Tracking fitness in marine vertebrates: current knowledge and opportunities for future research

For more than 60 yr, electronic tags (including acoustic transmitters, archival loggers, and satellite tags) have been applied to free-ranging marine vertebrates to track their behaviour and characterize their spatial ecology. However, only recently have researchers begun using electronic tags to elucidate the processes that relate directly to fitness, i.e. the ability of organisms to survive and reproduce. We briefly review the history of tracking studies focused on marine vertebrates and then provide a general overview of studies that have used tracking to address fitness-related questions. Although many studies have used at-sea movement and activity data to better understand feeding ecology, physiology, and energetics, there is growing interest in the coupling of electronic tracking techniques with other disciplines to resolve the mechanisms underlying individual fitness, or more precisely the proxies thereof (survival, timing of reproduction, foraging success, etc.). We categorized studies into 4 general fitness-related areas: (1) foraging dynamics, energetics, and growth; (2) migration and other non-breeding season activities; (3) survival; and (4) reproduction. Despite recent advances in tracking technologies, which include multi-sensor loggers, tri-axial accelerometers, and miniaturized geopositioning systems, etc., very few studies on wild marine vertebrates truly measure individual fitness or proxies thereof. There is thus a need to design experimental, multi-disciplinary, and longitudinal studies that use genetics, individual-based modeling, and other techniques in an effort to resolve the mechanisms responsible for individual variation in fitness in marine vertebrates.