In the Pacific Northwest and British Columbia, scientists have been sounding the alarm about the plight of the southern orca population. Annual statistics show that population numbers, already precarious, have fallen to levels of the mid-1970s. Most pregnancies end in miscarriage or the death of the newborn. Maybe they don’t catch enough food. Many elderly orcas have died – especially postpartum mothers, who are a source of knowledge and help for younger generations.
With only 73 individuals left, conservationists and the general public alike worry that the southern resident orcas may not survive.
However, during the same period, the northern orca population in the region, which has a similar diet and nesting area, has grown steadily in population. Today, there are more than 300 orcas residing in the North, leaving scientists to wonder why these two similar but distinct groups have had such divergent fates over the past half century.
A new study led by scientists at the University of Washington and NOAA Fisheries finds that populations differ in how they catch salmon, their primary and preferred food source. The research, which was conducted by an international team of government, academic, and nonprofit researchers, was published March 4 Behavioral ecology.
“For northern resident orcas, females hunted and captured more prey than males. For southern resident orcas, we found the opposite: males hunted and hunted more than females,” said lead author Jennifer Tennessen, a senior researcher at Britain’s University of Orcas. . University of Washington Center for Ecosystems. “We also found that if their mother was alive, adult males residing in the north hunted less, which is consistent with previous work, but we were surprised to see that adult males residing in the south hunted more. Adult females in both groups hunted less if they had calves, but the effect was stronger for southern residents.”
Five years of observational data show that males residing in the South catch 152% more salmon per hour than females. In other words, for every two fish a southern female catches, a southern male will catch five. As for the growing northern resident population, the trend has reversed: females caught 55% more salmon per hour than males.
This is the first study to track the underwater stalking, hunting and prey-sharing behaviors of both northern and southern resident orcas. Their findings reveal that although the two groups overlapped significantly in area and had similar social structures and reproductive behaviour, they should not be treated identically for conservation purposes.
“In the past, we’ve made assumptions about these populations and filled in gaps when designing interventions, particularly to help southern orca populations,” said Tennessen, who conducted the study while a research scientist at NOAA’s Northwest Fisheries Science Center. . “But what we found here are strikingly different patterns of behavior with something as crucial to survival as foraging. As we develop management strategies, we really need to look at these populations differently.”
NOAA scientists and an international team of collaborators non-invasively tracked the movement, sounds, depth, and feeding behaviors of 34 adult northern orcas and 23 southern resident orcas from 2009 to 2014 using “Dtags,” cellphone-sized digital devices. Dtags are attached by suction to the back of an orca and, in this study, are programmed to drop off hours later and resurface so researchers can collect them and download their data.
As the name suggests, northern resident orcas have a more northerly distribution, preferring the waters around Vancouver Island and Queen Charlotte Strait. In contrast, the southern resident orcas’ core areas hug the southern reaches of Vancouver Island and the inland waters surrounding the San Juan Islands, Puget Sound, and the Washington coast. Both groups were destroyed by the capture of orcas in theme parks, a practice that ended in the 1970s. Since then, the northern orca population has increased steadily, and has seen growth of at least 50% since 2001.
Both groups 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 flows of major rivers and streams in British Columbia and Washington, and have been severely affected by dams that reduce salmon runoff. Increased ship traffic and noise in the Salish Sea—from tourism, recreation, and shipping—has also negatively impacted these populations, particularly the southern Orca population, according to Tennessen.
This new study showed that southerners had fewer successful hunts overall, indicating that they were hunting less food. This effect is especially evident with young mothers.
“In both groups, the mother with young calves fed less than the other females, possibly because of the risk of leaving the calf temporarily with a ‘babysitter’ — another adult — while hunting, or because of the time demands of nursing,” Tensen said. In the South, who are more susceptible to disruption and stress from ship traffic, there was a significant effect: Our study found no case of Southern females with young calves that successfully made a hunt.”
The study also has a lot to say about the effect of an aging female orca on her adult offspring. Both northern and southern resident orcas are grouped into matrilineal clans, often led by post-reproductive females. They also help feed their adult offspring, a recent study led by the nonprofit Center for Whale Research shows, at the expense of their own reproductive capacity.
The new study adds complexity to the role of elderly females. Among northern orca populations, adult males with a live mother hunted less than adult males without a live mother, possibly because the mother still provided food. But the opposite is true among southern resident orcas: adult males with a live mother hunt more.
“These unexpected differences left us baffled. It is possible that the adult males residing in the south may partner with other members of their group, including their mothers, to help, especially since the survival of the adult male is strongly linked to the survival of his mother,” Tennessen said. Similarly, southern-based mothers may lead the group into areas where their adult offspring may be able to capture more prey, because healthier offspring may be more successful at mating and passing on some of their mother’s genes. We need more studies to determine what role this plays. Played by the presence — or absence, for southern resident orcas — of males’ foraging behavior.”
Future studies of the behaviors of northern and southern orcas could bring these differences to the surface, as can studies of Alaskan resident populations feeding on salmon farther north, where salmon stocks are generally healthier. Such comparative studies can help isolate cause and effect, Tennessen said.
“Understanding how healthy populations behave can provide guidance and goals for managing unhealthy populations,” Tennyson said. “Future comparisons with healthy orca populations eating fish can help us understand whether the divergent behavior we see in the southern population is indicative of the population trying to survive.”
Co-authors on the paper are Maria Holt, Bradley Hanson, and Candace Emmons of the NOAA Northwest Hunting Science Center. Brianna Wright and Sheila Thornton of Fisheries and Oceans Canada; Deborah Giles with UW Friday Harbor Labs; Jeffrey Hogan with Cascadia Research Collective; and Volker Dick of the University of Cumbria in the UK. The research was funded by the National Oceanic and Atmospheric Administration (NOAA), Fisheries and Oceans Canada, the University of Cumbria and the University of British Columbia.
