The role of refugia in reducing predation risk for Cape fur seals by white sharks

Wcisel, Michelle A.; O’Riain, M. Justin; Vos, Alta De; Chivell, Wilfred

doi:10.1007/s00265-014-1825-5

Cited by 27 publications

(47 citation statements)

References 51 publications

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“…For example, white shark attack rates on seals at the seal colony in False Bay (1.97 attacks/h) are 18 times higher than at Geyser Rock (0.1 attacks/h), because the latter is surrounded by dense kelp beds and reefs that serve as a refuge from sharks when the seals traverse from their colony (Wcisel et al. ; Fig. B).…”

Section: Introductionmentioning

confidence: 99%

Physiological stress responses to natural variation in predation risk: evidence from white sharks and seals

Hammerschlag

Meÿer²,

Seakamela³

et al. 2017

Ecology

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Predators can impact ecosystems through consumptive or risk effects on prey. Physiologically, risk effects can be mediated by energetic mechanisms or stress responses. The predation-stress hypothesis predicts that risk induces stress in prey, which can affect survival and reproduction. However, empirical support for this hypothesis is both mixed and limited, and the conditions that cause predation risk to induce stress responses in some cases, but not others, remain unclear. Unusually clear-cut variation in exposure of Cape fur seals (Arctocephalus pusillus pusillus) to predation risk from white sharks (Carcharodon carcharias) in the waters of Southwestern Africa provides an opportunity to test the predation-stress hypothesis in the wild. Here, we measured fecal glucocorticoid concentrations (fGCM) from Cape fur seals at six discrete islands colonies exposed to spatiotemporal variation in predation risk from white sharks over a period of three years. We found highly elevated fGCM concentrations in seals at colonies exposed to high levels of unpredictable and relatively uncontrollable risk of shark attack, but not at colonies where seals were either not exposed to shark predation or could proactively mitigate their risk through antipredatory behavior. Differences in measured fGCM levels were consistent with patterns of risk at the site and seasonal level, for both seal adults and juveniles. Seal fGCM levels were not correlated with colony population size, density, and geographic location. Investigation at a high risk site (False Bay) also revealed strong correlations between fGCM levels and temporal variation in shark attack rates, but not with shark relative abundance. Our results suggest that predation risk will induce a stress response when risk cannot be predicted and/or proactively mitigated by behavioral responses.

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Section: Introductionmentioning

confidence: 99%

Physiological stress responses to natural variation in predation risk: evidence from white sharks and seals

Hammerschlag

Meÿer²,

Seakamela³

et al. 2017

Ecology

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“…Foraging attempts and success by sharks on seals are considerably lower at Geyser Rock than at nearby Seal Island, likely due to the extra kelp refuge provided at Geyser Rock (Wcisel et al . ). The added protection provided by kelp causes seal departure locations at Geyser Rock to be more diffuse (i.e.…”

Section: Discussionmentioning

confidence: 97%

“…; Wcisel et al . ). As in other areas, selection of ARS sites or predation hotspots is going to be related to a number of factors including prey behaviour, habitat and intraspecific competition (Martin, Rossmo & Hammerschlag ).…”

Section: Discussionmentioning

confidence: 97%

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Sex‐specific and individual preferences for hunting strategies in white sharks

et al. 2016

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Summary Fine‐scale predator movements may be driven by many factors including sex, habitat and distribution of resources. There may also be individual preferences for certain movement strategies within a population which can be hard to quantify. Within top predators, movements are also going to be directly related to the mode of hunting, for example sit‐and‐wait or actively searching for prey. Although there is mounting evidence that different hunting modes can cause opposing trophic cascades, there has been little focus on the modes used by top predators, especially those in the marine environment. Adult white sharks (Carcharhodon carcharias) are well known to forage on marine mammal prey, particularly pinnipeds. Sharks primarily ambush pinnipeds on the surface, but there has been less focus on the strategies they use to encounter prey. We applied mixed hidden Markov models to acoustic tracking data of white sharks in a coastal aggregation area in order to quantify changing movement states (area‐restricted searching (ARS) vs. patrolling) and the factors that influenced them. Individuals were re‐tracked over multiple days throughout a month to see whether state‐switching dynamics varied or if individuals preferred certain movement strategies. Sharks were more likely to use ARS movements in the morning and during periods of chumming by ecotourism operators. Furthermore, the proportion of time individuals spent in the two different states and the state‐switching frequency, differed between the sexes and between individuals. Predation attempts/success on pinnipeds were observed for sharks in both ARS and patrolling movement states and within all random effects groupings. Therefore, white sharks can use both a ‘sit‐and‐wait’ (ARS) and ‘active searching’ (patrolling) movements to ambush pinniped prey on the surface. White sharks demonstrate individual preferences for fine‐scale movement patterns, which may be related to their use of different hunting modes. Marine top predators are generally assumed to use only one type of hunting mode, but we show that there may be a mix within populations. As such, individual variability should be considered when modelling behavioural effects of predators on prey species.

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“…Unlike other protected marine taxa (e.g. 1D) may expose sea otters to fatal bites from white sharks Carcharodon carcharias (Ames and Morejohn 1980, Wcisel et al 2014, Tinker et al 2016. 1B), where spatial gaps in giant Macrocystis pyrifera and bull Nereocystis luetkeana kelp forest canopies (Fig.…”

Section: Introductionmentioning

confidence: 99%

Gaps in kelp cover may threaten the recovery of California sea otters

et al. 2018

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Despite more than a century of federal protection, the California sea otter Enhydra lutris nereis remains threatened under the U.S. Endangered Species Act (ESA), and the population has not appreciably expanded its range in two decades. Here, we examine a novel dataset of 725 sea otter live strandings from 1984-2015 to gain insights into demographic and environmental factors underlying threats to sea otter recovery. Using multinomial logistic regression to evaluate spatiotemporal patterns of stranding causes, we demonstrate that increases in stranding rates, particularly outside the range center, are related to a substantial increase in shark bites. By contrast, trauma linked to human activities has declined dramatically, and now accounts for less than 5% of stranding cases. Within the range core, where the sea otter population seems regulated by prey availability, symptoms of energetic stress represent more than 63% of all strandings and are strongly associated with high sea otter density. Conversely, in range peripheries, the majority of strandings are caused by shark bite and neurological disease. Notably, these threats are virtually absent where nearshore habitat is characterized by at least 10% kelp canopy cover. Our analyses reveal that declining kelp cover may therefore constrain the population's spatial expansion and recovery in two key ways. Absence of kelp intensifies density-independent threats in the range peripheries, and likely limits dispersal of reproductive females, which depend on kelp canopy for nursery habitat. These results highlight the significance of both top-down and bottomup processes in population dynamics, and inform an ecosystem-based approach to conservation planning.

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The role of refugia in reducing predation risk for Cape fur seals by white sharks

Cited by 27 publications

References 51 publications

Physiological stress responses to natural variation in predation risk: evidence from white sharks and seals

Physiological stress responses to natural variation in predation risk: evidence from white sharks and seals

Sex‐specific and individual preferences for hunting strategies in white sharks

Gaps in kelp cover may threaten the recovery of California sea otters

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