Whales off Argentina face unlikely enemy in voracious seagulls

Whales have had to endure plenty over the years, from harpooning to ship strikes to pollution. But for southern right whales off Peninsula Valdes in Argentina, there’s a new and increasingly serious threat: attacking seagulls, which are preying on the gargantuan mammals.

If this sounds a bit like an Alfred Hitchcock tale, it is.

Hungry gull takes bite from the back of a southern right whale off Peninsula Valdes, Argentina. Credit: Mariano Sironi / Instituto de Conservacion de Ballenas

When the cetaceans arrive for the breeding season between June and September, the scavenging birds swarm out and land on the backs of surfacing whales, hurriedly tearing away skin and blubber, leaving wounds that can measure eight inches across.

Scientists are concerned because attacks are altering the whales’ behavior; they may be jeopardizing calf survival, and could be facilitating the transmission of infectious disease.

This is the only location in the world where the phenomenon is known to occur. The BBC and Discover magazine report that gull attacks on southern right whales were first documented in 1972, and for a couple of years they affected about 1% of the whale population.

But today about 78% of Peninsula Valdes whales carry wounds inflicted by gulls. The increase probably stems from a population explosion among the gulls, perhaps because of an abundant food source in nearby landfills and discarded refuse from the fishing fleet (as the gull population increases, so does the fierce competition for food).

Scientists have found that some gulls are more problematic than others, but are concerned that attacking whales is becoming instinctive among new generations.

Biologist Mariano Sironi of the Institutio de Conservacion de Ballenas states on the Ocean Alliance website: “They alter the whales’ normal behavior, interfere with lactation, reduce resting time, and increase the whales’ swimming speeds.”

Sironi was the lead author of a study that determined the gulls prefer to prey on mother-calf pairs, focusing largely on the babies. The gulls are less likely to go after juvenile whales unless the juveniles are interacting with mother-calf pairs.

Biologist Ana Fazio of the Argentinian group CONICET confirmed this in a separate study, in which she voiced concerns about gulls transmitting bacteria from rubbish dumps into whales. Some whales have developed unusual skin patterns caused by lesions.

It’s possible, but not yet determined, that calf survival is being jeopardized by attacking gulls. There has been unusually high calf mortality in recent years, but reasons are unknown.

The whales, which are not endangered, have learned how to reduce the frequency of attacks “by changing their resting posture at the surface, arching their backs to keep them underwater,” Sironi’s study reveals.

But real help may have come in the form of a government management plan that includes killing problematic birds and closing rubbish dumps to reduce the amount of waste and, hopefully, shrink the gull population.

That process will take time, however. Meanwhile, the whales will simply have to endure.

— Images are courtesy of Mario Sironi, via the BBC