New studies have shown that human-made underwater noise might be a reason for the failure of southern resident orca populations to bounce back from previous declines. According to researchers, noise pollution from various vessels, big and small, causes both northern and southern resident orcas to spend more effort and time searching for fish. This interference also reduces how successful their hunting attempts are. Southern resident orcas, in particular, seem to be more impacted by ship noise as they often inhabit areas of the Salish Sea that are heavily trafficked by boats.
The Salish Sea — the coastal waters between Washington and British Columbia — is inhabited by two distinct groups of fish-eating orcas, known as northern resident and southern resident orcas. Throughout the 20th century, human activities, such as dwindling salmon populations and capturing orcas for entertainment, dramatically decreased their numbers. While the northern resident population has been steadily increasing to over 300, the southern resident population remains stagnant at about 75 individuals, keeping them in a state of critical endangerment.
Recent research from the University of Washington and the National Oceanic and Atmospheric Administration has shed light on how human-generated underwater noise could explain why southern resident orcas continue to struggle. In an article published on September 10 in Global Change Biology, the research team found that noise pollution from vessels forces both northern and southern resident orcas to spend more time and effort hunting fish, which ultimately lowers their success rate. The sound of ships disproportionately affects southern resident orca pods, as they frequent areas in the Salish Sea with greater ship activity.
Lead author Jennifer Tennessen, a senior research scientist at UW’s Center for Ecosystem Sentinels, explained, “Vessel noise negatively impacts every step in the hunting behavior of northern and southern resident orcas: from searching, to pursuing and finally capturing prey. This highlights a reason why southern residents specifically have not shown improvement. One obstacle to their recovery is the availability of their preferred food: salmon. The presence of noise makes the already tough task of finding and catching prey even harder.”
To locate food, northern and southern resident orcas use echolocation. They emit short clicking sounds through the water that bounce back from nearby objects, providing them with information about prey, including its type, size, and location. If orcas identify salmon, they will engage in a complex process to chase and catch it, utilizing enhanced echolocation and diving deep.
The research team, which also includes scientists from Fisheries and Oceans Canada, Wild Orca, Cascadia Research Collective, and the University of Cumbria in the U.K., examined data from orcas equipped with digital tracking devices known as “Dtags.” These Dtags, which attach non-invasively beneath an orca’s dorsal fin, gather data on their movements, depth, and environmental sounds in their surroundings.
“Dtags provide essential insights into the conditions that orcas encounter,” Tennessen remarked. “They allow us to understand the sounds orcas detect, their echolocation habits, and the specific maneuvers they take while hunting.”
The study examined data from 25 Dtags placed on northern and southern resident orcas for several hours on selected days between 2009 and 2014. The thorough analysis revealed that vessel noise, especially from boat engines, increased the background noise in the water, which interfered with the orcas’ ability to use echolocation effectively. For each extra decibel increase in noise levels around the orcas, the researchers noted:
- Higher likelihood of orcas searching for prey
- Reduced chances of females pursuing prey
- Lower likelihood of both genders actually capturing prey
Moreover, Dtags also captured details about the orcas’ deep dive hunting attempts. Out of 95 attempts, most took place in quieter environments. However, six hunts occurred amid much louder conditions, with only one resulting in success.
The study revealed that noise had a particularly detrimental effect on female orcas, who were less likely to chase prey detected during noisy situations. Although the underlying reasons weren’t identified, possible explanations could include a hesitation to leave their calves exposed at the surface during lengthy pursuits that may yield no results and a need for lactating females to conserve energy. Despite southern resident orcas often sharing their catches, the added stress from noise could contribute to nutritional deficits in females, a link that previous studies have tied to increased pregnancy failures in this group.
Reducing the speed of vessels results in quieter waters for orcas. Voluntary speed reduction programs are in place on both sides of the U.S.-Canada border, including the Echo Program launched in 2014 by the Vancouver Fraser Port Authority and Quiet Sound, which started in 2021 in Washington state waters. However, mitigating noise pollution is just one piece of the puzzle in saving southern resident orcas and aiding the continued recovery of northern ones.
Tennessen emphasized, “Considering the complicated legacy we’ve built for the resident orcas, including habitat destruction, water pollution, and risks of vessel collisions, adding noise pollution intensifies an already serious situation. To reverse this trend, significant effort and collaboration are essential.”
Co-authors of the study include Marla Holt, Brad Hanson, and Candice Emmons from NOAA’s Northwest Fisheries Science Center; Brianna Wright and Sheila Thornton from Fisheries and Oceans Canada; Deborah Giles from Wild Orca and the UW’s Friday Harbor Laboratories; Jeffrey Hogan from Cascadia Research Collective; and Volker Deecke from the University of Cumbria. The research received funding from NOAA, Fisheries and Oceans Canada, the University of Cumbria, the Marie Curie Intra-European Fellowship, the University of British Columbia, and the Natural Sciences and Engineering Research Council of Canada.
