Ecological adaptability: Size-dependent foraging of European perch at the coastal-freshwater interface of the Baltic Sea

Elucidating the feeding ecology of predatory fish in coastal-freshwater ecosystems is crucial for effective conservation and management, especially considering ever-changing predator-prey relationships. This study explores the dietary patterns and trophic interactions of European perch ( Perca fluviatilis , hereafter “perch”) in the coastal-freshwater environments of the Baltic Sea. We combined stomach content and stable isotope analyses to examine the foraging strategies and habitat use of perch populations inhabiting the interface between coastal marine and inland freshwater lagoon-like environments, characteristic of the southeastern Baltic Sea. Our findings indicate that perch in these ecosystems exhibit significant dietary variability and occupy distinct trophic positions, influenced by the differing ecological characteristics of lake and sea habitats. The freshwater environment exhibits more consistent feeding patterns for perch, while the marine environment appears to offer a more dynamic food supply influenced by the migratory patterns of marine prey. This research highlights the ecological importance of habitat diversity in supporting perch populations and the need for targeted conservation strategies to preserve these vital ecosystems. Further studies are recommended to enhance our understanding of the long-term ecological dynamics and the impact of environmental changes on perch foraging behavior and habitat use. • European perch may obtain dietary benefits induced by spawning migrations. • Freshwater prey base for perch remains stable throughout the year. • The marine prey base for perch more closely reflects prey migratory patterns. • Capture environment and size interact to influence perch trophic level dynamics.