All that noise
1 May 2017
The walrus is an emblematic species of the Arctic. They are also highly social, gregarious mammals that rely on vocal cues. But researcher ISABELLE CHARRIER has found increasing noise pollution is having an adverse effect on the pinnipeds.
WALRUSES CAN FORM herds of several hundreds of individuals. Even if they stay in groups all year round, the group size and composition varies between the two subspecies (Atlantic and Pacific walrus), and changes with the time of year and gender. For instance, in summer, Atlantic walruses form mixed groups of individuals whereas Pacific walruses form large sex-segregated herds.
Walrus produce vocalisations in air and under water in many social contexts including mother-calf interactions, adult-adult interactions, courtship display, predator or danger alert. With climate change, the extreme loss of sea ice gives more opportunities to develop maritime traffic which in general will increase human activity in the Arctic. The absence of ice during summer has lead walruses to come ashore to rest more often. This has been seen for several consecutive summers in Alaska, with the most striking example being the largest aggregation of Pacific walruses at Point Lay in September 2014 where 40,000 animals were observed, mainly females with calves. Walruses are very sensitive to aerial noise such as boat engines, but also noise from low flying aircrafts, so the risk of stampede induced by aerial noise is extremely high and results in a tremendous number of deaths, mostly young animals crushed by congeners running to the water.
If aerial noise clearly affects walrus behaviour and thus threatens the survival of the youngest animals, the same can be said of underwater noise due to human activities such as boat traffic, drilling, air guns, etc. This underwater noise is loud and composed of low frequencies that can propagate over long distances. During movements from foraging grounds to resting periods on ice in summer, walruses swim in groups and communicate by sounds (both in air and under water) to stay together. Mothers and calves recognize each other by voice, and the bond between the mother and her calf is among the strongest in mammals, persisting several years after weaning. Underwater noise can impair their vocal communication which can be particularly dramatic for calves if they are separated from their mothers and cannot reunite using vocal cues.
The amplitude level of traffic noise can be extremely loud, and it has been recently shown in Scotland that shipping noise levels are above those known to induce temporary hearing loss in harbour seals. Impact studies in Arctic waters are thus needed to evaluate the increasing risk of shipping traffic and human activities on walrus behaviour and physiology.
Auditory masking of their social vocalisations and temporary hearing loss due to human-activity noise can have a great impact on both walrus reproduction and survival rate. Finally, such impact studies will help in developing rules or laws to limit the level of human-made noise in Arctic waters especially in areas where walrus and any other marine mammals are known to come for foraging or breeding.
Pacific walruses are segregated by gender for much of the year. Adult females and young follow the ice edge as it recedes through the Chukchi Sea in summer and they return to the Bering Sea in winter, while most males stay in the Bering Sea year-round. XAVIER MOUY says studies are underway to assess the effects of industrial activities on walruses.
Human activity coupled with diminishing sea ice means walrus herds are being forced to look for new platforms for feeding, mating and resting. MADS PETER HEIDE-JØRGENSEN says walrus distributions around Greenland have changed alongside past and present human activities and climate change.