White sharks as oceanographic sentinels: assessing apex predator tag data for ocean monitoring in the Greater Agulhas Current System

Abstract

This study hypothesizes that resident and migratory white sharks along the South African coast can effectively collect surface and subsurface temperature observations to complement conventional ocean observing platforms. To evaluate this potential, temperature data from three individuals (Nemo, Monwa, and Shark05), tagged with Wildlife Computers MiniPATs, were analyzed along their trajectories through the Greater Agulhas Current System (GACS). These biologging records were compared to two satellite-derived SST products (OSTIA and ESA CCI) and two global reanalyses (GLORYS 1/4° and 1/12°). All products exhibited reduced accuracy near the coast, where environmental gradients are strongest and variability is high. In contrast, offshore agreement with shark-derived surface temperatures was significantly better. Satellite SST products generally showed the best performance overall, while GLORYS reanalyses displayed systematic subsurface biases in coastal and frontal regions, with discrepancies reaching up to ±5 °C. These results confirm the utility of biologged sharks as complementary oceanographic sensors, especially for revealing limitations in model products. Despite inter-individual behavioral differences, all sharks generated highresolution datasets spanning a wide range of depths and thermal environments. Most dives occurred within the epipelagic zone (0–200 m), with thermal preferences between 12–16 °C. Shark05 and Monwa concentrated sampling in coastal waters, while Nemo conducted deeper vertical excursions reaching 1,000 m. Overall, this study demonstrates that biologged white sharks can enhance ocean observing systems by providing reliable, vertically resolved data in regions where satellite and model outputs remain uncertain. Their integration into operational frameworks could improve environmental monitoring in climate-sensitive marine systems like the GACS