Jaspers, Cornelia ... et. al.-- International Council for the Exploration of the Sea Annual Science Conference (ICES ASC 2014), Sustainability in a changing ocean, 15-19 September 2014, La Coruna.-- 2 pages

Ecological network analysis (ENA) was used to study the effects of Pomatoschistus microps on energy transport through the food web, its impact on other compartments and its possible role as a keystone species in the trophic webs of an Arenicola tidal flat ecosystem and a sparse Zostera noltii bed ecosystem. Three ENA models were constructed: (a) model 1 contains data of the original food web from prior research in the investigated area by Baird et al. (2007), (b) an updated model 2 which included biomass and diet data of P. microps from recent sampling, and (c) model 3 simulating a food web without P. microps. A comparison of energy transport between the different models revealed that more energy is transported from lower trophic levels up the food chain, in the presence of P. microps (models 1 and 2) than in its absence (model 3). Calculations of the keystone index (KSi) revealed the high overall impact (measured as εi) of this fish species on food webs. In model 1, P. microps was assigned a low KSi in the Arenicola flat and in the sparse Z. noltii bed. Calculations in model 2 ranked P. microps first for keystoneness and εi in both communities, the Arenicola flat and the sparse Z. noltii bed. Taken together, our results give insight into the role of P. microps when considering a whole food web and reveal direct and indirect trophic interactions of this small-sized fish species. These results might illustrate the impact and importance of abundant, widespread species in food webs and facilitate further investigations.

We analysed feeding interactions between Mnemiopsis leidyi and juvenile Clupea harengus in the Wadden Sea. Biomass, diet overlap, prey selectivity, predation impact and stable isotope composition (C, N) of both species were assessed from June to September 2010. High biomass of C. harengus was found in June and July (wet weight 3.0 ± 1.8 g m−3) followed by a steep decline from August to September (wet weight 0.01 ± 0.01 g m−3), coinciding with a dramatic increase in M. leidyi biomass (wet weight 18.3 ± 16.1 g m−3 during August). These two species showed a high overlap in their respective diets (copepods, meroplankton) during the study period. Predation impact of C. harengus on calanoid copepods was highest in June and July where 84 and 41 % of the standing stock were eaten per day in June and July, respectively. Predation impact of M. leidyi on calanoid copepods was highest in September (16 %). Based on stable isotope analysis C. harengus and M. leidyi were assigned to a trophic level of 3.08 and 2.47, respectively. Furthermore, we assessed the potential of competition between M. leidyi and C. harengus in a mesocosm experiment. Results indicated that at present zooplankton densities intraspecific competition in C. harengus seemed to be greater than interspecies competition with M. leidyi. Due to the low predation impact of M. leidyi and the reduced temporal overlap, competition between M. leidyi and C. harengus during the study period was estimated as low. Nevertheless, considering the high dietary overlap and the inter-annual variation in biomass and occurrence of both species and their zooplankton prey, competition in the Wadden Sea area cannot be excluded.

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-Romo 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.

Species composition, abundance, feeding relationships and guild structure of the fish assemblage in the Sylt-Romo bight, a tidal basin in the northern Wadden Sea, were investigated to show seasonal differences and the importance of functional groups in this area. The tidal flats and in shallow subtidal areas were sampled using a beach seine and a bottom trawl net was used for deeper subtidal areas and tidal gullies. Species richness of fish was highest in summer where 26 species were caught, while the lowest richness was recorded in winter (17 species). Clear differences in species richness and abundance were found between shallow areas and deeper parts of the bight. Clupea harengus and Ammodytes tobianus were the most abundant species in deeper areas, while Pomatoschistus microps and Pomatoschistus minutus dominated shallower waters. Gut contents of 27 fish species were identified and the guild structure analyzed by UPGMA clustering of niche overlaps. Calanoid copepods (19.9%), Crangon crangon (18.2%) and mysid shrimps (8.4%) were the most abundant prey items of all fish species combined. Seven feeding guilds were present in the fall and winter, and eight and six in spring and summer, respectively. Fish feeding on calanoid copepods and C. crangon were present year round, whereas the occurrence of other guilds varied between seasons. Species composition of prey changed through seasons and, for some fish species, even the feeding mode itself varied with season. Most noticeable, 11 fish species changed guilds between seasons. We found a convergence in summer towards abundant prey items, whereas in winter diet overlap was lower. This is the first investigation of guild structure of almost all fish species present in a Wadden Sea area, and shows that consideration of seasonal differences is essential when determining feeding relationships of fish in temperate areas. (C) 2011 Elsevier Ltd. All rights reserved.

Genetic variability of marine fish species is much higher than in most other vertebrates. Nevertheless, some species with large population sizes including flatfish such as European plaice Pleuronectes platessa show signs of population collapse and inbreeding. Taking plaice as a flagship example for fisheries-induced genetic changes also affecting the Wadden Sea, we determined the amount of genetic variability at antigen-presenting genes of the Major Histocompatibility Complex (MHC) and its potential interaction with the microbiota associated to gill tissue using a next-generation parallel tag sequencing approach. Genetic variation at MHC class IIB genes was extremely large, with 97 alleles found in 40 fish from different age cohorts. Although a strong signal of positive selection was present (dN/dS = 4.01) and we found significantly higher allelic diversity in 0+ fish than in older age classes, the amount of genetic variation maintained within the population may not have exceeded neutral expectations derived from mitochondrial markers. Associated microbes revealed significant spatiotemporal structure with 0+ fish displaying the highest microbial diversity as well as the highest diversity of potentially pathogenic genera. Overall the correlation between MHC genotypes and bacterial abundance was weak, and only few alleles significantly correlated with certain bacterial genera. These associations all conferred susceptibility (i.e. presence of an allele correlated to higher number of bacteria), either suggesting age-dependent selection on common alleles or weak selection on resistance against these bacterial genera. Taken together, our data suggest that selection coefficients of balancing selection maintaining immunogenetic diversity may be relatively small in large marine populations. However, if population sizes are further reduced by overharvesting, the response to increasing balancing selection coefficients will be largely unpredictable and may also negatively influence the adaptive potential of populations. (C) 2011 Elsevier Ltd. All rights reserved.