Bottlenose dolphins in the Adriatic Sea are increasingly relying on trawlers for food, with baby dolphins learning the scavenging technique from their parents, according to a new study published in Frontiers. The research highlights how overfishing has depleted natural prey, forcing dolphins to follow fishing boats to feed on discards and unwanted organisms.
Study finds high levels of dolphin-trawler association
The scientists monitored trawlers offshore in the Adriatic for 148 days, conducting 859 inspections across two regions and different types of trawlers. In one area, 76% of fishing boats were followed by dolphins, a significant increase from a 1990s study that found only 10% of trawlers were followed.
Giovanni Bearzi, co-author of the study and president of Dolphin Biology and Conservation in Italy, said: “These days the easiest way to find [bottlenose dolphins] is to look for trawlers. Many of them are followed by the dolphins that go to forage and scavenge in their wake. When I say scavenge, I mean that they also feed on discards and unwanted organisms. Dolphins are following the trawlers because natural prey is very hard to find. Why is it hard to find? It’s overfished.”
Overfishing in the Mediterranean
The Mediterranean Sea has one of the highest rates of fishing worldwide, with fishing pressure twice the level considered sustainable by the UN Food and Agriculture Organization. Bearzi noted that the Adriatic ecosystem once included many more common dolphins, but they have virtually vanished except for a few hotspots. “One species has gone and that is not a good sign. It’s a warning sign,” he said.
Risks and cultural transmission
While following trawlers provides a simple food source, it comes with risks. Dr Randall Reeves, study co-author and chair of the Committee of Scientific Advisors at the US Marine Mammal Commission, said: “It is known that bottlenose dolphins are occasionally injured or killed by trawl gear, and that foraging behind trawlers can affect dolphins’ diet, social organisation and communication. Dolphins may also suffer hearing damage that results from chronic exposure to the noise of trawlers. However, finding sufficient prey away from trawlers in an overfished sea may be too difficult. It appears that for these animals, taking the risks is better than going hungry.”
Bearzi added: “The baby dolphins follow the trawlers with their mothers, watch the adults foraging and learn that kind of behaviour. It is transmitted culturally.”
Call for action
The scientists are calling for action to protect marine biodiversity and advise against continued use of trawlers. Bearzi said: “When I was younger, I just wanted to study the behaviour of dolphins. I wouldn’t even think about human encroachment back then. But what we saw [in past research] is entire communities of dolphins disappearing. If you stop destructive fishing, the marine resources would bloom very quickly. Dolphins would be able to feed again on their natural prey, as they did for centuries in this same area. Fish less. Fish in less destructive ways. This is really the message.”
