Vertical movements of coastal pike (Esox lucius)-On the role of sun basking

Knowledge of patterns and drivers of the spatiotemporal distribution of top predatory fish is key to understand ecological dynamics and to successful management. Here, we integrated field and laboratory approaches to study vertical movements of pike (Esox lucius) in relation to season, light regimes and body temperature. We tagged pike from the Baltic Sea with data storage tags during spawning migration and retrieved them during migration the following years to obtain high-resolution data from full year of movements. The results showed seasonal and diel patterns of activity and body temperature that conformed to distinct patterns of crepuscular activity and diel vertical migrations. The latter manifested as two different patterns, either a stationary phase in the surface water during day followed by night-time in deeper water or vice versa. The occurrence of these two behaviours varied among individuals and within individuals among seasons. Diel vertical migration has previously not been described for this shallow-dwelling species, but was a common and consistent behaviour among individuals in this study. We suggest that the function of the daytime surface behaviour in pike is to increase body temperature through sun basking. This thermoregulatory role of surfacing was supported by the laboratory study where individuals sought the surface layer, exposed themselves to infrared light and thereby attained body temperatures in excess of ambient water. These results support sun basking as a mechanism for heat gain and further suggest that access to sunlight in the surface layer could be an important driver of vertical migrations.