Northern and southern resident orcas hunt differently, which may help explain the decline of southern orcas

Link to Google Drive folder containing images with caption and credit information:

https://drive.google.com/drive/folders/1Ye7QXkoTHfq7L4qEcJ33r6I6w7aB2-Mn?usp=share_link

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 University of Washington and NOAA Fisheries 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, was published 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 Jennifer Tennessen, a senior research scientist at the University of Washington’s Center for Ecosystem Sentinels. “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 Northwest Fisheries Science Center. “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 recent study 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 UW 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.