Mohm, Clarissa (2014) Feeding ecology of Baltic cod assessed by stable isotope analysis. (Bachelor thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 42 pp.

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Abstract

Baltic cod (Gadus morhua L.) is the subject of important fisheries and a keystone predator in the Baltic Sea. Despite this important role, the feeding ecology of cod is only incompletely understood. Here, I used stable isotope analysis (SIA), a tool increasingly used in food web studies but surprisingly, not yet for commercial fish species in the Baltic, to obtain a complementary dataset to existing stomach content data (SCA). For this purpose, I analyzed a large set of carbon (C) and nitrogen (N), and a pilot set of sulfur (S) muscle stable isotope data of the top predator cod, the key plankton feeders herring (Clupea harengus L.) and sprat (Sprattus sprattus L.), and other pelagic and benthic fish species of the Baltic. Analyses were based on 392 samples of 10 species from 19 sites covering the Kiel Bight (SD22), Arkona Basin (SD24), Bornholm Basin (SD25), and Gdansk Deep (SD26) that were obtained on a 2- week cruise with the research vessel Alkor in April 2014. As expected from prior stomach content data, cod showed an ontogenetic shift in δ15N ratios. However, the expected shift in δ13C ratios from benthic to pelagic diet was overall surprisingly weak. This supports the previous hypothesis that the importance of benthic diet for cod may have decreased due reduced availability of demersal invertebrates as prey for cod due to increasing anoxic condition in deeper water. Confirming previous studies, small herring and sprat displayed a dietary overlap due to a strict zooplanktivor diet, which indicates strong potential for dietary competition. However, herring, in contrast to the presumed strict zooplanktivor sprat, showed an ontogenetic shift because it changes its diet with increasing fish size to nektobenthos. Furthermore, the results of this study demonstrated that there are clear spatial differences in isotopic baselines and therefore an overall isotopic shift in the entire food web between basins
of the Baltic, which may be useful for studies in stock mixing and migrations, and which needs to be considered in future SIA studies in the Baltic Sea. Besides, additional SIA of benthos might help to get a better understanding of the importance of benthic prey and its potential consequences for cod’s condition.

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