Bottlenose dolphins (Tursiops truncatus) in the northern Adriatic Sea now spend a large share of their feeding time trailing trawlers. Rather than catching prey themselves, they eat fish disturbed by the nets.
A recent study found that dolphins followed more than three in every four trawlers examined by researchers along one section of the Italian coast.
This behaviour indicates that the fish these dolphins would ordinarily pursue may have become so limited that following fishing vessels offers an easier source of food.
It also reveals a population exceeding a thousand animals whose everyday lives have become tied to one of the most damaging forms of fishing.
Dolphins trail fishing vessels
Over eight years, a small research team navigated survey boats through fishing areas off two parts of Italy’s Adriatic coastline: Veneto in the north and Marche, roughly 145 kilometres farther south.
They approached every active trawler to determine whether dolphins were swimming in its wake. Altogether, the researchers travelled more than 17,700 kilometres and assessed 859 vessels during 148 fishing days.
Marine biologist Giovanni Bearzi, president of Dolphin Biology and Conservation, a northern Italian research organisation, led the project.
Across both regions, dolphins followed about one-quarter of the vessels. That overall proportion, however, concealed just how widespread the practice was among local dolphins.
Local dolphin population follows trawlers
Images of individual animals, identified through the notches and scars on their dorsal fins, indicated that the behaviour was close to universal.
In each region, between 86 and 90 percent of familiar adult dolphins were photographed feeding behind a trawler on at least one occasion.
“We estimated that the bottlenose dolphin populations of Veneto and Marche, combined, exceed 1000 individuals,” said Silvia Bonizzoni, a co-author of the study.
The photographs suggest that most of this community incorporates feeding around nets into its regular routine.
For the researchers, an entire community feeding in this manner revealed something about the condition of the sea itself.
“When one sees a majority of wild predators resort to opportunistic foraging and scavenging, one can be quite certain that human encroachment has reached a tipping point,” Bearzi told Earth.com.
A question of fishing gear
Whether dolphins were found behind a vessel depended almost entirely on the type of gear it towed.
Dolphins were present behind beam trawlers, which pull heavy metal-framed nets over the seabed, on only 1.5 percent of occasions.
The other two gear types produced markedly different results. Dolphins followed 41 percent of otter trawlers – vessels towing a single broad-mouthed net along the seafloor.
They also trailed 35 percent of midwater trawlers, which operate in pairs and pull a large net through open water.
The difference lies in the nets themselves. Beam trawls collect scallops and flatfish, prey that dolphins seldom target. The wider nets used by the other vessels, by contrast, catch the fish species dolphins do eat.
Fish that lag behind or are stunned are scattered across a broad wake behind these boats, creating easy prey for dolphins.
Trawler size also matters to dolphins
Larger vessels attracted more dolphins than smaller ones, and the animals appeared to actively seek them out.
Previous research in the same waters showed dolphins detecting a trawler from more than 2.4 kilometres away before swimming directly towards it.
One finding surprised the team. In Veneto, dolphins were seen behind around one-quarter of otter trawlers.
In Marche, just 145 kilometres away, they followed more than three-quarters. The researchers could not account for this threefold difference.
Environment damaged by overfishing
The Adriatic Sea gives dolphins compelling reason to take this shortcut. Decades of overfishing have made it the Mediterranean’s most depleted area and one of the world’s most intensively trawled seas.
Years of bottom trawling – in which weighted nets are dragged over the seafloor – have levelled habitats that were once complex and stripped life from the seabed. It ranks among the most severely damaged seafloors on record.
Most large predators have disappeared. Shark and ray populations declined by more than 90 percent during the second half of the twentieth century.
The common dolphin (Delphinus delphis), formerly abundant in these waters, has almost entirely disappeared.
Scavenge or starve
Bearzi considers these dolphins part of a broader pattern. Large predators deprived of their normal prey turn instead to whatever humans leave accessible.
“Polar bears become reliant on food around human settlements, and often scavenge in garbage dumps,” Bearzi noted in an e-mail to Earth.com.
Over time, that ecological collapse appears to have driven surviving dolphins towards the fishing fleet.
A survey of the same waters in the early 1990s recorded dolphins behind only about 10 percent of the otter trawlers inspected. That was a small proportion compared with the current rate.
A dangerous feeding habit
Given today’s depleted prey stocks, fishing nets provide a reliable meal – but they are not without danger.
Dolphins can become entangled and die in trawl gear, while continual engine noise may harm their hearing.
“It appears that for these animals, taking the risks is better than going hungry,” said Randall Reeves, the study’s senior author and chair of the scientific advisory committee to the U.S. Marine Mammal Commission.
This feeding strategy may also alter what dolphins consume, as well as the ways they travel and communicate.
Caught in an evolutionary trap
The behaviour is being passed between generations. Across multiple groups, calves and even newborn dolphins were observed diving over the towed nets with adults. In some cases, younger animals remained slightly farther from the vessels.
Young dolphins seem to acquire the practice by observing their mothers, allowing it to spread and become an established way of life.
Biologists describe this as an evolutionary trap – a behaviour that brings benefits now but could leave the animals vulnerable if the nets were ever removed.
Depending on a fishery is not necessarily harmful in every case. A study of killer whales in the Strait of Gibraltar found that those feeding around a tuna fishery survived and reproduced more successfully than animals that did not.
For Adriatic dolphins, however, any additional food could be offset by the broader damage caused by the same vessels.
Trawlers disrupt the seabed and reduce the prey dolphins must depend on whenever nets are not operating.
Dolphins need protection from trawlers
The researchers maintain that the solution is not to manage the dolphins, but to reduce trawling. These animals are adaptable, after all.
The team expects dolphins would resume hunting independently as the sea recovered if trawling were reduced or stopped in certain areas.
There is evidence that such a recovery can happen. After trawling was substantially cut in one Australian bay, a study found that local dolphins rebuilt their fragmented social groups within a decade.
Achieving this has been difficult. A 2023 European proposal to phase out bottom trawling within marine protected areas encountered substantial opposition from fishing lobbies.
The method remains authorised across much of the sea, including in locations intended to receive protection.
Protection could bring other benefits
The study shows that an entire community among the Adriatic’s remaining dolphins has reorganised itself around trawler fleets, at a scale not previously documented.
It remains unclear whether this leaves them better nourished or subtly worse off, and the outcome depends on decisions about how the sea is fished.
In Bearzi’s view, protection could restore something to the Adriatic that it has already lost.
“The benefits of wise marine conservation measures might even include the return of the common dolphin,” he told Earth.com.
Comments
No comments yet. Be the first to comment!
Leave a Comment