Abstract
Over the last two decades, satellite tagging of adult and sub-adult white sharks Carcharodon carcharias off the west coast of North America has revealed a predictable onshore-offshore migratory cycle. Our current understanding of the vertical movements exhibited by white sharks while in their coastal foraging phase in the California Current, however, remains limited. Here, we used recovered datasets from 31 archival satellite tags to quantify vertical habitat use. Tags were deployed on individuals between 2000 and 2018 and recorded depth and temperature data at continuous 1–120 s intervals before being recovered up to a year after deployments. Four satellite-tagged individuals were concurrently tagged with acoustic tags, providing precise location data when detected by acoustic receivers that allowed us to explore how reported vertical habitat use varied spatially. While in the coastal shelf waters, white sharks moved at a mean depth ± SD of 14.3 ± 4.0 m and occupied significantly deeper depths during the day than the night. High individual, temporal and spatial variation was evident in vertical movements, while consistent diel and lunar effects emphasized the importance of light-level driving vertical behavior around hunting sites. The vertical movement behaviors reported here provide knowledge of how white sharks may directly and indirectly interact with their mammalian prey in a dynamic three-dimensional system during their capital foraging phase. Temporal patterns in vertical behavior, for instance, indicated that surface waters during early morning hours are the riskiest place for prey. Combining these novel findings with higher-resolution biologging techniques in future studies will allow us to further contextualize fine-scale vertical movement behaviors of white sharks and examine the specific foraging events that could not yet be isolated in the tagging data.
Highlights
Electronic tagging has revealed that many species of marine megafauna are migratory and exhibit site fidelity, returning to the same site for foraging and/or reproduction purposes
This phase typically occurs during autumn and early winter months and represents a period of capital foraging, where sharks bulk up their lipid stores before heading to oligotrophic offshore waters, where they spend the majority of the year (Jorgensen et al, 2009; Del Raye et al, 2013)
We explore vertical movement behaviors that have been associated with hunting behaviors, such as movements associated with seabed depths and oscillatory diving
Summary
Electronic tagging has revealed that many species of marine megafauna are migratory and exhibit site fidelity, returning to the same site for foraging and/or reproduction purposes. Adult and subadult white sharks (Carcharodon carcharias) forage on marine mammals that aggregate at rookeries within cool, nutrient rich waters during the coastal phase of their annual migration (Ainley et al, 1981; LeBoeuf et al, 1982; Long et al, 1996). This phase typically occurs during autumn and early winter months and represents a period of capital foraging, where sharks bulk up their lipid stores before heading to oligotrophic offshore waters, where they spend the majority of the year (Jorgensen et al, 2009; Del Raye et al, 2013). Despite the decades of field-based research into the movement, life history and predatory behaviors of white sharks in California (e.g., Klimley, 1994; Goldman and Anderson, 1999; Boustany et al, 2002; Weng et al, 2007a; Jorgensen et al, 2009, 2019; Chapple et al, 2016), comprehensive investigation into the fine-scale vertical movement behaviors during their critical coastal foraging phase remains limited
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