THE presence of great white sharks off the Cape sends seals’ stress levels rocketing.
US scientists who studied seals on six islands for three years say levels of a stress hormone in their faeces rose sharply when the oceans’ apex predator was lurking.
In particular, researchers from the University of Miami found that stress levels peaked when seals were at risk of unpredictable and lethal attack from great whites as they left the safety of an island’s inner perimeter and passed through a gauntlet of sharks to reach offshore feeding grounds.
“These results underline the ecological importance of apex predators,” said study leader Neil Hammerschlag, who specialises in ecosystem science.
“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,” he wrote in the journal Ecology.
Hammerschlag said Cape fur seal colonies were the ideal place to study the hypothesis that predators could exert control over their prey using a stress response because they existed among some of the world’s densest great white shark populations.
Cape fur seal colonies are the ideal place to study the hypothesis that predators could exert control over their prey using a stress response
His team compared stress hormone levels in seal faces with residency patterns of great whites based on satellite tagging data. The team also compared seal faecal cortisol concentrations with measured shark attack rates on seals at one of the sites.
“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 Hammerschlag.
Co-author Scott Creel, a professor at Montana State University, said: “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 [Geyser Rock in Gansbaai] where seals could reduce their risk of attack by using kelp beds and reef as underwater refuges, despite the presence of hunting great whites.”
At Seal Island in False Bay, seals’ faecal stress levels were highly correlated with weekly shark attack rates, and 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 behavioural responses.