Kelp gulls have learned to feed on the skin and blubber that they peck from the backs of living southern right whales at Península Valdés.
“Gull attacks are very painful to the whales and repeated attacks to the same area cause large, deep lesions, particularly on the backs of young calves,” says Dr. Mariano Sironi, the Scientific Director of the Instituto de Conservación de Ballenas in Argentina. Whales try to escape from the gulls, but the attacks last for hours. Such attacks interfere with the time that young whales could be resting and nursing.
It’s possible that losing significant portions of skin due to gull attacks leads to dehydration and an inability to maintain a constant body temperature, or that attacks may have other physiological effects that are not yet understood. “As a result, right whale mothers and their calves are expending much precious energy during a time of year when mothers are fasting and at a site where little to no food is available to replenish fat reserves,” says Dr. Sironi.
Older whales learn to avoid gulls by keeping their backs under water, but it takes time for calves to learn this behavior. “Young calves come to the surface to breathe every 20 seconds,” says Dr. Vicky Rowntree, who is Co-Director of the Southern Right Whale Health Monitoring Program and has researched southern right whales for 37 years with Ocean Alliance. “A gull can attack a calf each time it surfaces.” The first three months of a whale’s life are a critical time for calf development and growth. According to Dr. Rowntree, harassment during this time stresses the calves, which could lead to reduced fitness or death.
Península Valdés is the only place where gull attacks occur and they have been studied in the area since the 1980s. A combination of causes may be responsible for the recent rise in the number of southern right whale deaths. Since evidence for the other hypotheses is limited, researchers are looking to gull harassment as one of the contributors to whale mortality.
The kelp gull population near Península Valdés has grown dramatically in recent decades, in part because of food available at landfills and other sources of human waste. - By Jasjeet Dhanota