Pacific killer whales have enough food, but are still struggling

Marine biologists have challenged the claim that lack of food is driving a population crash in killer whales in the Pacific Ocean saying boat noise may be the issue.

Killer whales (Orcinus orca) frequent the waters of British Columbia and feed on Chinook salmon.

Researchers from the University of British Columbia in Canada used echosounder data to determine prey salmon densities, as well as discussions with local anglers and whale watching crews in British Columbia, Canada.

There are 2 different populations. One population is local to northern waters. These killer whales have tripled their numbers to about 300 individuals since monitoring began in the 1970s.

The other population inhabits the waters between British Columbia and California to the south. Their numbers fluctuated between 66 and 98 individuals with the latest census putting their numbers at just 73.

“The differing trajectories of these two populations of fish-eating killer whales have been attributed to ecological and biological differences between regions such as prey availability, diet breadth, competition, physical disturbance, underwater noise, contaminants and inbreeding,” the authors write. “However, food availability likely plays the greatest role in limiting their carrying capacities.”

Previous research has shown a correlation between salmon numbers and killer whale population health. But the authors say these studies have never been able to show why the southern population was struggling.

In fact, the southern population of killer whale is the only marine mammal that is struggling in the region. Harbour seals, sea lions, other types of whale and porpoises are all thriving.

Nevertheless lack of access to the Chinook salmon was always put down as the reason for the killer whales’ woes.

But sport anglers told the researchers that they have noticed no drop in salmon numbers. And whale watchers have reported that they have regularly seen the endangered orcas swimming among salmon.

The researchers suggest that the issue isn’t lack of salmon, but that the southern population of killer whales are having trouble catching their prey. This, they say, is likely due to noise from boats. The area where the southern orca population lives has far higher sea traffic than the regions further north.

It’s also possible the orca struggle to hunt at different times of year. They may find enough salmon in summer, but have trouble during spring.

The findings are presented in a paper published in PLOS ONE.