Size-dependent diet composition and feeding of Eurasian perch (Perca fluviatilis) and northern pike (Esox lucius) in the Baltic Sea

To understand food web dynamics, knowledge about factors influencing trophic interactions is fundamental. Using stomach content analysis, we investigated size-dependent predator-prey relations of two coastal predatory fish in the Baltic Sea: perch (Perca fluviatilis) and northern pike (Esox lucius). Perch undergo two ontogenetic diet shifts, from zooplankton to macroinvertebrates at ca. 50 mm; and then to fish at ca. 250 mm. For pike, all sizes (103–810 mm) fed almost exclusively on fish. The fish prey of perch and pike was predominantly three-spined stickleback in spring, and gobiids in late summer. The mean and maximum prey:predator size ratio was larger, while the minimum was smaller for pike compared with perch. Perch and pike fed on smaller-sized gobiids, three-spined and nine-spined stickleback compared within the environment. Our results on size-dependent diets of perch and pike is useful for quantitative analyses of food-web interactions and for ecosystem-based management.