The climate crisis has forced many animals to change their ways of life, and bowhead whales (the only baleen whale species native to the Arctic) are no exception. These whales typically undertake a 6,000-kilometre migration during the winter, but a few years ago they chose to stay put — and according to new research, climate change may be to blame.
The research included contributions from the Wildlife Conservation Society Canada and the University of Victoria, and was published in Royal Society Open Science.
Strange behaviour from Arctic whales
Canada’s bowhead whale population has a well-known annual migration pattern. They spend their winters in the Bering Sea, which has large areas of open water, and return to Canada’s Beaufort Sea in the spring.
This migration pattern is dictated by sea-ice coverage in the Arctic: whales need to come up to the surface to breathe, but large areas of the Beaufort Sea freeze over during the Arctic winters. This means that until recently, bowhead whale populations have had to journey south to warmer, ice-free waters for survival.
Throughout the 2018-2019 season, however, residents of the Canadian Arctic reported bowhead whale sightings well into the winter months. These unusual sightings prompted a team of scientists to investigate the whale population in more detail, with the hopes of learning what could cause this migratory species to overwinter in the Canadian Arctic.
To do this, the team made use of audio data from underwater tape recorders around the region. They searched for sounds matching the typical frequencies of bowhead whale vocalizations, and compared the 2018-2019 recordings with those taken around the same time in previous years.
The 2018-2019 recordings contained clear evidence of the whales’ presence throughout the winter. This was a stark contrast to previous years, where bowhead whale sounds dwindled off as the population journeyed south for the winter.
“The evidence is clear that […] bowheads overwintered in their summer foraging region in the eastern Beaufort Sea,” the authors said in their paper. “[A]s far as we know, this is the first time it has been reported.”
Climate change is a likely culprit
While there were likely a number of factors at play in the whales’ decision to overwinter, the authors suspect that most of them have to do with climate change.
For example, lower levels of sea-ice coverage in the Canadian Arctic could mean easier access to the surface. This might have prompted the whales to stay in the Beaufort Sea, rather than expending large amounts of energy to journey to their usual winter homes.
The authors went on to investigate how sea-ice levels had changed in the region over time. They found that while the 2015-2016 season had the lowest levels on record, 2018-2019 was a close second, potentially explaining the whales’ behaviour.
Rising water temperatures could also be to blame. Bowhead whales have the thickest blubber of any whale species, and prefer water temperatures between -0.5 and 2 degrees Celsius to avoid overheating. Water temperatures further south may have risen above these levels and become uncomfortably warm for this Arctic species.
The researchers aren’t sure if this behaviour was a one-time occurrence, or if it marks the beginning of a new way of life. Future studies of data from the past two winter seasons will help the team understand whether 2018-2019 was a turning point for the whale population.
Going forward, the authors also plan to expand their underwater recording network to more locations and include measurements of ocean temperatures in their studies. This will help them understand the whales’ strange behaviour and pinpoint the greatest threats faced by this species.
“If the avoidance of warm ocean temperatures were the primary driver of this anomalous behaviour,” the authors said, “it may be a significant warning sign for bowhead whales.”