Aggregation behavior of pacific nurse sharks (Ginglymostoma unami) associated with seasonal water temperature changes in a tropical upwelling bay

Aggregations form as individuals respond to common drivers like prey availability, predator avoidance, reproduction, or environmental conditions. Pacific Nurse Sharks ( Ginglymostoma unami ) form aggregations in the shallow waters of Santa Elena Bay, north Pacific of Costa Rica, a region where seasonal upwelling (from December to April) causes water temperatures to decrease as low as 16 °C from mean annual temperatures of 28 °C. We expected G. unami aggregations to form when individuals seek out warmer pockets in shallow sites to avoid colder upwelling conditions in non-aggregation sites. Using unoccupied aerial vehicle surveys (UAV; drone) and in situ seafloor temperature loggers, we test whether the occurrence of G. unami aggregations are associated with water temperature changes between upwelling and non-upwelling seasons. We tested this hypothesis at two spatial resolutions: (1) “site-wide” resolution, comparing mean temperatures between sites; and (2) “within-site” resolution, considering temperature variations in a single aggregation site. At the site-wide scale, water temperature variation was associated with aggregating behavior. During upwelling events, aggregations were most often observed at sites with warmer waters, suggesting behavioral thermoregulation to avoid cold upwelling conditions. However, temperature had a limited effect explaining the fine-scale occurrence of aggregations within sites. Hence, we expect other factors, such as conspecific attraction, to drive this behavior as well. Understanding where and why these aggregations occur is essential for management, especially considering the potential alterations to G. unami' s behavior under changing upwelling patterns related to climate change. • Changing water temperatures through the year are associated with aggregations. • Aggregations occurred when water temperature at aggregation sites was 27–28 °C. • Higher aggregation occurrence was observed during tropical upwelling conditions. • Water temperature explains presence at aggregation sites, but not within sites. • Within-site spatial selection may be related to drivers like conspecific attraction.