Birds with a taste for meat threaten whale calves
Thousands of colossal southern right whales travel to the calm waters of Península Valdés off the coast of Argentina each year to breed and give birth. The cetaceans, which can reach 17 meters in length, are a sight to behold, especially with their calves in tow. But if you venture out to see them, your stomach may sometimes turn for a reason that has nothing to do with seasickness.
For the past 50 years, Peninsula Valdés kelp gulls have relentlessly pecked at any southern right whale that dares to swim to the surface to breathe. The birds gorge themselves with skin and blubber torn from the backs of the whales. In recent decades, the problem has escalated and is now so severe that it is causing young southern right whale calves to die prematurely, according to a study published Wednesday in the journal Biology Letters.
While kelp gulls and other seabirds are known to occasionally steal meat (and even eyeballs) of marine mammals, the study found that the number of southern right whale calves dying before their first birthday has increased in recent decades, as has the frequency and severity of the wounds the gulls inflict on them.
“It’s really sad to see,” said Macarena Agrelo, a marine ecologist at Brazil’s Federal University of Santa Catarina and author of the study.
Although southern right whales and kelp gulls have long lived side by side, their relationship took a bizarre turn in the 1970s. Until then, the birds seemed to be content with the skin sheets that the whales naturally shed on a regular basis. Somehow the birds realized they could get more satisfying bits by going straight to the source. And since then, the birds have passed on this knowledge from generation to generation.
“The attacks are very painful and cause large, deep lesions, especially on the backs of young calves,” said Mariano Sironi, scientific director of the Instituto de Conservación de Ballenas in Argentina and co-author of the study. While some cocks are small, he said, “in the most extreme cases, the largest wounds can cover much of the back of the calves and can be three feet long or even larger.”
At first, the gulls attacked both calves and adults, but over time the adults changed the way they surfaced for air, arching their backs so that only their heads left the water. Young whales cannot do this.
Kelp gulls’ constant attacks not only cause young southern right whale calves to suffer painful injuries, but also hinder their ability to rest and suckle. That, combined with other stressors, causes young southern right whales to die prematurely.
After analyzing thousands of documented sightings and aerial photographs collected between 1970 and 2017, the researchers found that the number of juvenile southern right whale injuries in Península Valdés has increased about tenfold over the past two decades. During that same period, they have linked a decline in calf survival to the serious injuries inflicted by the birds.
“The fact that gull harassment has population-level implications for these whales is quite surprising,” said Matthew Leslie, a conservation biologist with the U.S. Geological Survey who was not involved in the study.
The southern right whale was on the verge of extinction and has recovered since hunting for it was banned in 1935. But as is the case for nearly all whales today, that recovery is threatened by dwindling food resources, frequent entanglements in fishing gear and ship strikes. .
“For these whales, it’s death by 1,000 cuts,” said Dr. Leslie, “and these seagulls add another cut.”
The scientists behind the study claim that humans are partly responsible for the plight of the Patagonian whales, pointing to poorly managed landfills and the waste generated by fishing fleets, which are increasing the population of kelp gulls.
“By providing scientific evidence that gull strikes have an impact on whale survival, we hope people can change attitudes and become more involved in improving waste management,” said Dr. Sironi.