Ocean warming increases residency at summering grounds for migrating bull sharks (Carcharhinus leucas)

Abstract

Globally, climate change is driving warming ocean temperatures, increasing the frequency and severity of extreme temperature events and altering current systems. Consequently, distributions and movement patterns of marine species are shifting, causing changes to ecosystem functioning. Migration patterns of large-bodied species are also expected to be affected by climate change. However, contemporary evidence of such changes to the spatio-temporal dynamics of movement and residency in migratory marine predators is rare, consisting mainly of predictions of distributions under future climate change scenarios because long-term tracking/catch data is difficult to obtain. Here, we use passive acoustic telemetry data spanning 15 years (2009–2024), in combination with remotely sensed and in-situ temperature data, to investigate how shifts in climate influence residency duration and migration timing in migratory bull sharks (Carcharhinus leucas) on their temperate summering grounds. Our results show that sharks have delayed departure from their temperate summering grounds off Sydney, Australia by an average of 1 day per year over a 15-year period, while also for the first time recording arrival times in October rather than November, as previously published. As a result, sharks are increasing residency duration, shifting the timing of migrations back to tropical latitudes for the colder months. Bull sharks depart temperate summering grounds upon possible long-term exposures to temperatures of ∼19–20.5 °C and below. Increases in sea temperatures are likely reducing the species' exposure to this thermal limit. In concordance, we found an average warming of 0.67 °C based on remotely sensed sea-surface temperature data (1982–2024) and of 0.57 °C based on in situ data (2006–2024) at our study site. As bull sharks are high-trophic level predators and implicated in shark-human interactions, more time spent at temperate summering grounds has the potential to impact local ecosystems through intensified predation pressure, while increasing temporal overlap with people in estuarine and coastal areas.