Thanks to humans, the waters of the Salish Sea are too noisy for resident orcas to hunt successfully.

The Salish Sea, which includes the inland coastal waters of Washington and British Columbia, is home to two unique populations of fish-eating orcas: the Northern Resident and the Southern Resident. Human activity over much of the 20th century, including reduced salmon runs and the capture of orcas for entertainment, decimated their numbers. During this century, the Northern Resident population has steadily increased to over 300 individuals, but the Southern Resident population has stagnated at about 75. These orcas remain critically endangered.

New research from the University of Washington and the National Oceanic and Atmospheric Administration has revealed how human-made underwater noise could help explain the plight of people in the South. In a paper published September 10 in Biology of global changeThe team reports that underwater noise pollution from both large and small vessels is forcing both northern and southern resident orcas to devote more time and energy to hunting fish.

The noise also reduces the overall success of their hunting efforts. Ship noise likely has an outsized impact on southern resident orca pods, which spend more time in areas of the Salish Sea where boat traffic is high.

“Vessel noise negatively impacts every stage of northern and southern resident orcas’ hunting behavior: from searching to chasing and finally capturing prey,” said lead author Jennifer Tennessen, a principal investigator at the UW Center for Ecosystem Sentinels who began the study as a postdoctoral researcher at NOAA’s Northwest Fisheries Science Center.

“This sheds light on why southern residents in particular have not recovered. One of the factors hindering their recovery is the availability and accessibility of their preferred prey: salmon. When you introduce noise, it becomes even more difficult to find and catch prey that are already difficult to find.”

Northern and southern resident killer whales search for food by echolocation. Individuals transmit short clicks through the water column that bounce off other objects. These signals return to the killer whales as echoes that encode information about the type of prey, its size, and its location. If the killer whales detect a salmon, they can initiate a complex pursuit and capture process, which includes heightened echolocation and deep dives in an attempt to trap and capture fish.

The team, which also includes scientists from Fisheries and Oceans Canada, Wild Orca, the Cascadia Research Collective and the University of Cumbria in the United Kingdom, analyzed data from northern and southern resident killer whales, whose movements were tracked using digital tags, or “Dtags.” The cellphone-sized Dtags, which attach non-invasively just below the orca’s dorsal fin using suction cups, collect data on three-dimensional body movements, position, depth and other environmental data, including, crucially, noise levels where the whales are located.

“Digital tags are a critical innovation in allowing us to understand firsthand the environmental conditions that resident orcas face,” Tennessen said. “They provide a window into what orcas hear, their echolocation behavior, and the very specific movements they initiate when hunting prey.”

The researchers analyzed data from 25 Dtags placed on northern and southern resident killer whales for several hours on specific days between 2009 and 2014. The team’s in-depth analysis of the Dtag data showed that vessel noise, particularly from boat propellers, increased ambient noise levels in the water. The increased noise interfered with the killer whales’ ability to hear and interpret echolocation information about prey. For every additional one-decibel increase in peak noise levels around the killer whales, the researchers observed:

• Increased risk of prey seeking by male and female orcas
• Less likely for females to pursue their prey
• Less chance of males and females actually capturing prey

Dtags also recorded deep-diving hunting attempts by orcas. Of 95 such attempts, most took place in low- to moderately noisy environments. But six deep-diving hunting dives took place in particularly noisy environments, only one of which was successful.

The team found that noise had a disproportionately negative impact on females, who were less likely to pursue prey detected in noisy conditions. The Dtag data did not indicate the reason, although possible explanations include a reluctance to leave calves vulnerable on the surface while engaging in long, fruitless pursuits with prey, and pressure on lactating females to conserve energy.

Although Southern Resident orcas often share captured prey with each other, the impact of noise may contribute to nutritional stress in females, which previous research has linked to high rates of pregnancy failure in Southern Residents.

Reducing vessel speeds allows orcas to navigate in calmer waters. Both sides of the Canada-U.S. border have voluntary vessel speed reduction programs: the Echo program, launched in 2014 by the Vancouver Fraser Port Authority, and the Quiet Sound program, launched in 2021 for Washington state waters. But reducing noise is just one factor in saving southern resident orcas and helping northern residents continue their recovery.

“When you consider the complex legacy we’ve left for resident orcas – salmon habitat destruction, water pollution, ship strike risk – adding noise pollution just makes an already dire situation worse,” Tennessen said. “It could be reversed, but only with a lot of effort and coordination on our part.”