The stranding of humpback whales on the Brazilian coast reached an atypical number in 2021. According to a survey by the Humpback Whale Project, until in December, there were 122 episodes, record in the historical series started in 1986.
The distant second place got 2002, with 122 strandings.
The growth in the number of strandings still does not have a precise explanation. But, according to Milton Marcondes, research coordinator of the Humpback Whale Project, the main hypothesis is the decrease in krill.
Krill (tiny crustaceans) is one of the main foods of humpbacks, which also eat small fish. Whales suck seawater and “filter” their prey.
In addition to the record in 2002, there was another unusual factor in the records. Almost 95% of the stranded animals were juveniles — animals from 1 to 5 years of age, when they reach sexual maturity.
“We’ve never had such a large percentage of a category,” says the specialist.
The stranding of thin juveniles is another clue, according to Marcondes, that the lack of krill may be responsible for the situation . This is because, in the event of a food shortage, more experience (ie more years of life at sea) can help to catch food. As a result, younger ones would find it more difficult in this scenario and would reach the Brazilian coast without having properly fed.
Adult animals also have a greater capacity to store fat, another advantage over juveniles.
Humpbacks (Megaptera novaeangliae) that swim along the Brazilian coast, in general, feed in the Antarctic waters of South Georgia and come to Brazil to reproduce and have little whales. By the way, the Abrolhos National Marine Park is known as an important nursery for these whales in the South Atlantic Ocean, which provides whale watching tourism in the area.
Another sign of the lack of krill are the whales approaching the shore in search of fish. São Paulo and Santa Catarina, states with large schools, are the leaders in stranding. There were records, says Marcondes, of animals entering fishing pens at sea looking for food and becoming trapped in nets.
Another record for the year was the —underestimated — number of animals trapped in fishing nets. : 58 (but not all the animals died). With fishermen and whales going after the same resource, accidents of this type end up becoming more likely.
Finally, another sign of the lack of krill came from far away from Brazil. Researchers from Namibia got in touch with the researcher from the Humpback Whale Project and stated that they also had higher-than-normal records of strandings and young individuals. from Namibia belong to another population that, in general, feeds in a different region (Bouvet Island and near Cape of Good Hope) from the feeding area of Brazilian whales.
The lack of krill can possibly be explained by the effect of the climate crisis on these crustaceans, which have a complex life cycle and an important role in the food chain.
Research published in the journal Nature Climate Change pointed out that the change in climate can shift the areas where krill develop further south and also trigger a mismatch between its life cycle and ocean conditions during the year.
There are records of associations, by example, from the low availability of krill to the drop in the reproductive success of fur seals, penguins and albatrosses and species that prey on these crustaceans.
“Each year, humpback whales migrate from the tropics to the poles to feed on the huge amount of krill from summer. If the krill peak occurs earlier, the whales must adapt by arriving earlier or they will end up hungry,” the authors of the research wrote on The Conversation website.
Despite the high numbers of strandings and accidents with fishing nets, humpbacks are not included in the latest ICMBio (Chico Mendes Institute for Biodiversity Conservation) list of threatened species.
According to the institute, the ban on hunting and conservation initiatives are essential for the species to remain out of risk.
Commercial whale fishing had a strong impact on populations of humpbacks in the world. A moratorium on whale hunting, instituted by the International Whaling Commission and in force since 1985/1986, has helped in the recovery of humpbacks, according to NOAA (National Oceanic and Atmospheric Administration, US agency).