A spate of attacks by seals around Cape Town in South Africa may be due to the marine mammals having been poisoned by domoic acid.
The domoic acid, a toxin released during a red tide algal bloom, may have made its way into the seal’s system if they fed in the bloom’s vicinity and caused swelling in their brains and hearts, leading to increased aggression, Brett Glasby, wildlife management program coordinator at Two Oceans Aquarium, told local outlet News24.
“Domoic acid is produced by the phytoplankton genus Pseudonitzschia, a type of diatom,” Pete Strutton, a biological oceanographer at the University of Tasmania, told Newsweek. “Diatoms are a group of plankton (more specifically a type of phytoplankton, the photosynthetic plankton). They are common in very productive parts of the ocean, like coastal upwelling regions. Cape Town is close to the large Benguela upwelling system. California and Peru are other prominent examples of upwelling systems where the phytoplankton are dominated by diatoms.”
Under certain conditions, Pseudonitzschia produces domoic acid, but they don’t produce it all the time. Scientists aren’t sure why.
“There are some theories about it being useful for taking up nutrients. Domoic acid is a powerful mammalian neurotoxin. That is, it can accumulate in shellfish or fish without them experiencing any ill effects, but if mammals (seals, humans etc) eat those fish or shellfish, they suffer amnesic shellfish poisoning (ASP) from the domoic acid,” Strutton said.
Glasby said that usually when seals are poisoned by domoic acid, they will die, with survivors experiencing long-term neurological damage. He suggested that a mass-death event of Cape fur seals occurred due to domoic acid poisoning in late 2021 in the Cape Town area.
“We also know from similar events that have been documented in California and New Zealand that for roughly 18 months after the event, some seals will exhibit unusual behaviors, predominantly excessively aggressive defensiveness,” Glasby said. “This means that their response to stress or a possible stress is to respond aggressively.”
One of the attacks occurred on January 3, when actress LouLou Taylor, who recently played in the series Raised by Wolves, was bitten six times by a seal on Clifton’s 4th Beach in Cape Town. Other incidents occurred around the same time in Hout Bay and Yzerfontein, the South African National Sea Rescue Institute stated.
WATCH | Seal attack at Clifton beach, Cape Town could have been avoided if Swimmers didn’t attempt to chase it away, says Hout Bay Seal rescue.
On this received video the seal was making its way back into the water when it chased and started biting a child that was splashing pic.twitter.com/tQOQkL0bG3
— MDN NEWS (@MDNnewss) January 4, 2023
“The neural mechanisms that drive aggressive behaviors can differ between species,” Jake Robinson, an ecologist and science author, told Newsweek. “But, it is entirely possible that domoic acid poisoning could result in increased aggressive behavior in the seals when the toxin interacts with their neural circuitry. It has been shown to elicit these types of behavior in rodent models.”
“For instance, domoic acid can interact with the amygdala and ventromedial hypothalamus––both known to play key roles in the regulation of aggression in mammals. Domoic acid can damage these brain regions through the activation of […] receptors,” Robinson said.
Strutton said that he too has known domoic acid to change seal behavior, but that he has never seen it cause increased aggression.
“In 1998 I worked at the Monterey Bay Aquarium Research Institute in California,” he said. “That year there was a huge Pseudonitzschia bloom from California to Canada, and many seals/sea lions were affected by domoic acid. They were seen wandering the parking lot of the institute. The only treatment was to confine the animals in a recovery facility and let the toxin gradually depurate. I recall studies of seals that got better. [They] were released and tagged and they were unable to complete their usual migrations, likely due to brain damage. So it’s possible that domoic acid/ASP is causing strange behaviors in seals. I can’t say if it’s likely. I don’t recall any reports in 1998 of California seals being aggressive, mostly just disoriented.”
Whether the aggressive seals are being spurred on by domoic acid poisoning or not, they can be very dangerous to the humans they encounter, and will attack if threatened.
“Keep your distance from seals on the beach, if the animal does not feel threatened then there is no reason for it to respond aggressively,” Glasby said. “The primary trauma will result in puncture wounds, bruising and deep tissue trauma,” The secondary effect is very possibly bacterial infections. Seals have a very high bacteria load in their mouth and this can lead to a very bad infection. If bitten, seek medical attention.”
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