Thermosaline niche, resource use and growth of a stenohaline freshwater predator (northern pike, Esox lucius) in brackish ecosystems and adjacent tributaries of the southern Baltic Sea

Niche variation and subsequent eco-evolutionary adaptations to different habitats, environmental conditions and resources drive intraspecific diversification and speciation. While niche variations have been documented in multiple different taxa, most studies have focused on only a few specific axes of a species’ ecological niche. Much less is known about the interactions between niche axes, and how multidimensional niche variations relate to intrapopulation diversification. I used a stenohaline freshwater predator, northern pike (Esox lucius), inhabiting interconnected brackish lagoons and freshwater tributaries around Rügen island in the southern Baltic Sea of Germany, as a model. I studied multidimensional niche variation and intraspecific diversification patterns, leveraging individual-level data on thermal, saline and dietary niche use in combination with telemetry, genetics and growth data as a fitness surrogate. Pike displayed variation along multiple niche axes, corresponding with genetic differentiation, forming three ecotypes with distinct thermosaline and feeding niches. I detected a habitat-specialized ecotype each in freshwater and brackish environments, which were connected by an intermediary migratory ecotype. Reproductive isolation is likely facilitated by environmental factors, specifically salinity, geographical distance, and anthropogenic habitat blockage. Niche overlap and differences in individual-level and subpopulation-level specialization suggested intraspecific competition in parts of the pike metapopulation, likely contributing to niche differentiation and diversification. I detected growth benefits through specialization on marine resources, in particular pulses of Atlantic herring (Clupea harengus), especially in large brackish-adapted individuals. However, the absence of differences in lifelong growth indicated similar lifelong fitness among pike ecotypes. Reasons for this are likely tradeoffs between osmoregulatory costs in brackish habitats and growth benefits through marine resource use. Equal growth, as well as resource partitioning, are likely important mechanisms for the coexistence of the pike ecotypes. My results indicate a behaviorally and genetically highly diversified metapopulation that displays strong local adaptation and specialization, which has important consequences for conservation and management. To increase general abundance and resilience of the pike metapopulation against environmental change, habitat connectivity in the system should be increased. Furthermore, protecting large fish that are able to profit most from marine resources may increase the growth performance of trophy pike. The coexistence of multiple discrete ecotypes with distinct habitat and resource specialization in the pike metapopulation highlights the need to integrate individual life histories and intraspecific diversity into adaptive management approaches, which holds the key for healthy pike stocks in the future.