The Salish Sea is too noisy for the critically endangered southern resident orcas to hunt successfully, according to a new study led by the University of Washington.
In the study published this week, researchers say underwater noise from vessels on the Salish Sea, a name for inland waters in southern B.C. and northern Washington, forces both northern and southern resident orcas to spend more time and energy looking for food.
Lead author Jennifer Tennessen, a senior research scientist at the university, said vessel noise is hurting every step in the hunting behaviour of the orcas, including finding, pursuing and, finally, capturing prey.
“It shines a light on why southern residents in particular have not recovered. One factor hindering their recovery is availability and accessibility of their preferred prey: salmon. When you introduce noise, it makes it even harder to find and catch prey that is already hard to find,” she said in a statement.
The study included scientists from the U.S. National Oceanic and Atmospheric Administration and Fisheries and Oceans Canada, and was published in the scientific journal Global Change Biology.
It notes that southern resident orca pods spend more time in parts of the Salish Sea with high ship traffic than other orcas. The noise from these ships disrupts their ability to use echolocation to dive deep and find salmon.
The researchers analyzed data from 25 digital tags on northern and southern resident orcas. They then studied the deep-dive attempts by orcas — 462 dives by females and 1,265 dives by males — and compared the likelihood of searching in an area where the noise level exceeded 110 decibels to areas with noise under 95 decibels.
All but one deep-foraging attempt with noise over 110 decibels resulted in failure to catch prey, researchers said.
Tennessen told Postmedia the underwater noise levels they detected came from a variety of sources, such as cargo ships, cruise ships, ferries, whale-watching vessels and private vessels, but they couldn’t differentiate between the ships.
They do know that most of the underwater vessel noise comes from propellers and that faster vessels produce more noise.
Researchers measured noise in a narrow frequency range relevant to a range of orcas’ greatest hearing sensitivity, so 110 decibels is actually not the full noise that the whales received.
Tennessen said all ships with propellers are capable of emitting over 110 decibels in noise, so the problem is widespread. While there are no regulations on noise levels, she noted that some programs have encouraged ships to slow down in areas where there are orcas.
The sound of a shipping freighter’s propeller at close range, for example, can be more than 170 decibels, which is louder than a jet engine, say UBC experts.
The study says echolocation clicks were more likely to occur as dive depth and noise level increased, and the orcas spent more time searching in noisier conditions, resulting in reduced foraging efficiency.
They also found that females were less likely to pursue prey that had been detected during noisy conditions. Researchers say this could be contributing to nutritional deficiency, which other research has linked to high rates of pregnancy failure among southern residents.
The study says that for every one decibel increase in noise level, there was a 58 per cent decrease in the odds of pursuit by females, and a 12.5 per cent decrease in the odds of prey capture by both sexes.
The findings demonstrate the effects of vessel noise on foraging, and underscore the need to manage noise to achieve conservation objectives for acoustically sensitive species, the researchers wrote.
“When you factor in the complicated legacy we’ve created for the resident orcas — habitat destruction for salmon, water pollution, the risk of vessel collisions — adding in noise pollution just compounds a situation that is already dire,” said Tennessen.
While the northern resident population has steadily grown to more than 300 individuals, researchers say the southern resident population remains critically endangered at around 75.
Another study earlier this year found there are warning signs of an accelerating decline for these orcas.
That study, by the B.C. conservation group Raincoast and an international team of scientists, said they are suffering from habitat degradation from noise, high concentrations of industrial chemicals, and declining quality and quantity of salmon.
A 2023 UBC study found chemicals from wildfire smoke and oil emissions in muscle and liver samples from transient and resident orcas between 2006 and 2018.
