Seals feel stressed in presence of sharks

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Seals feel stressed in presence of sharks

December 06, 2017 - 19:57
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A study examines the impact of predators on their prey's stress levels in the wild.

Colony of brown fur seals at Cape Cross on the Skeleton Coast, Namibia.

Being a seal swimming in the ocean surrounded by great white sharks is definitely stressful. In a three-year study, researchers sought to find out whether living in high-stress conditions can have an impact on the seal population.

Led by University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science, they focussed their attention on the fur seals living off South Africa's Western Cape, where one of the densest populations of great white sharks are found. The findings of the study was published recently in the Ecology journal.

At the different seal colonies, they compared the glucocorticoid metabolite concentrations (fGCM) in seal fecal samples with the residency patterns of great white sharks. (fGCM is a cortisol stress hormone.) At one of the sites, shark attack rates were also considered.

“Our findings showed that seals exhibited high stress in the places and at the times when great whites were hunting and the seals had no way of anticipating or effectively preventing a predation attempt from any shark that decided to attack,” said lead author Neil Hammerschlag, a research assistant professor at the UM Rosenstiel School and UM Abess Center for Ecosystem Science and Policy.

“Comparable stress responses were not detected in places and times where sharks were not hunting. Interestingly, stress responses were also not detected at one island where seals could reduce their risk of attack by using kelp beds and reef as underwater refuges, despite the presence of hunting great whites,” said co-author Scott Creel, a professor at Montana State University.

The team thus concluded that in cases where the risk cannot be predicted or controlled by behavioural responses, predation risk will lead to an increased level of stress for the fur seals.

Hammerschlag concluded that the study's findings underlined the ecological importance of apex predators, saying: "Any resulting loss in health or survival of prey due to predator-induced stress could have cascading effects on the entire ecosystem and food web."

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