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Marine Ecology Progress Series

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MEPS 644:215-228 (2020)  -  DOI: https://doi.org/10.3354/meps13346

Daily and seasonal movements of Cape Cod gray seals vary with predation risk

Jerry H. Moxley1,4,*, Gregory Skomal2, John Chisholm2, Patrick Halpin1,3, David W. Johnston1

1Division of Marine Science and Conservation, Nicholas School of the Environment, Duke University Marine Lab, Beaufort, NC 28516, USA
2Massachusetts Division of Marine Fisheries, New Bedford, MA 02744, USA
3Division of Marine Science and Conservation, Nicholas School of the Environment, Duke University, Durham, NC 27708, USA
4Conservation Research, Monterey Bay Aquarium, Monterey, CA 93940, USA
*Corresponding author:

ABSTRACT: White sharks Carcharodon carcharias and gray seals Halichoerus grypus are re-establishing their ecological roles within the Northwestern Atlantic Ocean, presenting an opportunity to understand gray seal movement and at-sea behavior under predation risk. As with other shark-seal hotspots, movements to and from terrestrial haul outs can be risky for gray seals, thereby eliciting antipredator strategies. We investigated the movement and coastal behavior of gray seals on Cape Cod (USA) in relation to seasonal and diel changes in white shark activity. Analyzing 412 trips to sea by 8 seals and more than 25000 acoustic detections from 23 individual white sharks, we observed seasonally homogeneous movements in seal behavior during months with greater shark presence. During riskier months, seal behavior manifested in near-exclusive nocturnal foraging, reduced offshore ranging, and limited at-sea activity. On these nocturnal trips to sea, seals returning to haul outs tended to avoid daybreak and traversed during diel minima in shark activity. However, seals tended to depart haul outs at dusk when shark presence was maximal. As conservation efforts succeed in rebuilding depleted populations of coastal predators, studying re-emerging predator-prey interactions can enhance our understanding about the drivers of movement and behavior.


KEY WORDS: Food-safety tradeoff · Gray seal · Halichoerus grypus · White shark · Carcharodon carcharias · Predation risk · Behavioral ecology


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Cite this article as: Moxley JH, Skomal G, Chisholm J, Halpin P, Johnston DW (2020) Daily and seasonal movements of Cape Cod gray seals vary with predation risk. Mar Ecol Prog Ser 644:215-228. https://doi.org/10.3354/meps13346

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