Southern right whale calf wounding by Kelp Gulls increased to nearly all over four decades

Over 600 southern right whale calves died at the Península Valdés calving ground, Argentina, between 2003 and 2014. This is a vastly larger number than seen over any similar period and in any other right whale calving ground. Kelp Gull harassment -- they feed on skin and blubber pecked from the backs of living whales -- has also increased in recent years, implicating the wounding as a potential contributing cause of the increased mortality.

Mother-calf pairs are the primary targets for Kelp Gull attacks and pairs attacked by gulls appear to spend less time nursing, resting, and playing than pairs not under attack. An increase in wounds could potentially lead to dehydration, impaired thermoregulation, and energy loss due to wound healing. Using aerial survey photographs, the authors of this study assessed the presence, number, and total area of gull-inflicted lesions on 2680 living mother-calf pairs in 1974-2011 and ~200 stranding photographs of dead calves in 2003-2011 around Península Valdés, Argentina.

The authors observed the percentage of mothers and calves with gull lesions increase from 2% in the 1970s to 99% in the 2000s. Additionally, in the 1980s and '90s, mothers and calves had a similar numbers of lesions, but by the 2000s, calves had more lesions covering a greater area of their backs compared to their mothers. The number and area of lesions increased during the calving season with older calves having more lesions than younger ones.

"It is tempting to look at the correlation in time and think the gull-inflicted wounds must be a contributing factor to calf deaths, but despite a lot of work we still don't have convincing evidence for any plausible mechanism," says Dr. Marón. "One possibility is that increased stress is making young calves more vulnerable to a variety of other factors. We like that idea, but it won't be easy to prove."