The role of the Antarctic silverfish Pleuragramma antarcticum in the Antarctic waters

Maj Wetjen¹, Kai Wätjen¹, Rainer Knust¹ Grant No: AWI_PS82_03

1AWI

Objectives

Studies have shown that the Antarctic fish communities differ between the north-eastern Weddell Sea shelf areas and the southern shelf of the Filchner area (Hubold 1992). The pelagic fish biomass and production is quite high in the southern Weddell Sea (1.3 g/m²) compared to the north-eastern shelf (0.10 g/m², Hubold 1992) and is mostly borne by one single species, the Antarctic silverfish Pleuragramma antarcticum (Hubold 1992, Hubold & Ekau 1987). This zooplankton-feeding species inhabits both, the ice-free and the pack ice waters over the Antarctic continental shelf and distinctly dominates the fish communities on the eastern Weddell Sea shelf. Therefore, it is the most important food source for upper consumers (e.g.

seals, pinguins) (Hubold 1992, Plötz et al. 2001) and contributes to the bentho-pelagic coupling by vertical feeding migrations (Knust et al. 2012) from the seafloor towards upper water layers (Fulman et al. 2002, Plötz et al. 2001). Recent studies have shown that besides high abundances of adult Pleuragramma specimen, high densities of larvae and postlarvae could be observed off the Filchner Ice Shelf in the upper water layers over the Filchner Trough (Ekau & Hubold 1985, Keller 1983, Boysen et al. 1983, Hubold 1984). Therefore, the Filchner area might probably not only be a feeding ground for adults, but also a nursery ground and retention area for larval P. antarcticum (Hubold 1984). Moreover, in the eastern part of the Weddell Sea, foraging behavior of Weddell seals is known to be linked to the occurrence of P. antarcticum (Plötz et al. 2001). It is an important food source, not only for Weddell seals, but also for southern elephant seals from King George Island (Daneri & Carlini 2002) and other high-level predators in the Antarctic waters. The dense stock of P. antarcticum in the southern Weddell Sea may also explain the tendency of male southern elephant seals to migrate from King George Island to the Filchner area for feeding. The dominant role of P. antarcticum in the high-Antarctic shelf areas evinces that this key species represents a central and essential position in the Antarctic food webs.

Work at sea

Biomass, abundance and size distribution of P. antarcticum was determined from bottom trawl catches as it could not be caught via Agassiz trawls or baited traps. The bentho-pelagic trawl could not be operated due to the very difficult ice situation. Pleuragramma specimens were selected from the trawl catches in order to collect morphological and biological data of each individual. Directly after the haul Pleuragramma specimens were measured, weighed and dissected in order to obtain the following information: total and standard lengths (TL and SL); total and gutted weights (TW and EW); and sex. Moreover, from around 20 – 40 specimens

3.3.4 Antarctic fishes

per station tissue samples (stable isotope analysis), stomachs (food web analysis), and otoliths (age determination) were extracted. After proceeding, tissue samples were labeled and directly frozen at -20 °C. Additionally to the sampling activities, several Pleuragramma specimens were also labeled and frozen at -20 °C for later investigation in our home laboratories at the AWI in Bremerhaven.

Preliminary (expected) results

Overall around 450 specimens of P. antarcticum could be processed directly on board to extract the otoliths, stomachs and tissue samples, whereas further individuals were frozen to be processed and analysed in the laboratories of the AWI. Biomass and abundance data of P. antarcticum were converted to an area of 1,000 m² to allow a direct comparison of the different investigated stations.

In most investigated areas P. antarcticum takes in a great portion of the whole biomass and abundance determined. At the stations PS82/129-1, PS82/166-1, and PS82/306-1 it`s biomass accounts for more than 70 % and at station PS82/188-1 even PS82/188-100 % of the entire biomass determined. Abundance was also quite high at the stations PS82/166-1, PS82/248-1, PS82/282-1, and PS82/306-1. At these stations the abundance was greater than 90 % of the total catches. Besides the stations where no Pleuragramma specimens could be obtained (PS82/011-1, PS82/053-1, PS82/088-1, PS82/175-1, PS82/201-1, PS82/244-1, PS82/249-1), abundance was relatively low at the stations PS82/316-1 and PS82/341-1 with less than 1 individual per 1,000 m². In 12 out of 23 bottom trawls P. antarcticum showed an abundance > 50 % of the total individuals caught.

Fig. 3.3.4.2.1.: Abundance (N/1,000 m²) and biomass

(g/1,000 m²) of P.

antarcticum in comparison to the total abundance and biomass of the investigated

stations (only bottom trawls) during the cruise

PS82

Data management

All samples taken during this expedition will be analyzed in the laboratories at the AWI in Bremerhaven. Tissue samples will be analyzed for stable isotopes, stomach contents will be investigated to get a better insight in the prey taxa and food webs, and otoliths will be examined to determine the age and growth of P.

antarcticum in the Filchner area. After data analyses, all results generated from this expedition will be published in the AWI data base PANGAEA Data Publisher for Earth & Environmental Science (http://www.pangaea.de/). In collaboration with the colleges from the University of Padova, data of P. antarcticum are likely to be joined together to get a full overview and a better understanding of the ecological role of this key species in the Antarctic waters.

References

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Ekau W, Hubold G (1985) Fish and fish larvae. In: Hempel G (ed.) Die Expedition ANTARKTIS III mit FS „Polarstern“ 1984/85. Reports on Polar Research, 25, 121-129.

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Keller R (1983) Fish larvae. In: I. Hempel G, Hubold B, Kaczmaruk R, Keller R, Weigmann-Haass. Distribution of some groups of zooplankton in the inner Weddell Sea in summer 1979/80. Reports on Polar Research 9, pp. 11-13.

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3.3.4 Antarctic fishes

3.3.4.3 Reproductive traits in Antarctic fish: a comparative analysis