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Significance of fish for food webs in the Wadden Sea

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Citation:
Pockberger, M. , Kellnreitner, F. and Asmus, H. (2011): Significance of fish for food webs in the Wadden Sea , Wadden Sea Day 2011, Wilhelmshaven – Wattenmeerhaus, 25 August 2011 - 25 August 2011 .
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Abstract:

Significance of fish for food webs in the Wadden sea Moritz Pockberger, Florian Kellnreitner, Harald Asmus The Wadden Sea, an UNESCO world heritage site since 2009, is widely known as a nursery ground for commercially important fish of the North Sea, and also provides a habitat for small-sized fish species. Recently the fish fauna of the North Sea shows profound alterations respecting species compositon, standing stocks and migration behavior. Therefore, in the frame of long term observations of biological parameters in the Sylt-Rømø Bight (=SRB), a fish survey was conducted since 2007, to document possible changes of species composition and stock size, considering the rise of temperature on a local scale. Sampling takes place monthly on seven locations inside SRB using a bottom trawl, which is designed to be deployed also as flyde trawl for pelagic fishing. On every location a benthic and on deeper sites additionally a pelagic haul is performed. To achieve a clear picture of trophic positions and interactions between fish species inside the SRB gut content analysis and feeding experiments are carried out. Most of the long term data will be used to portray a food web for the total SRB as well as for the particular habitats of this area. This food web will reflect certain scenarios of environmental conditions and biological status to give a holistic view of the current as well as the expected changes. Until now, fish species are underrepresented in this food web and due to the lack of basic data only eight fish species could be included. Increasing temperatures in summer may lead to immigration of lusitanian species, such as the sand smelt (Atherina presbyter), anchovy (Engraulis encrasicolus) and the striped red mullet (Mullus surmuletus) into the Wadden Sea. Additionally a higher proportion of thermophile species is expected to visit the Wadden Sea, whereas boreal species may move to more northern regions. During a warm summer period in 2003 a high immigration of small thermophile fish, such as sand smelt and anchovy and emigration of indigenous fish was observed in the SRB. Considering the above described changes and varying residence times during seasons, shifts in the food web can be expected. In winter, higher temperatures may delay the migration of fish species into depths that have more stable environmental conditions. Several findings indicate ongoing changes in the SRB, e.g. sand smelt was found rarely, but is now found regularly in summer during sampling and it is assumed that reproduction takes place inside the SRB. In autumn, hatchlings of grey mullets (Chelon labrosus) were caught both in beach seine and bottom trawl. This species was only recorded as a summer guest before. Furthermore, catches of black goby (Gobius niger), may be an example for ongoing faunal changes occurring in the area. To identify potential seasonal differences in species abundance and the role of southern species in the Wadden sea food web, an analysis of guild structures was conducted. Another subject of investigation will be the impact of new established habitats, like reefs of the Pacific cupped oyster (Crassostrea gigas) and Japanese Sargasso weed (Sargassum muticum) on fishes. These two species occur densely in high numbers and are able to change abiotic factors. Identification of feeding relationships and of key prey species provide important information for coastal area management and may help to explain structural variations of fish community in an area that is subject to constant change.

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