It’s official: Seals get stressed out over great white sharks

If you’re a seal and have great white sharks constantly chasing you, wanting to eat you, it stands to reason that you’re going to be stressed out.

Well, now it can be stated unequivocally.

In a three-year study, researchers tested the stress levels in seals inhabiting six islands in South Africa’s Western Cape where Cape fur seal colonies have varied exposure to great white sharks hunting them.

The research team from the University of Miami Rosenstiel School of Marine and Atmospheric Science collected seal fecal samples to test for concentrations of the cortisol stress hormone glucocorticoid metabolite.

“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 the study’s 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.”

Great white sharks cause seals great stress. Photo: Courtesy of Chris Fallows/Apex Shark Expeditions and the Rosenstiel School of Marine and Atmospheric Science

More from the Rosenstiel School of Marine and Atmospheric Science press release:

In one location, called Seal Island in False Bay, the seals’ fecal stress levels were highly correlated with weekly shark attack rates. However, seals did not show comparable signs of stress at another location known as Geyser Rock in Gansbaii, which contains kelp beds and reefs that the seals use as natural safe passageways from sharks as the move about the island.

Based on the findings, the authors suggest that predation risk will produce physiological costs in the form of a stress response when risk cannot be adequately predicted or controlled by behavioral responses.

“These results underline the ecological importance of apex predators,” said Hammerschlag. “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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