Trophic structure and resource utilization of the coastal fish community in the western Wadden Sea: evidence from stable isotope data analysis

The trophic structure of the western Wadden Sea fish community was studied by means of stable isotope analysis (δ13C and δ15N) of 1658 samples from 57 fish species collected between 2012 and 2016. Stable isotope values differed between species but did not vary between years or between seasons, and only for some species with fish size. Stable isotope values were not different between immigrating (spring) and emigrating (autumn) fish suggesting a similar trophic niche of the various fish species in the coastal zone and inside the Wadden Sea. For the majority of the species, average δ13C values were within the range of -12‰ to -20.5‰, showing that both (marine) pelagic and benthic primary producers were at the base of the food web. Average δ15N values varied among species from 13‰ to 18‰, resulting in estimated trophic positions (TP) between 2.1 to 5.5 with the majority between 2.2 to 3.5. Thick-lipped grey mullet (Chelon labrosus), golden grey mullet (Chelon aurata), greater pipefish (Syngnathus acus) and pilchard (Sardina pilchardus) had the lowest trophic position (2.2 – 2.4). Among the common species (> 10 observations), highest TP values (3.4 – 3.5) were found for the twaite shad (Alosa fallax), smelt (Osmerus eperlanus), bull-rout (Myoxocephalus scorpius), bass (Dicentrarchus labrax) and cod (Gadus morhua). For all species, estimated trophic positions based on isotope values were lower than those based on stomach content composition (2.0 – 4.7), which could be explained by species-specific differences in trophic fractionation or by underestimation of the contribution of smaller prey species in the stomach content analysis. The trophic niche space of benthopelagic species was the smallest and overlapped with that of the pelagic and benthic species. In terms of use of the area, trophic niche space was smaller for juvenile marine migrant species (nursery-type species) and overlapped with that of the (near)-resident species and marine seasonal visitors. Potentially, trophic competition is highest for the functional group of benthopelagic species and the guild of juvenile marine migrant species (nursery-type species)