Die trophische Struktur einer Fischgemeinschaft des Wattenmeeres und deren Fraßinteraktionen mit gebietsfremden Arten.

The Wadden Sea is an important nursery area for commercial fish species of the North Sea and habitat for many small fish species. It is a very variable habitat with high variations in salinity and temperature. The Wadden Sea ecosystem has been modified by the utilization of humans since millineries. Species composition and food web of the fish of the Wadden Sea are influenced by destruction and alterations of habitats, fisheries, introduced species and climate change. The species distribution shows marked differences in species occurrence between inter- and subtidal areas and between the 4 different seasons of a year. Pomatoschistus microps was the most abundant fish species in intertidal areas and Clupea harengus and Ammodytes tobianus were abundant in subtidal areas. Variation was also discovered in the occurrence of feeding guilds found by diet overlap calculations. Only calanoid copepod eaters and Crangon crangon eaters were present year round. Fish of the Sylt-Rømø Bight show a broad spectrum of prey and opportunistic feeding behavior. Convergence towards abundant prey organisms in summer and divergence of feeding in winter was observed. An increase in the abundance of zooplanktivorous fish compared to previous investigations was found. Stable isotope analysis of carbon and nitrogen revealed a lower mean trophic position for zooplanktivorous fish than for fish feeding on zoobenthos. Merlangius merlangus was found at the highest trophic position (3.84) and Limand limanda at the lowest (2.94). The overall low trophic position of the fish of the Sylt-Rømø Bight pointed to short food chains and therefore to an immature ecological state of the bight. Immature systems are characterized by a high degree of unutilized resources and are therefore more susceptible to introductions of new species. The influence of these species on native fish was shown in the example of the herring (Clupea harengus) and 2 new species in the food web, the sea walnut (Mnemiopsis leidyi) and the sand smelt (Atherina presbyter). The sea walnut was introduced into the North Sea most likely between the year 2000 and 2006. It first occurred in samples in early summer and reached its highest abundance in fall (10.3 ± 8.2 inidviduals m-3). Herring and sea walnut showed temporarily high diet overlap which was related to the high amount of ingested calanoid copepods by both species. High amounts of bivalve larvae were found in the diet of the sea walnut in August, which could not be found in the guts of herring. In an experiment it was demonstrated that in herring intraspecific competition was significantly higher than competition from the sea walnut. Nevertheless, competition between both species could occur if their feeding resources should be limited. The distributional range of the sand smelt was possibly extended to the North due to increasing sea water temperatures induced by climate change. Only recently increasing abundance of this fish species in the Sylt-Rømø Bight is observed. The results of our investigation show only a temporarily high diet overlap due to ingestion of nauplius and cypris larvae of both species. The sand smelt shows positive selection of these meroplankter and avoidance of calanoid copepods, which are the main diet of herring. There is still place for new species in the food web of the Sylt-Rømø Bight. Its biology determines how a new species will integrate into the food web. The sea walnut poses a potential threat to native zooplanktivores due to overlapping diets and its high fertility and abundance. This potential increases should the competition pressure from native fish be reduced. The sand smelt uses mainly different habitats than native zooplanktivorous fish and could therefore integrate into the fish food web.Das Wattenmeer ist ein wichtiges Aufwuchsgebiet vieler kommerzieller Fischarten der Nordsee und Lebensraum für viele kleinbleibende Fischarten. Es stellt einen sehr variablen Lebensraum dar der sich durch große Salinitäts- und Temperaturunterschiede auszeichnet. Das Wattenmeer ist seit Jahrtausenden durch die Nutzung des Menschen geprägt. Zerstörung und Veränderung von Habitaten, Fischerei, eingeschleppte Arten und Klimawandel üben Einflüsse auf Artenzusammensetzung und Nahrungsnetz der Fische des Wattenmeeres. Die Untersuchung der Artenverteilung der Fische der Sylt-Rømø Bucht zeigt deutliche Unterschiede zwischen inter- und subtidalen Bereichen und innerhalb dieser zwischen den verschiedenen Jahreszeiten. Über das gesamte Jahr gemittelt sind Pomatoschistus microps in den intertidalen und Clupea harengus und Ammodytes tobianus in den subtidalen Bereichen am häufigsten. Auch die durch Berechnung von Nahrungsüberlappungen gefundenen Nahrungsgilden variieren stark zwischen den Jahreszeiten. Nur calanoide Kopepoden Fresser und Crangon crangon Fresser waren das ganze Jahr hindurch vorhanden. Die Fische der Sylt-Rømø Bucht zeigten generell ein breites Beutespektrum und opportunistisches Fressverhalten. Es wurde eine Konvergenz zu häufigen Beuteorganismen im Sommer und einen Divergenz im Fressverhalten im Winter beobachtet. Ein Anstieg in der Anzahl zooplanktivorer Fische im Vergleich zu vorangegangenen Untersuchungen konnte gezeigt werden. Die Analyse der Isotopenzusammensetzung von Kohlenstoff und Stickstoff zeigte eine durchschnittlich niedrigere trophische Position für zooplanktivore Fische als für benthivore Fische. Die höchste trophische Position wurde für Merlangius merlangus gefunden (3,84), die niedrigste für Limanda limanda (2,94). Die generell niedrige trophische Position der Fische der Sylt-Rømø Bucht deutet auf kurze Nahrungsketten und somit auf ein unreifes ökologisches Stadium der Bucht hin. Unreife Systeme zeichnen sich durch ein hohes Maß an ungenutzten Ressourcen aus und sind somit anfälliger für eingeschleppte Arten. Der Einfluss dieser Arten auf die heimische Fischfauna sollte anhand des Herings (Clupea harengus) und zweier neuer Arten im Nahrungsnetz, der Meerwalnuss (Mnemiopsis leidyi) und des Ährenfisches (Atherina presbyter) gezeigt werden. Die Meerwalnuss wurde vermutlich kurz nach der Jahrtausendwende in die Nordsee eingeschleppt. Sie war ab Juni in den Planktonproben und erreichte ihre größte Dichte im Herbst (10.3±8.2 Individuen m-3). Hering und Meerwalnuss zeigten eine zeitweise hohe Nahrungsüberlappung die auf die starke Nutzung beider Arten von calanoiden Kopepoden zurückzuführen war. Die Meerwalnuss zeigte im August eine starke Nutzung von Bivalvialarven die beim Hering nicht zu finden war. Durch ein Experiment wurde gezeigt, dass beim Hering die intraspezifische Konkurrenz significant höher ist als die Konkurrenz durch die Meerwalnuss. Konkurrenz zwischen diesen beiden Arten könnte jedoch auftreten sollten die Nahrungsressourcen limitiert sein. Der Ährenfisch hat sein Verbreitungsgebiet vermutlich durch den Klimawandel und den damit verbundenen Temperaturanstieg in den Norden erweitert. Im Bereich der Sylt-Rømø Bucht wird diese Fischart erst seit kurzem in hohen Dichten beobachtet. Die Ergebnisse unserer Untersuchungen zeigen eine teilweise hohe Nahrungsüberlappung, die hier allerdings auf Nauplien- oder Cypris-Stadien von Crustaceen zurückzuführen ist. Der Ährenfisch zeigte positive Selektion von diesen Meroplanktonarten und Vermeidung von calanoiden Kopepoden, die meist den Hauptbestandteil der Nahrung des Herings ausmachen. Zusammenfassend kann davon ausgegangen werden, dass noch Platz für neue Arten im Nahrungsnetz vorhanden sind. Wie sich eine neue Art in das Nahrungsnetz eingliedert hängt von ihrer Biologie ab. Die Meerwalnuss kann durch ihre hohe Reproduktionsfähigkeit potentiell eine Gefahr für die Zooplankton Fraßgilde darstellen, vor allem wenn der Konkurrenzdruck durch heimische Fische wegfallen sollte. Der Ährenfisch nutzt hauptsächlich andere Habitate al

Diversity of immune genes and associated gill microbes of European plaice Pleuronectes platessa

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

Significance of fish for food webs in the Wadden Sea

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