Size‐dependent prey availability affects diet and performance of predatory fish at sea: a case study of Atlantic salmon

Identifying factors determining the performance of individuals is an essential part of resolving what drives population dynamics. For species undergoing ontogenetic shifts in resource and habitat use, this entails assessing individual performance in all habitats used. Whereas survival and growth of anadromous Atlantic salmon, Salmo salar L., in its juvenile, river habitat are known to depend on size‐dependent foraging and food availability, individual performance of salmon in the growth habitat out at sea is commonly explained only by abiotic factors. Still, individuals undergo this habitat shift to grow large, suggesting performance should be food‐dependent also in the growth habitat. Because fish communities are highly size‐structured, the link between predators and their prey may depend on their respective body sizes. Here, we study whether the performance of Baltic Sea salmon in its growth habitat is food‐ and size‐dependent, by combining extensive diet and body size data of Baltic salmon with spatially resolved monitoring data on abundance and size distribution of their main prey, herring, Clupea harengus L., and sprat, Sprattus sprattus L. We found that both the species and size composition of prey in the diet varied with salmon body size. By accounting for this size‐dependent predation and the spatially varying size distribution of prey species, we could explain the variation in salmon diet composition among salmon individuals in different Baltic Sea basins and of different length. The proportion of sprat in diet of salmon was better explained by size‐specific prey availability (SSP) than total prey biomass, especially for small salmon. Further, salmon body condition increased with SSP, whereas total prey biomass could not explain variation in the condition of salmon. These findings demonstrate that food‐ and size‐dependent processes indeed can influence the performance of anadromous fish also in large marine systems. Thus, we argue that consideration of these processes, stretching across habitats, is important for understanding performance and dynamics of predatory fish in open aquatic systems, as well as for successful management of species such as Atlantic salmon.