Eating with a grain of salt: Trade-offs between habitat use and individual foraging specialisation drive growth across ecotypes of a freshwater piscivore (pike, Esox lucius) inhabiting brackish habitats

Freshwater fishes living in brackish environments often diverge into ecotypes displaying different habitat use. Salinity levels common to brackish habitats incur osmoregulatory costs to freshwater fishes, but also offer opportunities in the form of access to marine prey, creating trade-offs. We investigated foraging specialisation of coastal northern pike (Esox lucius) ecotypes, ranging from migratory to brackish- or freshwater- residents, sampled from coastal lagoons and freshwater tributaries in the southern Baltic Sea. We combined resource use with individual growth as a fitness surrogate to identify mechanisms of coexistence. Individual foraging specialisation was highest in behavioural phenotypes with high trophic overlap in shared foraging grounds, brackish resident pike and cross-habitat pike. Larger brackish-adapted individuals specialised more on marine resources with positive effects on growth, but higher-than average salinities reduced growth. Trophic overlap of anadromous and freshwater pike phenotypes was low and differed from cross-habitat and brackish habitat phenotypes. Age-specific effects of marine resources and salinity on individual growth suggest coexistence of pike ecotypes is facilitated by dynamic trade-offs between foraging specialisation and habitat costs.