Water temperature is a key driver of horizontal and vertical movements of an ocean giant, the whale shark Rhincodon typus

Abstract

Integrated analysis of the vertical and horizontal movements of epipelagic fishes requires high-resolution data from tags that have been attached to animals for long periods. The recovery of a SPLASH tag deployed on a whale shark Rhincodon typus for 3 mo enabled access to archival data of horizontal and vertical movements of the shark that travelled 5380 km from Christmas Island to the Banda Sea. We examined water temperature and bathymetry to compare movements of the shark with key oceanographic features. Over 89 d, we identified 1931 dives and grouped these into 5 broad categories depending on dive shape. To investigate the potential for these dives to reflect behavioural thermoregulation, we used boosted regression trees to model the relationship between pre- and post-dive surface durations and dive characteristics. The movements of the shark were correlated with water temperatures, with the animal following frontal systems while remaining in a sea surface temperature (SST) range of 24-29°C. Across the continental shelf off north-western Australia, the shark mostly remained near the seafloor, likely avoiding very warm SSTs and strong currents at the surface. U- (foraging) and V- (searching) shaped dives accounted for ~78% of dives. Foraging dives during the day descended to 200-500 m and were preceded and followed by extended periods at the surface, whereas at night, the shark rarely dived below 200 m, likely foraging near the thermocline. Our results show how water temperatures influence the movement of this whale shark, with ‘basking’ at the surface during the day likely to gather environmental heat for thermoregulation, aiding to maintain body temperatures.