Inter- and Intra-Estuarine Comparison of the Feeding Ecology of Keystone Fish Species in the Elbe and Odra Estuaries

Food webs in estuarine ecosystems serve as important biological indicators. The feeding ecology of four keystone fish species, pikeperch (Sander lucioperca L.), smelt (Osmerus eperlanus L.), ruffe (Gymnocephalus cernua L.) and flounder (Platichthys flesus L.), in the Elbe and Odra estuaries was analyzed using stomach content analyses. Important prey of pikeperch were fishes and mysids in both estuaries. Amphipods were especially important as prey for smelt in the Elbe estuary, whereas smelt caught in the Odra estuary mainly consumed mysids. Ruffe fed mainly on amphipods in the Elbe estuary, while annelids (lower section) and insect larvae (upper section) were the most important prey in the Odra estuary. Flounder favored copepods as prey in the Elbe estuary, while bivalves were preferred in the Odra estuary. Higher dietary overlaps were found in the Elbe estuary between smelt vs. ruffe, pikeperch vs. ruffe, and pikeperch vs. smelt. In the Elbe estuary, a shift in the diet composition of pikeperch, smelt, and ruffe was observed from 2021 to 2022 compared to food analyses from the 1990s. These shifts included an increased consumption of amphipods, while mysids and copepods had recently decreased in their diets. These changes indicate a restructuring of the food web, potentially linked to environmental changes, which highlights the sensitivity of estuarine ecosystems.