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Marine Ecology Progress Series

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MEPS 622:219-230 (2019)  -  DOI: https://doi.org/10.3354/meps13051

Cape fur seals (Arctocephalus pusillus pusillus) adjust traversing behaviour with lunar conditions in the high white shark (Carcharodon carcharias) density waters of Mossel Bay, South Africa

P. Morse1,*, M. A. Mole2, M. N. Bester2, R. Johnson1, U. Scacco3, E. Gennari1,4,5

1Oceans Research Institute, PO Box 1767, Mossel Bay 6500, South Africa
2Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Private Bag X20, Hatfield 0028, Gauteng, South Africa
3ISPRA-Italian National Institute for Environmental Protection and Research, Via Vitaliano Brancati, 60, 00144 Roma, Italy
4South African Institute for Aquatic Biodiversity (SAIAB), Private Bag 1015, Grahamstown 6140, South Africa
5Department of Ichthyology and Fisheries Science, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
*Corresponding author:

ABSTRACT: White sharks Carcharodon carcharias are highly visual predators, leading to the hypothesis that the predation risk for foraging Cape fur seals Arctocephalus pusillus pusillus might differ with ambient light conditions. This study investigated the relationship between environmental fluctuations of ambient light and the traversing behaviour of Cape fur seals in and out of their colony at Mossel Bay, South Africa, to better describe potential predator avoidance strategies. A total of 12 144 traversing events were observed over a 4-yr period and there was an overall trend for Cape fur seals to traverse less often but in relatively larger group sizes during periods when white sharks are suggested to be more active. Specifically, Cape fur seal activity was reduced during winter when white sharks are most actively hunting, and most traversing behaviour occurred at night when Cape fur seals were less likely to be detected by white sharks. However, among nocturnal observations, Cape fur seal group sizes increased significantly with moonlight. Although nocturnal predations of Cape fur seals by white sharks have been observed before in Mossel Bay, this is the first study to indicate Cape fur seals might respond to the increased risk of improved white shark visual acuity during moonlit nights by seeking safety in numbers while foraging. Further investigations are needed to assess the effect of the lunar cycle on white shark nocturnal hunting behaviour, but observations presented here suggest that white sharks may pose a bigger threat to Cape fur seals under the light of a full moon.


KEY WORDS: Predator avoidance · Pinniped · Lunar cycle · Safety in numbers · Mobbing · Predator-prey


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Cite this article as: Morse P, Mole MA, Bester MN, Johnson R, Scacco U, Gennari E (2019) Cape fur seals (Arctocephalus pusillus pusillus) adjust traversing behaviour with lunar conditions in the high white shark (Carcharodon carcharias) density waters of Mossel Bay, South Africa. Mar Ecol Prog Ser 622:219-230. https://doi.org/10.3354/meps13051

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