US government scientists have launched an investigation what has caused the deaths of an unusually high number of gray whales found washed up on the west coast of North America.
About 70 whales have been found dead so far this year on the coasts of California, Oregon, Washington and Alaska, the most since 2000. About five more have been discovered on British Columbia beaches.
Scientists believe that is a very small fraction of the total number of whales that have died because most simply sink and others wash up in such remote areas they are not recorded.
The National Oceanic and Atmospheric Administration fisheries agency on Friday declared the deaths an “unusual mortality event”, providing additional resources to respond to the deaths and triggering the investigation.
“Many of the whales have been skinny and malnourished, and that suggests they may not have gotten enough to eat during their last feeding season in the Arctic,” agency spokesman Michael Milstein told reporters.
The eastern North Pacific gray whales were removed from the endangered species list in 1994 after numbers recovered from the whaling era.
The population has grown significantly in the last decade and is now estimated at 27,000, the highest since surveys began in 1967. That has raised questions about whether their population has reached the limit of what the environment can sustain. Another theory suggests that the loss of Arctic sea ice is a culprit.
The whales spend their summers feeding in the Arctic before migrating 10,000 miles (16,000 km) to winter off Mexico. Though they eat all along their route, they are typically thinning by the time they return north along the west coast each spring.
They eat many things, but especially amphipods, tiny shrimp-like creatures that live in sediment on the ocean floor in the Arctic.
For many years, researchers noted that fewer calves tended to be born following years when the ice in the Chukchi sea, north of the Bering Strait between Alaska and Russia, was late to melt. The whales had less time to feast because they couldn’t access the feeding area, and thus had less blubber to sustain them on their next migration.
Last year, though, the Artic was unusually warm. The whales were not blocked from the feeding area, and yet are still struggling this year. Scientists believe the loss of sea ice could have led to a loss of algae that feed the amphipods. Surveys show the amphipod beds moving farther north, said Sue Moore, a biological oceanographer at the University of Washington.
In an average year, about 35 whales wash up in the US.
More than 100 washed up in 2000, prompting NOAA to declare an “unusual mortality event”. The resulting investigation failed to identify a cause. The die-off followed strong changes in ocean conditions in the mid-1990s, suggesting that warmer water patterns affected the availability of prey.
John Calambokidis, a research biologist with the Cascadia Research Collective, noted that as the whales searched farther afield for food, they have entered areas where they are not normally seen, including San Francisco Bay and Puget Sound. Four of the 10 gray whales found dead near San Francisco this year were struck by ships.