Thousands of colossal southern right whales travel to the calm waters of Península Valdés off the coast of Argentina every year to breed and give birth. The cetaceans, which can reach 56 feet in length, are a sight to behold, especially with their calves on the back. But if you head out to take a look, sometimes your stomach turns over for reasons that have nothing to do with seasickness.
For the past 50 years, sea grass gulls on the Peninsula Valdés have mercilessly pecked any southern right whale that dared to surface to breathe. Birds eat the skin and blubber ripped from the whale’s back. Over the past decades, the problem has intensified, and is now so severe that it is causing young southern calves to die prematurely, according to a study published Wednesday in the journal Biology Letter.
While kelp gulls and other seabirds have been known to occasionally steal meat (and even eyeball) of marine mammals, the study found that the number of southern right whale calves that die before their first birthday has increased in recent decades, as has the frequency and severity of injuries inflicted by seagulls.
“It’s really sad to see,” said Macarena Agrelo, a marine ecologist at the Federal University of Santa Catarina in Brazil and the study’s author.
Although southern right whales and sea grass gulls have long coexisted, their relationship took an odd turn in the 1970s. Until then, the birds had seemed content to eat the strips of skin that whales naturally shed. Somehow the birds realized they could get a more satisfying morsel by going straight to the source. And since then, birds have passed this knowledge from generation to generation.
“The strikes were very painful and caused large, deep injuries, especially to the back of the young calf,” said Mariano Sironi, scientific director of the Instituto de Conservación de Ballenas in Argentina and one of the study’s authors. While some bites are minor, he says, “in the most extreme cases, the largest wounds can cover a large part of the back of the calf and can be a meter or even larger in length.”
At first, the seagulls attack the calves and adults, but over time the adults have changed the way they surface for air, arching their backs so that only their heads leave the water. The young pope cannot do this.
Constant attacks by sea grass gulls not only cause young southern right whale calves to suffer painful injuries but also interfere with their ability to rest and nurse. That, combined with other stressors, causes young southern right whales to die prematurely.
After analyzing thousands of documented sightings and aerial photographs collected from 1970 to 2017, researchers found that the number of injuries suffered by juvenile southern right whales at Península Valdés has increased approximately tenfold in the last two decades. Over the same time period, they attributed reduced calf survival to the severe injuries inflicted by the birds.
“The fact that gull harassment is causing population-level impacts on these whales is quite surprising,” said Matthew Leslie, a conservation biologist with the US Geological Survey who was not involved in the study.
Once on the verge of extinction, the southern right whale has recovered since hunting was banned in 1935. But, as is the case for almost all whales today, that recovery is threatened by dwindling food sources, regular attachment to fishing gear and ship strikes. .
“For this whale, the death is 1,000 injuries,” said Dr. Leslie, “and this seagull added another wound.”
The scientists behind the research argue that humans are partly to blame for the plight of Patagonia whales, pointing to poorly managed landfills and waste generated by fishing fleets, which increase populations of sea weed gulls.
“By providing scientific evidence that seagull attacks have an impact on the survival of whales, we hope that society can change attitudes and be more involved in improving waste management,” said Dr. Sironi.