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Fish display a fascinating variation in behavior, morphology and physiology among species, among individuals within species, and within individuals over time. A central quest in ecology and evolution is to understand causes and consequences of such variation. This thesis aims to contribute to this knowledge by: (1) investigating the evolutionary processes that shape intraspecific variation among sympatric subpopulations of pike in coastal areas of the Baltic Sea; and (2) exploring whether fish can utilize sun-basking to regulate body temperature and whether this has any consequences for fitness.

Identifying barriers that delineate populations is a first step towards evaluating the evolutionary origin of intraspecific variation. This thesis reports on genetic population structures among co-existing pike in the coastal Baltic Sea separated by homing behavior, different spawning strategies and geographic barriers. Field studies revealed that these subpopulations also show phenotypic divergence in reproductive and meristic traits. Experimental studies suggested that differentiation among subpopulations likely was a result of divergent selection and local adaptations to spawning grounds. These adaptations that may further reinforce barriers among subpopulations due to a reduced success of immigrant genotypes.

For the second aim of the thesis, we first studied seasonal and diel patterns of activity and vertical migration among the Baltic Sea pike. The results suggested that pike exposed themselves to sunlight during spring and summer, and that body temperatures were positively correlated with sun exposure during these basking events. This was followed by experimental studies on inanimate physical models and a field study on carp which demonstrated that fish can become warmer than ambient water when exposed to sun light, a previously overlooked mechanism for fish thermoregulation, and that the heat gain was positively correlated with growth.

This thesis contributes to our understanding of the origin and maintenance of intraspecific variation among coexisting populations with direct implications for management of pike. It also establishes sun-basking as a novel mechanism for fish to obtain body temperatures in excess of ambient water which could motivate adaptations, both evolutionary and plastic, that optimize heat gain, affect spatiotemporal distributions and biotic interaction within and among species.