Environmental drivers of abundance and residency of a large migratory shark, Carcharhinus leucas, inshore of a dynamic western boundary current

Abstract

Understanding the environmental drivers of movement of potentially dangerous shark species can help inform mitigation strategies. Bull sharks are known to undertake seasonal migrations from tropical to temperate waters along the east coast of Australia. However, the environmental drivers of their movements from sub-tropical to temperate waters are unknown. Using multiyear (2010−2016) acoustic telemetry data from 68 bull sharks and generalised additive models, we evaluate the (1) temporal and (2) environmental variables that drive shark abundance, presence/ absence and residency along the south-eastern coast of Australia. Bull sharks were detected in sub-tropical waters (~28°S) almost year-round but were most abundant in the southern latitudes in the austral summer and autumn. Abundance, presence and residency were all highest around the latitudes that sharks were tagged, indicating a bias to tagging location, and at estuary mouths and mid-shelf (20−60 m water depth) habitats. Bull sharks were present when sea surface temperature (SST) was 20−26°C, with peak abundance at 24°C, and low chlorophyll a (chl a). There was a higher abundance of sharks in months when SST was higher than the long-term average. Residency duration was longest when SST was <22°C or >24°C, and in areas of low SST slope (<3°C) and chl a slope. Although no sex bias in residency time was detected, sharks <200 cm TL had the longest residency times. These results provide the power to predict when and where bull shark abundance may be higher, which can help management authorities deploy mitigation strategies for bull shark interactions along eastern Australia.