Few people believed Tess Gridley when she said seals were behaving strangely, chasing swimmers, biting beachgoers, convulsing and then dying along the coast from Namibia to Mossel Bay.
Gridley had spent enough time with seals to know this was unusual. Turns out she was right — our seal population is infected with rabies and the Cape Town-based marine scientist is being hailed for being first to raise the alarm.
“It started with noticing seal die-offs, then it morphed into strange behaviour,” said Gridley, whose work involves recording and interpreting animal sounds such as whale song and dolphin clicks.
Disturbed by what she saw, Gridley, who co-manages a maritime research company with her husband, started collecting seal carcasses, recording data and sending brain samples for testing. Their painstaking efforts initially received little recognition.
At the time there was an underlying assumption that the attacks on people were a case of seals defending their territory. “There was a little resistance to the idea that something unusual was going on. It was much easier to blame people, that they shouldn’t be coming so close to the seals, than it was to say the species had somehow changed its behaviour,” she said at the Muizenberg office of their Sea Search company — a collective of marine experts and researchers.
Initially the results pointed to a form of brain disease caused by algal blooms, commonly known as “red tide”. The team photographed about 500 dead seals in a day at two locations on the West Coast. At one small beach in Lamberts Bay there were 58 dead seals, many malnourished.
But it was an alarming increase in seal attacks on humans that raised further questions, particularly as they involved seemingly normal seals that were not malnourished. Gridley flagged one incident — a violent attack on a spearfisherman who was repeatedly bitten while swimming back to shore. “I remember that very clearly — it just didn’t make sense.”
So violent was the attack that many thought it was a visiting leopard seal, a bigger species endemic to Antarctic waters known for its aggression. But her own investigations pointed to a local Cape fur seal — suggesting highly an unusual behaviour on the part of the culprit.
Cape Town-based US actress Loulou Taylor, who was attacked at Clifton, is one of more than 71 people who have been bitten or scratched by Cape fur seals since 2021. A seal-diving operation in Hout Bay closed down due to concern for the staff and customers after repeated seal attacks .
Despite multiple attacks, researchers were unsure how to prove their suspicions. “If you have an animal behaving oddly, how do you track it? They all kind of look the same. That was a big issue,” Gridley said.
The breakthrough came earlier this year when a seal attacked several Muizenberg surfers, before washing up dead. Its carcass was sent for testing with three other samples from Gridley’s collection. “We had four brain samples and because each of those seals had something a little bit strange, we decided to box them off. Three out of the four animals tested positive for rabies.”
The result took everybody by surprise. Apart from a solitary case in Norway more than 40 years ago, it was the first time rabies had been identified in a marine mammal.
“Rabies is directly linked in this case to severe aggression, where behaviour is completely out of the norm,” said City of Cape Town head of coastal management Gregg Oelofse. He said it was unclear if rabies would remain endemic in the seal population or gradually fade away, as it did in some other species. “It’s a concern, and we are trying to figure things out as we go along.”
Oelofse paid tribute to Gridley and partner Simon Elwen for their research: “If it wasn’t for Sea Search I don’t think we would know what we now know.”
Elwen said Gridley and her team had persevered “against all odds”.
“There were a lot of people who have been dismissive, saying, ‘Oh well nothing has changed and why does it matter.’ It has taken a good behavioural biologist to see there is something unusual going on and that we really should get to the bottom of it.”
It has taken a good behavioural biologist to see there is something unusual going on and that we really should get to the bottom of it
— Researcher Simon Elwen
Scientists involved in seal research this week issued a joint statement confirming that rabies appears to be well-established in the Cape fur seal population. Future coastal management would include culling seals that exhibited rabies symptoms or behaved aggressively.
Experts said it was unclear how the disease would manifest, or whether it could be contained and prevented from spreading more widely. They advised anybody bitten by a seal to receive immediate medical treatment for rabies.
Gridley said the seal crisis had spurred collaboration that could have lasting benefit. “That is what you aim for as a scientist — to do something that has a conservation impact, a real impact. We managed to discover something that had never been discovered.”
Seal rabies is not the only discovery by Gridley and her colleagues. Previous bioacoustic work has shown how seals not only have distinct calls for different actions, they may have distinct dialects and accents as well, much like humans. They also discovered that visiting humpback whales spend up to 17 hours a day singing to each other, sometimes together, in sing-a-longs that can be heard at least 30km away.
During the pandemic lockdown, when Gridley was unable to go to sea, she and Elwen recorded geese congregating near her house and at Westlake Golf Course. Data analysis revealed a range of goose call types, from loud honking to a short, soft vocalisation.
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