Marine wildlife experts are baffled by the death of thousands of Cape fur seals on the west coast.
Video footage this week showed seals convulsing and dying in shallow water, with people reporting numerous carcasses between Cape Town and Elands Bay to the north.
The deaths coincide with those of thousands of coastal birds from avian influenza on the west coast.
Last year the deaths of many seals in Namibia were linked to malnutrition caused by a temporary collapse of the sardine and anchovy fisheries.
Experts say malnutrition makes seal populations more vulnerable to infectious agents, though the precise cause of the latest deaths is unknown.
Tess Gridley from Cape Town marine research consultancy Sea Search said a concern is that the deaths are of adults and pups, unlike smaller seasonal deaths in the breeding cycle when not all the dead animals were malnourished.
“In Lamberts Bay there were 58 dead seals on one small beach just 150m long. I don’t really understand it yet,” she said.
“It’s a funny year with El Niño and lots of changes going on. Maybe there is some underlying pathogen that comes out when animals are under stress.”
Gridley and her team photographed about 500 dead seals in a day in just two locations on the west coast this week.
It’s not just seals feeling the effects of dwindling fish resources. Coastal birds such as Cape cormorants and African penguins are also showing signs of malnutrition, making them more susceptible to other environmental stresses.
Steve Benjamin, who runs seal-diving adventures in Hout Bay, said the deaths are alarming. “I’ve never seen it this bad — the seals seem to be starving,” he said.
Former Johannesburg Zoo vet Brett Gardner, who is researching seal infectious abortions in Australia, said laboratory tests from the deaths will indicate whether the animals suffered the same protein malnutrition observed in Namibia, where seals were “so abnormally skinny that they were losing their pregnancies”.
He said human pressure on wild fisheries, particularly pelagic species like sardines and anchovies, is putting pressure on apex predators such as seals.
“When you see an apex predator dying of malnutrition then you can only imagine what has happened [to the food chain] below it. It is a big alarm bell,” said Gardner.
Department of environment, forestry & fisheries spokesperson Zolile Nqayi said those involved are “alarmed” by the deaths. The department has asked veterinarians at the Western Cape department of agriculture for help with post-mortems.
He said there is no indication of a link between seal mortality and the avian flu outbreak along the west coast.
Two Oceans Aquarium spokesperson Renée Leeuwner said government authorities and state vets are testing some of the dead animals.
Commenting on the avian flu outbreak, Western Cape environmental affairs MEC Anton Bredell said the province is most concerned about Dyer Island, where an estimated 500 birds are dying every day.
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