The waters around Argentina’s Valdes Peninsula are thebreeding and calving grounds for southern right whales (Eubalaena australis). These whales have become targets for the kelp gull (Larus dominicanus). Birds tear pieces of skin measuring several centimetres from the backs of the whales, causing wounds that are prone to infection. Mothers and calves are the main targets. These attacks have increased over the past 30 years, with the proportion of individuals injured having gone from 2 to 99%. Although females have developed new avoidance behaviours, newborns are still highly vulnerable. This harassment has even played a role, amongst other factors, in the significant calf mortality rate observed in this region since the 2000s.
A 40-year study
Southern right whales gather from June to December off the Valdes Peninsula in Argentina to mate and calve. Since the 1970s, these 15-metre long giants have become the target of kelp gulls inhabiting the region. Mothers and their young are the main targets. The birds peck at the skin and blubber on the whales’ backs. Repeated attacks lead to enlarged wounds prone to infection. Infection can lead to dehydration and thermoregulation problems. The whales also spend more energy trying to escape the birds rather than resting or caring for young while they are at a crucial period of their lives, completely dependent on their mothers for survival.
A team of researchers led by Carina Marón of the University of Utah was curious as to how this harassment has evolved over time. Thanks to aerial photographs taken between 1974 and 2011, the team noted changes in the presence, number and size of lesions on over 200 whales, mothers and newborns. They also studied the photos of 192 calves found dead between 2003 and 2011. The results of this study were published in October 2015 in the journal PLOS ONE.
In the 1970s, only 2% of individuals showed lesions caused by gulls. Forty years later, almost all individuals (99%) do. In the 1980s-90s, the number of lesions appeared to be comparable in adults and calves (between 1-5 lesions/individual). Since 2000, this number has become much higher in young whales (at least 9 lesions/individual). Females have since developed new surface behaviours to avoid attacks such as arching their back, leaving only their head and tail out of the water. Young, being less experienced, smaller and more often at the surface to breathe, remain highly vulnerable to bird attacks.
An additional stress factor, but which does not explain everything
According to the researchers’ findings, this harassment has some responsibility in the calf mortality observed in this region. Over 600 young southern right whales were found dead between 2003 and 2014. The causes behind this great mortality remain unknown, but some factors have already been raised such as the lack of food for females, toxic poisoning, an infectious disease, etc.
Birds attacking animals larger than themselves
The kelp gull is a sea bird that inhabits islands and coasts of the Southern Hemisphere. It has a very varied diet: mollusks, fish, reptiles, amphibians and other bird species and their eggs. Southern right whales are not the only marine mammal to be targeted by these birds. Over the past 15 years, scientists have identified nearly 500 kelp gull attacks on young South African fur seals (Arctocephalus pusillus pusillus) off of Namibia’s Dorob National Park. Their strategy is to target mainly the eyes of newborns. Blind, they are no longer able to find their mothers and become powerless to face future attacks. This “new” kelp gull strategy is believed to be attributable to the increase of fur seal populations in the region.