Dolphin suspected of increasing porpoise fatalities in Lillebælt

During the summer, Line Schock Jepsen takes tourists on whale-watching tours in Lillebælt multiple times a day. While she usually knows just where to navigate her boat, the ‘Mira III’, to spot porpoises, the recent arrival of a dolphin nicknamed ‘Skywalker’ has complicated matters. “When the dolphin is around, porpoises tend to retreat,” she explains.

The dolphin, several times larger than a porpoise, is suspected of causing an increasing number of porpoise fatalities found washed ashore in the area. Bjarne Christensen, a nature guide from Fredericia Municipality, has evidence that ‘Skywalker’ has killed at least one porpoise. New documentation also indicates another dolphin in Svendborgsund is responsible for at least two porpoise deaths.

Researchers now plan to listen underwater over the next few months to detect whether porpoises swim away when ‘Skywalker’ is nearby. They have installed microphones to capture the sounds of porpoises and dolphins. “The idea is to see if porpoise sounds diminish when dolphin sounds emerge,” says Magnus Wahlberg, a researcher at SDU.

In some cases, both species can coexist, but the theory is that ‘Skywalker’ is isolated, lacking playmates or potential mates, which leads to aggressive interactions with the more vulnerable porpoises. “Some porpoises do get beaten by the dolphin,” Wahlberg adds, though he reassures that there’s no immediate need to worry about porpoises disappearing from Lillebælt.

Onboard the ‘Mira III’, Jepsen has witnessed ‘Skywalker’ after porpoises, recalling an instance when she mistook one dolphin for another due to their size difference. In a short period, ten dead porpoises have washed ashore near Fredericia, with ‘Skywalker’ suspected of being involved. While she acknowledges this is a natural occurrence, it saddens her as she has grown fond of the individual animals she frequently encounters during tours. “We are deeply upset when we see a dead porpoise,” she laments.