The Salish Sea – the inland coastal waters of Washington and British Columbia – is home to two unique populations of fish-eating killer whales: the northern and southern resident killer whales. Human activities during much of the 20th century, including reducing salmon numbers and capturing orcas for entertainment purposes, decimated their numbers. This century, the population in the north has grown steadily to more than 300 individuals, but the population in the south has remained around 75. They remain critically endangered.
New research led by the University of Washington and the National Oceanic and Atmospheric Administration has revealed how human-generated underwater noise could help explain the plight of southern residents. In an article published on September 10 in Biology of global changethe team reports that underwater noise pollution – from both large and small ships – forces northern and southern killer whales to spend more time and energy searching for fish. The noise also decreases the overall success of their hunting efforts. Ship noise likely has an outsized impact on southern resident orca pods, which spend more time in parts of the Salish Sea with heavy ship traffic.
“Vessel noise negatively impacts every step in the hunting behavior of northern and southern resident killer whales: from searching to chasing and ultimately capturing prey,” said lead author Jennifer Tennessen, senior researcher at the UW’s Center for Ecosystem Sentinels , who started this research. study as a postdoctoral fellow at NOAA’s Northwest Fisheries Science Center. “It sheds light on why southern residents in particular have not recovered. One factor hindering their recovery is the availability and accessibility of their favorite prey: salmon. Introducing sound makes it even more difficult to find and catch prey, which is already difficult. find.”
Northern and southern killer whales search for food via echolocation. Individuals emit short clicks through the water column that bounce off other objects. These signals return to killer whales as echoes that encode information about the type of prey, its size and location. When the orcas detect salmon, they can initiate a complex chase and capture process, including more intensive echolocation and deep dives to try to catch and catch fish.
The team – which also includes scientists from Fisheries and Oceans Canada, Wild Orca, the Cascadia Research Collective and the University of Cumbria in Britain – analyzed data from northern and southern resident killer whales, whose movements were tracked using of digital tags, or “Dtags.” The cell phone-sized Dtags, which are attached non-invasively just beneath an orca’s dorsal fin via suction cups, collect data on three-dimensional body movements, position, depth and other environmental data, including – crucially – the noise levels at the whales’ locations.
“Dtags are a critical innovation for us to understand firsthand the environmental conditions that killer whales face,” says Tennessen. “They open a window into what killer whales 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 killer whales for several hours on specific days from 2009 to 2014. The team’s deep dive into Dtag data found that ship noise, especially from ship propellers, increased the level of ambient noise in the area. water. The increased noise disrupted the orcas’ ability to hear and interpret information about prey through echolocation. For each additional decibel increase in the maximum noise level around killer whales, the researchers noted the following:
- An increased chance that male and female killer whales will search for prey
- A lower chance of females chasing prey
- A smaller chance of both males and females actually catching prey
Dtags also recorded “deep dive” hunting attempts by killer whales. Of the 95 such attempts, most occurred in low or moderate noise conditions. But six deep hunting dives took place in particularly loud environments, of which only one was successful.
The team found that noise had a disproportionately negative impact on females, who were less likely to pursue prey detected during noisy conditions. Dtag data did not indicate the reason, although possible explanations include a reluctance to leave vulnerable calves on the surface while engaging prey in long chases that may not be fertile, and pressure on lactating females to conserve energy. Although southern resident killer whales often share captured prey, the impact of noise can contribute to nutritional stress in females, which previous research has linked to high rates of pregnancy failure among southern residents.
Lowering ship speed leads to quieter waters for the orcas. Voluntary vessel speed reduction programs have been included on both sides of the U.S.-Canada border: the Echo Program, initiated in 2014 by the Vancouver Fraser Port Authority, and Quiet Sound, launched in 2021 for Washington State waters. But reducing noise pollution is just one factor in saving southern resident killer whales and helping northern residents recover.
“When you take into account the complicated legacy we have created for the orcas – destruction of salmon habitat, water pollution, the risk of ship strikes – the situation is only made worse,” Tennessen says. “The situation can be turned around, but only with great effort and coordination on our part.”
Co-authors on the paper are Marla Holt, Brad Hanson and Candice Emmons of NOAA’s Northwest Fisheries Science Center; Brianna Wright and Sheila Thornton at Fisheries and Oceans Canada; Deborah Giles with Wild Orca and the UW’s Friday Harbor Laboratories; Jeffrey Hogan of the Cascadia Research Collective; and Volker Deecke at the University of Cumbria. The research was funded by 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.
