Aggressive dolphins attack porpoises in Danish waters

Svendborgsund is usually a safe and popular place for porpoises to live. But in recent years, kayakers and other water visitors have reported bloody attacks by a dolphin on porpoises.

More precisely: A dolphin has "played" so violently with a porpoise that the porpoise died, after which the dolphin, to the horror of onlookers, continued tossing around the dead body.

In a new scientific article, biologists from SDU, Fjord&Bælt, and DTU report that at least two porpoises have died in this way in Svendborgsund (in August 2020 and 2022), but according to local Facebook groups, there may be more, and similar incidents have been observed in other parts of the country.

Normal behaviour for solitary males

- This is normal behaviour for solitary males. The concern is more about whether the Danish porpoise populations can withstand this pressure from the dolphins. Right now, only a few dolphins live in Danish waters, and they are not enough to pose a threat to the porpoise populations, but the situation could change if more dolphins come to Denmark, says Olga Filatova, a marine biologist and whale expert at the Department of Biology.

Ecologically and biologically, nothing prevents more dolphins from living in Denmark. They have lived here before but were driven out by human activities. Now, fishing is more dolphin friendly, and the climate is getting warmer, so more dolphins in Danish waters is a realistic scenario.

Olga Filatova and her colleagues are therefore closely monitoring the movements and behaviour of the dolphins in Danish waters, as well as trying to observe how porpoises react to them. They are particularly focused on the young male, Skywalker, who is currently residing in the Lillebælt. He is the younger brother of Delle, who until April 2023 was in Svendborgsund but has now moved on to Travemünde in the northern Germany. Both Skywalker and Delle are known for hunting, maiming and killing porpoises.

Underwater microphones at work

- We are interested in learning more about how dolphins affect porpoises, and Skywalker will help us with that, just as Delle did when he was in Denmark, says Olga Filatova.

In their research, Filatova and her colleagues use underwater microphones. These are lowered to about 10 meters depth, and their job is to record all the sounds in the water. Since Skywalker is currently in Lillebælt, the microphones are now being installed in Lillebælt.

- When we listen to the recordings, we can tell if both Skywalker and porpoises are present at the same time near the microphone, or if the porpoises only come when Skywalker is away from the area. It could also be that the porpoises completely avoid the area when Skywalker appears. When we had the microphones in Svendborgsund with Delle, we could mostly hear his sounds. The absence of porpoise sounds suggested that they were trying to avoid him. Now we will see what happens around Skywalker, says Olga Filatova.

Porpoises withdraw but come back

In previous studies, researchers have seen that dolphins and porpoises find a way to share an area by each using the area at different times. Other times, the researchers have observed how porpoises stay completely away.

It is unclear whether porpoises have an instinctive fear of dolphins and their sounds, or if they learn that dolphins can be dangerous when they encounter one.

- But we can certainly see that porpoises often withdraw when a dolphin enters their area, and that they stay away as long as the dolphin is in the area. But we also see that they return to the area quite quickly after the dolphin leaves. We saw this in Svendborgsund, where porpoises returned a few days after Delle left the area, says Olga Filatova.

Not all males have this behaviour

According to the researchers, it would not take many dolphins in Danish waters before they could potentially impact the porpoise populations. Even a small number of top predators – like dolphins – can be enough to cause major changes in the local environments.

For example, researchers have seen this in South Africa, where two male killer whales in 2017 began hunting sharks and ended up displacing them from assembling spots along the Gansbaai coast. The pressure from the killer whales led to fewer sharks hunting seals, which in turn led to an increase in the number of seals to hunt the protected African penguins (Spheniscus demersus) living in the areas.

According to Olga Filatova, aggressive behaviour is common among male dolphins and has been observed in several places around the world. But that does not mean that all male dolphins exhibit this behaviour.

Skywalker’s play with the underwater microphone

- We see this in males during the mating season. But when winter comes, and the mating season is over, they calm down. We do not know why they do it. It may have something to do with the fact that there are no females around to interact with, she says.

She also mentions that not only porpoises get a beating when Skywalker is in a "playful" mood. One of the researchers' underwater microphones was also “attacked”.

- Our microphones are well anchored when we set them out. They are connected by ropes to both a heavy anchor and smaller side anchors, and one day when we went to retrieve one of our microphones, we could not find it. We found it two days later, completely entangled in ropes and anchors. At first, we thought there had been an accident, that a fishing boat had crashed into it, but then we listened to the recordings: There were hours of recordings where we could hear Skywalker dragging the ropes around and tangling everything into a big mess. It sounded like he perceived it as a toy, says Olga Filatova.