News release

From: Frontiers

Bottlenose dolphins in the Adriatic Sea seem to be heavily reliant on trawlers for food 

Up to 76% of bottom trawlers are followed by bottlenose dolphins scavenging in their wake, suggesting there isn’t enough prey to hunt elsewhere

Overfishing by trawlers, especially bottom trawlers which damage the seabed, can destroy ecosystems. Now a new study has shown that bottlenose dolphins in the northern Adriatic Sea, the only dolphin species that continues to live in this heavily overfished area, may have become dependent on the fish scooped up by these trawlers. The scientists suspect that this is because there aren’t enough fish left for the dolphins to hunt as they normally would.

Bottlenose dolphins in the Adriatic Sea are spending much of their time following trawlers to scavenge for food, scientists say. The Adriatic seabed has been ploughed by bottom trawlers for decades, resulting in ecosystem damage. Many apex predators are no longer present there. Only the bottlenose dolphins are left, and the frequency of their presence around fishing trawlers — up to 76% of the trawlers inspected by scientists off Marche, Italy, were followed by dolphins — suggests that they may be struggling to hunt normally.

“Long-term, consistent, and deliberate association with trawlers suggests a high degree of reliance on that fishery,” said Dr Giovanni Bearzi, the president of Dolphin Biology and Conservation, Italy, and the lead author of the article in Frontiers in Mammal Science. “While dolphins would still need to forage independently when trawling does not occur, on days of trawling they forage predominantly near trawl nets.”

Plenty of fish? 

The fish populations of the Mediterranean in general, and the Adriatic Sea in particular, are severely overexploited. Although we know dolphins can forage around trawlers, eating discarded fish or even sneaking into the moving net to snatch a meal, it’s not clear how reliant they are on the trawlers for food or what impact this could be having on their behavior and health.

To understand how bottlenose dolphins respond to trawlers, the scientists monitored trawlers offshore from Veneto and Marche in Italy. Between 2018 and 2024 they surveyed in Veneto, and in 2025 they surveyed in Marche, carrying out a total of 859 inspections of trawlers over 148 days. During this process, they photographed the dolphins to identify individual animals. They then analyzed this data, looking for differences in dolphin behavior between the two regions and across different types of trawler.

76% of otter trawlers in the Marche area and 26% of otter trawlers in the Veneto area were followed by dolphins. The scientists couldn’t account for this striking difference, but it’s unlikely to be caused by sampling bias, as they inspected more than 100 trawlers in Marche. On average, dolphins followed 41% of the otter trawlers and 35% of the midwater trawlers but only 1.5% of the beam trawlers. This difference may be because the nets used by beam trawlers in this area are harder for dolphins to access and don’t target suitable prey.

“We estimated that the bottlenose dolphin populations of Veneto and Marche, combined, exceed 1000 individuals,” said Silvia Bonizzoni of Dolphin Biology and Conservation, co-author of the article. “Between 86 and 90% of the dolphins, depending on region, were photographed one or more times while they were following trawlers. The evidence suggests that the majority of a relatively large community of dolphins regularly forage behind trawlers.”

Scavenge or starve 

Dolphins have always followed fishing boats. But this normal opportunistic behavior seems to have increased in intensity over time: a previous study in the Adriatic in the 1990s found that only 10% of trawlers were followed by dolphins. Many animals whose prey are depleted start pursuing food near humans, like polar bears scavenging in rubbish dumps. Trawlers could represent a simple source of nourishment in an impoverished ecosystem.

“It is known that bottlenose dolphins are occasionally injured or killed by trawl gear, and that foraging behind trawlers can affect dolphins’ diet, social organization, and communication,” said Dr Randall Reeves, the senior author of the study and the Chairman of the Committee of Scientific Advisors at the US Marine Mammal Commission. “Dolphins may also suffer hearing damage that results from chronic exposure to the noise of trawlers. It is risky behavior. 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.”

But the researchers emphasize that bottlenose dolphins are resilient. If trawling were reduced or even banned to give the ecosystem a chance to rebuild, dolphins would adapt and thrive as they used to before the age of trawling. Other species that long ago declined or disappeared in the region could also make a comeback, once the environment is protected from this destructive type of fishing.

“We advocate conservation action to protect a rich diversity of species,” said Bearzi. “As trawlers have been causing profound damage to Adriatic ecosystems, their continued use is inadvisable, not only to protect dolphins but also to conserve marine biodiversity more generally.”