Wildlife experts are concerned about an increase in unusually aggressive seals, with an unprecedented recent spike in biting incidents in and around Cape Town.
And a top marine scientist has warned that the attacks may be due to a naturally occurring toxin previously linked to unusual animal behaviour and found in seal carcasses along the coast after a recent seal die-off.
Last weekend a Cape fur seal attacked two people in separate incidents on Fish Hoek beach, with one victim hospitalised due to the severity of the bites. There has also been a marked increase in attacks on recreational divers, prompting concern among wildlife stakeholders.
One prominent Cape Town wildlife stakeholder said he had seen more seal bites in the past six months than in the past 20 years.
Dr Tess Gridley, founding director of Cape Town research group Sea Search, this week said seal aggression could be evidence of neurological damage caused by the powerful biotoxin domoic acid found in algal blooms that occur regularly along the coast. Seals may be eating seafood contaminated by the blooms, thereby exposing themselves to domoic acid, said Gridley, who has conducted extensive research on seal carcasses collected during the die-off.
Domoic acid has been found in one or more seals from 13 different locations — from Lamberts Bay on the west coast to Muizenberg in Cape Town, Gridley said. The toxin was found in all age cohorts and both sexes of animals tested.
“It is one potential explanation for this sudden change in behaviour. We obviously want to check this line of enquiry,” she said, adding that reports of seals chasing people on land were highly unusual.
• 2-million: The number of Cape fur seals in Namibia
• 25-40: The number of Cape fur seal colonies along coast of Southern Africa
— IN NUMBERS
A video circulating on social media of one of last weekend’s attacks shows the seal pursuing a startled woman across Fish Hoek beach before pouncing on her. “It is very unusual for an animal to come out of the water and purposefully chase a person. It is very, very unusual and doesn’t make sense,” Gridley said.
She said changes in behaviour had been documented in sea lions in California, also linked to domoic acid. The waters around southern California have seen an increase in naturally occurring algal blooms and domoic acid, with the world’s highest measurement of the neurotoxin recorded near San Pedro. The increasing frequency of algal blooms is a cause for concern due to possible links with ocean warming or increased nutrient runoff from land.
Domoic acid moves up the food chain as animals feed off each other — a process called bioaccumulation.
An increase in domoic acid in South African Cape fur seals could indicate a similar increase in algal blooms in South African waters.
Gridley said there were other possible causes of aggressive seal behaviour and more research was needed to determine a definite link to domoic acid. Higher testosterone levels in the animals or increasing human encroachment on seals’ natural habitat were other possible factors.
The Fish Hoek Surf Lifesaving Club has cautioned bathers to steer clear of seals in the area.
“This type of behaviour is more common than we thought with a number of similar incidents being reported recently,” the club said in a social media post. “Unfortunately there is no way this incident could’ve been avoided as there was an attempt to give way to the seal when it approached.”
Jerome Petersen, a 51-year-old spearfisherman who was attacked last year while diving in False Bay, said he had only been diving once since the incident. He said he was lucky to escape alive after the seal bit him repeatedly and attempted to drag him into deeper water.
He said two friends recently had to fend off an aggressive seal. “I can’t comment on whether seals have become more aggressive because I haven’t been back into the water since then,” Petersen said, adding that he had since bought himself a sailboat.
The City of Cape Town said it was aware of recent attacks but could not confirm whether there had been an increase in incidents along its shores. “Several of these incidents involved members of the public approaching seals,” said spokesperson Luthando Tyhalibongo. “Seals are wild animals and we want to caution residents and visitors not to approach, try to interact or touch or feed seals.
“This is not a new phenomenon — there have been incidences of aggression and bites in the past. The reports of some incidents being unprovoked is of concern to the city, something we will continue to monitor,” Tyhalibongo said.
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