The Salish Sea — 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 orcas. Human activity over much of the 20th century, including reducing salmon runs and capturing orcas for entertainment purposes, decimated their numbers. This century, the northern resident population has steadily grown to more than 300 individuals, but the southern resident population has plateaued at around 75. They remain critically endangered.

New research led by the 91̽�� and the National Oceanic and Atmospheric Administration has revealed how underwater noise produced by humans may help explain the southern residents’ plight. In a published Sept. 10 in Global Change Biology, the team reports that underwater noise pollution — from both large and small vessels — forces northern and southern resident orcas to expend more time and energy hunting for fish. The din also lowers the overall success of their hunting efforts. Noise from ships likely has an outsized impact on southern resident orca pods, which spend more time in parts of the Salish Sea with high ship traffic.

“Vessel noise negatively impacts every step in the hunting behavior of northern and southern resident orcas: from searching, to pursuing and finally capturing prey,” said lead author , a senior research scientist at the UW’s , who began this study as a postdoctoral researcher with NOAA’s . “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.”

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

The team — which also includes scientists at Fisheries and Oceans Canada, Wild Orca, the Cascadia Research Collective and the University of Cumbria in the U.K. — analyzed data from northern and southern resident orcas, whose movements were tracked using digital tags, or “Dtags.” The cellphone-sized Dtags, which attach noninvasively just below an orca’s dorsal fin via suction cups, collect data on three-dimensional body movements, position, depth and other environmental data including — critically — the sound levels at the whales’ locations.

“Dtags are a critical innovation for us to understand firsthand the environmental conditions that resident orcas experience,” said Tennessen. “They open a window into what orcas are hearing, their echolocation behavior and the very specific movements they initiate when they hunt for prey.”

The researchers analyzed data from 25 Dtags placed on northern and southern resident orcas for several hours on specific days from 2009 to 2014. The team’s deep dive into Dtag data showed that vessel noise, particularly from boat propellers, raised the level of ambient noise in the water. The increased noise interfered with the orcas’ ability to hear and interpret information about prey conveyed via echolocation. For every additional decibel increase in maximum noise levels around orcas, the researchers observed:

• An increased chance of male and female orcas searching for prey
• A lower chance of females pursuing prey
• A lower chance that both males and females would actually capture prey

Dtags also recorded “deep dive” hunting attempts by orcas. Out of 95 such attempts, most occurred in low or moderate noise. But six deep-hunting dives occurred in particularly loud settings, 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 that had been detected during noisy conditions. Dtag data did not indicate the reason, though potential explanations include a reluctance to leave vulnerable calves at the surface while engaging prey in long chases that may not be fruitful, and the pressure for lactating females to conserve energy. Though southern resident orcas often share captured prey with one another, the impact of noise may contribute to nutritional stress among females, which previous research has linked to high rates of pregnancy failure among southern residents.

Reducing vessel speeds leads to quieter waters for the orcas. Both sides of the U.S.-Canada border include voluntary speed-reduction programs for vessels: the , initiated in 2014 by the Vancouver Fraser Port Authority, and , launched in 2021 for Washington state waters. But reducing noise is only one factor in saving southern resident orcas and helping northern residents continue to recover.

“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. “The situation could 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 with NOAA’s Northwest Fisheries Science Center; Brianna Wright and Sheila Thornton with Fisheries and Oceans Canada; Deborah Giles with Wild Orca and the UW’s Friday Harbor Laboratories; Jeffrey Hogan with the Cascadia Research Collective; and Volker Deecke with 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.

For more information, contact Tennessen at jtenness@uw.edu.

In the Pacific Northwest and British Columbia, scientists have been sounding the alarm about the plight of southern resident orcas. Annual counts show that population numbers, already precarious, have fallen back to mid-1970s levels. Most pregnancies end in miscarriage or death of the newborn. They may not be catching enough food. And many elderly orcas — particularly post-reproductive matriarchs, who are a source of knowledge and help younger generations — have died.

With just 73 individuals left, conservationists and members of the public alike are concerned that southern resident orcas may not survive.

Yet over the same period, the region’s northern resident orcas, who have a similar diet and an overlapping territory, grew steadily in population. Today, there are more than 300 northern resident orcas, leaving scientists wondering why these two similar but distinct populations have had such dissimilar fates over the past half century.

A new study led by scientists at the 91̽�� and reveals that the two populations differ in how they hunt for salmon, their primary and preferred food source. The research, done by an international team of government, academic and nonprofit researchers, March 4 in Behavioral Ecology.

“For northern resident orcas, females were hunting and capturing more prey than males. For southern resident orcas, we found the opposite: The males were doing more hunting and capturing than females,” said lead author , a senior research scientist at the 91̽��’s . “We also found that if their mother was alive, northern resident adult males hunted less, which is consistent with previous work, but we were surprised to see that southern resident adult males hunted more. Adult females in both populations hunted less if they had a calf, but the effect was strongest for southern residents.”

The study’s five years of observational data show that southern resident males catch 152% more salmon per hour than females. In other words, for every two fish a southern female caught, a southern male would catch five. For the growing northern resident population, the trend is flipped: females caught 55% more salmon per hour than males.

This is the first study to track the underwater pursuit, hunting and prey-sharing behaviors of both northern and southern resident orcas. Their findings reveal that, though the two populations overlap significantly in territory and have similar social structures and reproductive behavior, they should not be treated identically for conservation purposes.

“In the past, we’ve made assumptions about these populations and filled in the gaps when designing interventions, particularly to help the southern resident orcas,” said Tennessen, who conducted this study while she was a research scientist with NOAA’s . “But what we found here are strikingly different patterns of behavior with something as critical to survival as foraging. And as we develop management strategies, we really need to consider these populations differently.”

NOAA scientists and an international team of collaborators temporarily tracked the movement, sounds, depth and feeding behaviors of 34 northern and 23 southern resident adult orcas non-invasively from 2009 to 2014 using “Dtags,” cellphone-sized digital devices. Dtags attach via suction to the back of an orca and, for this study, were programmed to fall off hours later and float back to the surface so the researchers could collect them and download their data.

As the name would suggest, northern resident orcas have a more northerly distribution, preferring waters around Vancouver Island and the Queen Charlotte Strait. In contrast, core areas for southern resident orcas hug the southern reaches of Vancouver Island, inland waters surrounding the San Juan Islands, Puget Sound and the Washington coast. Both populations were devastated by the capture of orcas for theme parks, a practice that ended in the 1970s. Since then, northern resident orcas have increased steadily, seeing at least 50% growth since 2001.

Both populations hunt for salmon using echolocation. Adult orcas can dive at least 350 meters — or 1,150 feet — to pursue fish on their own, though they often bring kills to the surface to share with others. Pods travel between the outflows of major rivers and streams in British Columbia and Washington, and have been heavily impacted by dams that have reduced salmon runs. Increased vessel traffic and noise in the Salish Sea — from tourism, recreation and shipping — have also negatively affected these populations, particularly the southern resident orcas, according to Tennessen.

This new study showed that southern residents had fewer successful hunts overall, indicating that they were presumably catching less food. This impact is particularly evident with young mothers.

“In both populations, a mother with a young calf foraged less than other females, possibly due to the risk of leaving the calf temporarily with ‘a babysitter’ — another adult — while she hunts, or because of the time demands of nursing a calf,” said Tennessen. “But for southern resident females, which are more prone to disturbance and stress from vessel traffic, there was an outsized effect: Our study found no instance of a southern resident female with a young calf who successfully carried out a hunt.”

The study also has much to say about the impact of elderly female orcas on their adult sons. Both northern and southern resident orcas are grouped into matriarchal clans, often led by post-reproductive females. They also help feed their adult sons even, as a led by the nonprofit Center for Whale Research showed, at the expense of their own reproductive capacity.

The new study adds complexity to the role of elderly females. Among northern resident orcas, adult males with a living mother hunted less than adult males without a living mother, perhaps because the mother still provides food. But among southern resident orcas, the opposite is true: Adult males with a living mother hunted more.

“These unexpected differences left us scratching our heads. It is possible that southern resident adult males could be sharing with other members of their group, including their mothers, to help out, especially since an adult male’s survival is strongly linked to his mother’s survival,” said Tennessen. “Relatedly, southern resident matriarchs may be leading the group to areas where their adult sons may be able to capture more prey, since healthier sons might be more successful at mating and passing along some of their mothers’ genes. We need more studies to determine what role the presence — or absence, for southern resident orcas — of matriarchs has on male foraging behavior.”

Future studies on the behaviors of northern and southern resident orcas could bring these differences to the surface, as could studies of Alaska resident orca populations, which forage for salmon farther north, where salmon stocks are generally healthier. Such comparative studies can help isolate cause and effect, said Tennessen.

“Understanding how healthy populations behave can provide direction and goals for management of unhealthy populations,” said Tennessen. “Future comparisons to healthy fish-eating orca populations could help us understand whether the divergent behavior we’re seeing in the southern residents is indicative of a population trying to survive.”

Co-authors on the paper are Maria Holt, Bradley Hanson and Candice Emmons with the NOAA Northwest Fisheries Science Center; Brianna Wright and Sheila Thornton with Fisheries and Oceans Canada; Deborah Giles with the 91̽��Friday Harbor Labs; Jeffrey Hogan with the Cascadia Research Collective; and Volker Deecke with the University of Cumbria in the U.K. The research was funded by NOAA, Fisheries and Oceans Canada, the University of Cumbria and the University of British Columbia.

For more information, contact Tennessen at jtenness@uw.edu.