A new study gives some insight into the forage behaviors of the Southern Resident orca whales, and how those behaviors may affect their population.
The study published this month by biologists from the National Oceanic and Atmospheric Administration and Canada’s Department of Fisheries and Oceans compares the foraging strategies of the Southern Resident and Northern Resident orcas.
While the Southern Resident population that is mostly located off the coasts of Washington, Oregon and British Columbia is endangered and numbers about 73, the Northern Residents range from Southeast Alaska to B.C. and, while threatened, have a much larger population.
Past research has shown Southern Residents contend with low numbers of chinook salmon, their preferred food, and substantial noise from vessels, which hinders their ability to locate and catch prey.
The two groups are genetically distinct.
Jennifer Tennessen, the lead author of the study while a postdoctoral researcher at NOAA, said the Northern Resident population has tripled in size over a 50-year period, while Southern Residents have shown no net growth.
“We were trying to understand how the whales behave while foraging, to see if that could shed any insight into why these populations have these divergent growth trajectories,” said Tennessen, who is now a senior research scientist at the Center for Ecosystem Sentinels at the University of Washington.
Tennessen and others who worked on the study found a few surprises.
Northern Resident females caught more fish as a food source than the males, were more efficient at catching fish and spent more time engaged in catching prey.
“We expected to see similar patterns for Southern Residents as well. But instead, we found the complete opposite pattern,” Tennessen said.
Female Southern Residents caught less fish than males and spent less time engaged in catching prey.
For both populations, the female orcas that had calves caught less prey than those without calves. However, for Southern Residents, the difference was striking.
For the duration of the study, sensor tags recorded no Southern Residents with calves catching prey.
“There’s a toll that reproduction takes, and rearing offspring takes, on the female (orcas). I think we may be seeing that in Southern Residents, females seem to have to choose between catching a fish and caring for their calves at the surface,” Tennessen said.
The study also showed that adult male Northern Residents who had living mothers caught less prey than those whose mothers had died. Northern Resident females often shared their prey with their sons.
On the other hand, Southern Resident adult males with living mothers caught more prey than those whose mothers had died.
Tennessen said one theory is that because the Southern Resident population is so fragmented, typical social relationships between the whales are unbalanced.
Male orcas could also be sharing prey with their mothers to reduce their foraging efforts and help them survive.
Tennessen said that the study helps show that Southern Residents differ from typical evolutionary behavior in orcas.
“I think we really need to lean away from general assumptions between populations and think about the Southern Residents as a distinct population that clearly behaves differently,” she said.
Previous studies have shown that female Southern Resident orcas with calves are especially sensitive to vessel traffic.
“The picture that’s beginning to emerge is that females are particularly vulnerable to disturbance and so we need to manage in a way that prioritizes females’ abilities to catch fish and care for their calves,” Tennessen said.
The SeaDoc Society Science Director Joe Gaydos said the study’s results can help inform actions to help the Southern Residents.
“If we already know that females are more likely to stop feeding when there’s boats present … and they’re not catching as much as the females in a population that’s doing well … I think that has big implications for really wanting to push boats as far away as we can from these animals,” he said.
Senate Bill 5371, which would expand the state-regulated distance between Southern Residents and vessels, is making its way through state Legislature. It has been passed by the Senate and is now in committee in the House.
The SeaDoc Society testified in front of the Agriculture and Natural Resources committee Wednesday about the bill.
“We should be doing everything we can do help these animals,” Gaydos said.
