analysis of the community structure of         like myctophids in other oceans, are primarily
migratory species, like mesopelagic fish, has to        zooplanktivores. They feed mainly on copepods
consider the day/night distribution of the species.      and euphausiids. The proportions of these main
Because of our restricted sampling program (no         prey categories differed in the diets of the species
                                examined. Large myctophids (460 mm SL), E.
samples above 273 m depth, only four night
                                antarctica, G. braueri and G. nicholsi fed mainly on
samples) conclusions about the vertical migration
                                euphausiids, while the smaller-sized P. bolini
patterns of the mesopelagic fish can be drawn only
from data in the literature (Lancraft et al., 1989;      (53 mm maximum SL) preyed mainly on cope-
Piatkowski et al., 1994; Duhamel et al., 2000).        pods. These results suggest niche separation by the
  The results of the multivariate statistical analy-     utilization of different prey taxa.
sis of the mesopelagic fish data can be best            More than 90% of the krill biomass is concen-
explained by diel migratory behaviour of the          trated in the upper 100 m of the water column in
species. The most characteristic species, E. antarc-      the South Shetland Island region (Siegel, 1985).
tica and G. braueri, in the group of deeper stations      The diet analysis indicated that krill was the main
                                prey item of E. antarctica and G. nicholsi. It can be
(as defined by SIMPER analysis) perform exten-
                                assumed that E. antarctica and G. nicholsi were
sive vertical migrations (Torres and Somero, 1988;
Lancraft et al., 1989; Piatkowski et al., 1994).        sampled below their main feeding depth, because
During daylight, they stay in the core of the CDW       the fishes examined in our study were taken in
(400–800 m) and migrate to the upper 200 m at         depths greater than 273 m. This assumption was
                                supported by the low stomach filling in E.
night. A third species characteristic of deeper
stations was P. gracilis, a species whose vertical       antarctica (57.9% empty stomachs) and the
distribution is probably confined to mesopelagic        advanced digestion stage of krill found in the
                                stomachs of G. nicholsi (46.3% digestion stage 3).
depths. The most characteristic species of shallow
samples, P. bolini, also migrates vertically, but       In contrast, the proportion of heavily digested
                                Thysanoessa macrura, a species characterized by a
over a more restricted range as described by
Piatkowski (1989) and Lancraft et al. (1989).         wider vertical distribution range (Lancraft et al.,
Therefore, a substantial part of the population        1989), was much smaller (9.1% digestion stage 3).
                                The high importance of M. gerlachei in the diet of
remains in the ASW (0–400 m depth) during
                                P. bolini in our study re-confirmed the results of
daylight.
  Duhamel et al. (2000) showed a clear change in       Ascenio and Moreno (1984). The vertical distribu-
                                tion maximum of M. gerlachei is in the upper and
the species composition of mesopelagic fish in the
vicinity of the Polar Front at Kerguelen Island. A       lower mesopelagic depths (Huntley and Escritor,
frontal system has also been described for the         1992). It can be assumed that this copepod species
shelf-break zone north of King George Island that       was available over the whole vertical distribution
                                range of P. bolini. In contrast to these findings,
results in increased krill densities in this area (Ichii
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Gaskett et al. (2001) described P. bolini as an         E. antarctica taken in samples from November
                               to January (Rowedder, 1979a) and P. bolini
euphausiid feeder, as this prey category made up
more than the half of the food wet weight in their      sampled in February (Ascenio and Moreno,
study that was undertaken close to the subantarc-       1984) have been shown to prey heavily on
tic Maquarie Island. Such differences support the       development stages of krill. The breeding season
findings of Pakhomov et al. (1996), namely that        of krill fluctuates in the Antarctic Peninsula region
the food composition of myctophids varies by         from the beginning of December until the end of
locality and probably by season. The same           March (Siegel et al., 1997). During our study, at
conclusion is true for the role of krill in the diets     the beginning of December, most female krill
of E. antarctica and G. nicholsi. The importance of      individuals were in a pre-spawning or early
adult krill in the diet of both species has been       spawning condition (Siegel et al., 1998a). There-
described previously for several areas of the         fore, it can be assumed that no krill development
Southern Ocean (Rembiszewski et al., 1978;          stages were available in the zooplankton commu-
Rowedder, 1979a; Lubimova et al., 1983; Kozlov        nity at the time of our study. In summary, our data
and Tarverdiyeva, 1989). However, Pakhomov et         indicate that krill was an important prey item for
al. (1996) found no krill in the diet of E. antarctica    E. antarctica and G. nicholsi over the slope of King
and only small quantities in the diet of G. nicholsi     George Island. It can be expected that the dietary
sampled in the Prydz Bay, South Georgia and          importance of krill for myctophids will be en-
Lazarev Sea (Indian Sector) regions. Therefore,        hanced during the course of the austral summer,
these authors suggest that krill only plays a         when krill abundance increases and their smaller
significant role in the diet of myctophids only        development stages become available in the
during certain time periods and within specific        zooplankton (Siegel et al., 1997; Ichii et al.,
                               1998). Our data also show that P. gracilis is an
areas. An additional reason for the minor im-
portance of krill in the diet of E. antarctica and G.     important predator of mesopelagic fish over the
nicholsi in their study could be result of size-       slope of King George Island, so confirming the
dependent feeding. Numerous studies have shown        results of Rembiszewski et al. (1978).
that larger individuals of a population are able to
                               4.3.2. Feeding chronology
consume larger prey items, which is often demon-
                                 Except for G. braueri, all myctophids in the
strated by an ontogenetic switch from copepods to
euphausiids as the main prey category (Young and       present study showed diurnal feeding patterns
Blaber, 1986; Williams et al., 2001). The size-        characterized by increased food intake during
ranges of E. antarctica (25–85 mm SL) and G.         night. Nevertheless the day-night differences in
nicholsi (69–139 mm SL) examined by Pakhomov         feeding activity were not significant for any
et al. (1996) were smaller than their size-ranges       species. Pakhomov et al. (1996) suggested an
in our study, namely 57–113 mm SL and             active nocturnal feeding period of 8–10 h for
123–172 mm, respectively. Nevertheless, the          Antarctic myctophids. Migratory myctophids,
difference in diet composition cannot be explained      especially in nutrient poor tropical and subtropical
entirely by predator length. In the present          waters, are reported to show a pronounced
study the diet of the smaller-sized E. antarctica       chronology of feeding activity, by feeding in the
(57–79 mm) and G. nicholsi (123–144 mm), that is       food rich epipelagic layer at night (Clarke, 1978;
size ranges represented in Pakhomov’s samples,        Kinzer and Schulz, 1985; Gartner et al., 1997). In
have a similar food composition to larger           contrast, in more productive areas, a number of
                               migratory myctophids such as Diaphus theta,
individuals and is dominated by euphausiids.
Therefore scarcity of E. superba in the diet of        Stenobrachius leucopsarus, Tarletonbeania crenu-
E. antarctica and G. nicholsi in Pakhomov’s          laris (Tyler and Pearcy, 1975), Benthosema glaciale
                               (Kinzer, 1977), Diaphus danae, Lampanyctus hec-
study may be due more to the effect of food
                               toris (Young and Blaber, 1986) and D. theta
availability than the effect of size-dependent
feeding behaviour.                      (Moku et al., 2000) tend to feed continuously. It
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should be noted that the Southern Ocean ecosys-       examined were empty, Table 5). Because of this
tem is characterized by a seasonal high productiv-      fact, our calculated daily rations could be under-
ity near the pack ice and in the shelf regions like     estimated. However, we suggest that our estimates
the slope region of King George Island (Hempel,       are realistic, because we have re-calculated prey
1985; Ichii et al., 1998).                  dry weights by length-weight regressions of un-
  In our study, G. nicholsi showed an increased       digested prey items. The use of re-calculated
feeding activity in the time period 2100–2400 h,       weight is expected to bias the SCI values in a
while in P. bolini SCI was above the average from      positive direction (Hyslop, 1980). It is for this
2000 to 0300 h. However, neither species ceased       reason that we felt justified in applying the daily
feeding during the day, and both showed a second       rations of myctophids in the calculation of their
feeding activity peak in the time interval          predation impact on the krill stock in the study
0800–0900 h. A daily feeding period in P. bolini       area. Most estimates of the predation impact by
appears to be possible due to the relatively shallow     mesopelagic fish on the zooplankton stock have
daytime depth they occupy (273–500 m). This         been performed in tropical and subtropical
range overlaps with the vertical distribution range     waters. Despite differences in regions and
of its major prey item, M. gerlachei (Hernandez-       methods of estimation used, all these studies
Leon et al., 2001). The SCI of E. antarctica         indicate that mesopelagic fish are able to remove
increased during the hours of darkness and in        two to three times the standing stock of herbivor-
the early morning. In contrast during summer and       ous zooplankton (Dalpadado and Gjoesaeter,
autumn at South Georgia, South Sandwich and         1988; Hopkins and Gartner, 1992; Gartner et al.,
South Orkney Island Rowedder (1979a) observed        1997).
the highest feeding activity of this species in the       There has been only one previous estimate of
afternoon (1400 h) and second peak in the morning      zooplankton consumption by Antarctic mesopela-
(0600 h). The different feeding patterns of E.        gic fish. Naumov (1985) as cited in Pakhomov et
antarctica observed by Rowedder suggest that         al. (1996) on the basis of an assumed mesopelagic
the feeding chronology of mesopelagic fish can        fish stock of 275 million tonnes in the Southern
show a high regional and seasonal variability.        Ocean, estimated an annual consumption of 1085
                               million tonnes of meso- and macro zooplankton
4.3.3. Daily ration and predation impact on the krill    (excluding E. superba). This equates to an esti-
stock                            mated predation impact equivalent to 40% of the
  Our estimates of daily rations (0.7–3.3% of dry      annual secondary production.
body weight, assuming a 24 h feeding period) were        We have calculated the annual removal of the
                               krill stock in the Southern Ocean region by E.
in good agreement with the estimates of Pakho-
                               antarctica and G. nicholsi, which are thought to be
mov et al. (1996) for Antarctic myctophids. The
latter ranged from 0.2% to 4.4% of dry body         two of the most abundant myctophids in the
weight.                           Southern Ocean, to be 11.1–26.7%. These results
  The estimates of daily ration given by Rowedder      confirm the important position of mesopelagic fish
(1979a) for E. antarctica (5% of dry body weight)      in the Antarctic oceanic system, and give support
and by Gerasimova (1990) for E. carlsbergi          to the suggestion of Lancraft et al. (1989) that they
(3.7–5.6 wet body weight) were higher than in        are the most prevalent predators on krill in the
our results. While feeding activity in migratory       Antarctic ecosystem. However, the figures given in
mesopelagic fish has been observed to reach          the present study should be treated with caution
maximum values in the food-rich epipelagic layers      since they represent a ‘‘snapshot’’ of the mid-water
(Gartner et al., 1997), the calculations of daily      fish community over the slope of King George
rations in our study were based on individuals        Island. Because the feeding impact of mesopelagic
sampled from mesopelagic depths. E. antarctica        fish on the krill stock is largely dependent on
and G. braueri, in particular, showed a low feeding     locality and season, future studies should be
activity (57.9% and 68.7% of all stomachs          carried out at different seasons of the year to get
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a more realistic picture of the Antarctic mesope-         Eggers, D.M., 1977. Factors in interpreting data obtained by
                                   diel sampling of fish stomachs. Journal of the Fisheries
lagic food web.
                                   Research Board of Canada 34, 290–294.
                                  Field, J.G., Clarke, K.R., Warwick, R.M., 1982. A practical
                                   strategy for analysing multispecies distribution patterns.
Acknowledgements                           Marine Ecology-Progress Series 8, 37–52.
                                  Gartner Jr., J.V., Crabtree, R.E., Sulak, K.J., 1997. Feeding at
 Special thanks to Dr. Volker Siegel (Sea Fisheries         depth. In: Randall, D.J., Farrel, A.P. (Eds.), Deep-Sea
                                   Fishes. Academic Press, San Diego, pp. 1–388.
Research Institute) for his great assistance in the
                                  Gartner Jr., J.V., Conley, W.J., Hopkins, T.L., 1988. Escape-
identification of zooplankton and his constructive
                                   ment by fishes from midwater trawls: a case study using
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by Craig Osenberg last modified 14-10-2006 17:36

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