Wednesday, March 29, 2023
March 29, 2023

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What researchers learned studying Northwest orcas hunting for salmon


SEATTLE — In a first-of-its-kind study of the hunting behaviors of salmon-eating orcas, researchers found stark differences in two populations of killer whales that may have implications for their survival.

University of Washington and NOAA Fisheries researchers used suction-cup tags to track how northern and southern resident orca populations forage — the depth they dive, the sounds they make and receive and how they move. The northern residents, who frequent the waters of Vancouver Island and Queen Charlotte Strait, relied heavily on females to bring dinner home. Yet among the endangered southern residents, females had far less hunting success.

The southern resident orcas who often grace the Salish Sea with their presence are a lot like their relatives up north. The northern and southern residents are both picky eaters — Chinook salmon, please! — and they have similar social structures, living in large matrilineal groups and sharing their food with their family.

But as the northern resident population has steadily grown, there are only 73 southern resident orcas left. For the southern residents, pregnancies often end in loss. They struggle to find adequate food as Chinook face human-made barriers, pollutants and hungry seals and sea lions.

The new study illuminates some of the orcas’ behavior that may help to explain the disparity.

Northern resident females were 257% more efficient than southern resident females at gathering food, spending less time on task to get the job done, the study shows. Northern residents spent more time resting and traveling than southern residents, who dived deeper for food.

According to the study, southern resident males caught five fish for every two a southern resident female caught. It’s the opposite for northern residents, where females caught 55% more salmon per hour than males.

Females without calves in both populations took more risks when looking for food, but the southern resident females with calves captured no prey in the duration of the study.

“It may make sense for females to hang back with calves that are vulnerable,” said lead author Jennifer Tennessen, a senior research scientist at the University of Washington’s Center for Ecosystem Sentinels. “They’ve had a really poor success at producing calves that survive recently, and so it may be that the population is taking a different strategy entirely out of necessity.”

Another recent study found adult male southern residents rely on their moms for food, and that comes at a reproductive cost. Mothers with an adult son were about half as likely to reproduce compared with females with a daughter or no offspring.

Both northern and southern resident populations were devastated by the capture of orcas for theme parks. But the northern resident population has grown by at least 50% since 2001, with more than 300 today.

Meanwhile, the latest census tallied one of the lowest population counts among the J, K and L southern resident pods since 1974. The southern resident population peaked at 98 in 1995 and declined by almost 20% in the late ‘90s, leaving 80 orcas in 2001.

In 2005, the southern residents were listed as endangered under the Endangered Species Act and a recovery plan was finished in 2008.

In 2015, they became one of NOAA’s “Species in the Spotlight,” an effort to raise awareness and save “the most highly at-risk marine species.”

In a perfect world, said NOAA research wildlife biologist Marla Holt, you might expect a similar recovery of the southern residents. But they don’t have the same food availability as the northern residents, and boat traffic is making it harder for them to hear each other and access food.

Estuaries — a refuge for growing juvenile salmon — have been diked and drained to create farmland. Freeways, cities and neighborhoods have been built atop wetlands. Rivers have been tamed for hydropower and flood control.

Now, Chinook, the southern residents’ primary food source, are in decline. Fewer than 500,000 adult Chinook were estimated to have passed through the Salish Sea in 2018 — a 60% reduction since 1984.

Holt said the southern residents’ situation can be likened to a parent living paycheck to paycheck. “If you have kids, as soon as your children are old enough, they’re going to have to start contributing to the household.”

“There’s no free lunch.”

The study is just one step toward better understanding the southern residents’ behavior and designing a successful conservation program, said Tennessen, the lead researcher.

Future studies will have to tease apart some of the social and environmental factors to help better interpret these findings.