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Why are we seeing an increase in aggressive behaviour b…

Seals regularly come out of the water to rest — often this will happen on beaches. They will usually need to rest for a day or two after a hunting trip, which can result in the seal spending three to four days out at sea. 

When a seal rests on a beach that is popular with human activities, the seal will be exposed to people and dogs, which both cause stress for the seal.

This has the potential, typically, of triggering the fight-or-flight response. We always advise people to keep their distance and allow the seal to return to the water, or if the seal fails to do so, to alert the appropriate authorities. 

Over the past few months, we have seen an increase in the number of incidents where seals react aggressively to people, including lunging in their direction or, in extreme cases, biting.

This situation is not within their normal behaviour.

Generally, when seals are resting out of the water, and they feel threatened or harassed, they will return to the water to escape. This is why their presence on crowded beaches has not normally been a problem in the past. The situation we are now seeing is that many seals are choosing to rather face the perceived danger aggressively.


The likely reason for this is a long-term side effect of domoic acid poisoning.

Domoic acid poisoning happens when seals eat prey such as fish and crayfish that have been living in or consuming red tide algae. Red tide algae are a natural phenomenon which periodically appears along our coastline.

As the domoic acid builds up quite quickly in the seal’s system, it causes them to be poisoned. Towards the end of 2021, we saw a mass poisoning event that resulted in numerous seals either dying or aborting their pregnancies. 

We saw the effects of this all along the Cape coastline.

Domoic acid poisoning causes swelling around the heart and brain, resulting in seals that survive the initial poisoning being left with long-term side effects, typically cardiac weakness and neurological disorders. 

The most notable effect that has been seen and documented internationally is heightened aggression, a neurological side effect caused by a swelling of the brain.

Fight response

It would appear that the neurological side effect of the poisoning is that it removes the animal’s need to escape when threatened. A normal fight-or-flight response to stress is no longer available and the animal resorts directly to a fight response. This change in behaviour is something the scientific community is still working to understand.

In combination with human activity in the vicinity of the seal, this has led to incidents where the seals bite people. The bite itself is very similar to a dog bite in terms of damage, with a greater risk of bacterial infections. 

Although not all bites result in infection, it is recommended that those who experience such a bite should seek medical attention as a cautionary action. 

In the long term, these affected seals will not survive for long as a direct correlation of the poisoning. Their body is weakened due to the side effects of the poisoning and if they face every perceived danger aggressively, they will likely be killed by larger predators or from being struck by boat propellers when they charge and attack moving vessels. 

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We do not know how many seals on our coastline are affected and therefore advise that, during this time, people keep their distance from any wild seals. 

While this is advice-specific to this situation, we follow this thought process in circumstances where humans find themselves in close proximity to any wild animal. My work with wildlife management with the Two Oceans Aquarium Foundation is based on this attitude towards shared human and animal environments. 

These animals are wild and should be respected for the risk they pose to people. If in doubt regarding a seal’s behaviour, it is always best to contact any of the seal rescue groups to assess the animal. DM

Bretty Glasby is the coordinator of the Marine Wildlife Management Programme for the Two Oceans Aquarium Foundation.




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