Combining acoustic telemetry and cardiac thermal physiology to determine environmental drivers of movements of a temperate coastal fish

Although animal movement is known to be driven by environmental cues, the underlying physiological mechanisms that are impacted by the environment to trigger movements are rarely explored. We used acoustic telemetry to follow the migrations of 72 gilthead seabream Sparus aurata over more than 2 yr, between a coastal lagoon and the sea in the Western Mediterranean and related the movements to prevailing environmental parameters. We then captured seabream from coastal lagoons and used implanted data storage tags to investigate cardiac responses of free-swimming animals to acute changes in temperature. This permitted a novel interpretation of patterns of seasonal migrations for spawning, and apparent refuge migrations in summer, with respect to thresholds of cardiac tolerance to warming or cooling. The telemetry data indicated that lagoon temperature was the main trigger of seaward seasonal departures in autumn and returns to forage in the lagoon in spring. Cardiac responses to cooling showed that 13°C was a lower sublethal temperature threshold, and seabream did not return to the lagoon until the average water temperature was 14.4°C. Cardiac responses to warming showed that 29°C was an upper sublethal threshold and, in summer, seabream performed apparent refuge migrations out to sea when lagoon temperature rose rapidly to exceed a mean of 27.4°C. Overall, this study demonstrates that combining acoustic telemetry and ecophysiology can provide insights into the mechanistic basis for migratory movements of valuable fish species, which can improve understanding of how ongoing climate change will impact them.