Sperm and bottlenose whales may have learnt to follow trawlers for easy meals

Sperm whales may specifically follow trawlers that are hauling in a favourite food.

Research onboard a fishing vessel off the coast of Newfoundland, Canada, found that whenever the ships targeted Greenland halibut, sperm and northern bottlenose whales would sometimes follow behind. Some whales followed the ship for over an hour as they ate fish escaping from the net, and would return to the boat on different days as it travelled.

When the ships targeted other fish species like redfish and thorny skate, however, the whales were nowhere to be seen, suggesting the animals are being selective in the meals they take.

Dr Usua Oyarbide, the study’s lead author, says, ‘It appears that sperm and northern bottlenose whales have learnt to identify the fishing vessels as a source of fish which are easier to catch.’

‘Halibut escaping from trawl nets are likely to be dazed, making them easier to catch and reducing the time whales need for foraging.’

Richard Sabin, the Museum’s Principal Curator of Mammals who was not involved in the study, adds, ‘While it’s been known for some time that orcas and other dolphins have become habituated to the presence of active trawlers, it’s interesting to see other cetacean species opportunistically targeting escaped fish, rather than those being disposed of over the side of boats.’

‘Papers like this demonstrate the complexity of cetacean behaviours and provide the groundwork for more research in the future.’

The findings of the study were published in the journal Plos One.

Whales and fisheries

As with many animals that live in the sea, whales and dolphins have a complex relationship with the fishing industry. At its most simplistic, the harvesting of fish from the sea leaves less for these animals, known collectively as cetaceans, to eat.

Deep-water trawling in particular is known to be damaging to the seafloor, and can harm fragile ecosystems that take decades to recover. The loss of animals like cold water corals can have knock-on impacts for fish nurseries, reducing the ability of fish stocks to recover.

The equipment also poses a risk. Entanglement in fishing nets can cause the animals a great deal of stress and in the worst cases can lead to drownings.

While the discovery that trawling can provide limited benefits to whales does not negate these issues, it demonstrates the animals’ ability to develop new feeding behaviours. Around 19 cetacean species have previously been seen scavenging the castoffs of trawlers, with some darting into trawl nets to take fish directly.

These records, however, tend to be of dolphins. While there have been sightings of whales interacting with trawlers before, these are much less common.

‘Sperm whales, as with all toothed cetaceans, are highly intelligent and socially structured so they are well placed to adapt to changing circumstances such as new prey opportunities,’ Richard says. ‘Toothed whales are also known to communicate these opportunities to each other, passing on information within and between groups.’

Signs of hope

The current study provides more concrete evidence of these feeding behaviours, with both sperm and northern bottlenose whales interacting with the ships.

Sperm whales were most commonly seen, and often swam with the boat while it was hauling in fish. While males are generally solitary, they clustered in groups of up to six around the vessel while it was trawling.

Usua took photos of the whales which approached the boat, and used a resource known as Flukebook to identify individuals. This uses pattern recognition and machine learning to recognise the unique shape and markings of the tails of individual whales.

Analysis of the photos suggest that some whales were returning day after day. One sperm whale reappeared over 230 kilometres away from where it was first seen, a distance more than twice the width of the English Channel.

‘The resighting of individual sperm whales in this study suggests they followed the trawler, and that some individuals may specialise in this associative behaviour,’ Usua explains. ‘Individual northern bottlenose whales were not seen more than once, and perhaps are less likely to specialise in this form of foraging.’

While they may not have recurred, the presence of northern bottlenose whales at all is a promising sign for their future, and reveals more about these elusive mammals.

‘Northern bottlenose whales are one of the least understood whales in the world, so any new information is useful,’ Richard says. ‘Not only do these observations provide new information on their behaviour and distribution, but they suggest numbers may be increasing after decades of exploitation from commercial whaling.’

The research may also have wider implications for whales in the region. While the northern bottlenose whale is considered to be Near Threatened overall, populations in Nova Scotia down the coast from Newfoundland, are classed as Endangered by the Canadian government.

The discovery of these interactions could feed into policy changes that ensure the conservation of the whales and fish stocks in this region.

‘When research projects such as this focus on interactions between cetaceans and humans in the field, the data gives us a much more finessed understanding of what’s going on in our world,’ Richard says. ‘It’ll be interesting to see how this information is received by the fishing industry, and how it informs future developments in fisheries management and marine conservation.’