Southern Resident killer whales are less efficient at hunting compared to another orca population living in the waters off the west coast of British Columbia and the United States.

That’s among the findings within a new study published in Behavioural Ecology, which looked at the behaviours of endangered southern residents and threatened Northern Resident killer whale populations.

Southern resident females captured less prey and spent less time hunting than their males and the females of northern residents, with the latter out-catching their male counterparts.

Female northern residents caught 167 per cent more per hour than SRKW females and 152 per cent more than SRKW males. Those southern male whales also spent 114 per cent more time engaged in prey capture dives than female SRKWs.

“This potential prey-sharing role of SRKW adult males challenges the existing paradigm that adult females are disproportionate provisioners in resident killer whale populations,” the study’s scientists wrote.

Across both sexes, the northern whales spent 62 per cent more time resting or travelling.

Both orca populations saw females catch a reduced amount of prey when they were accompanied by a calf aged three or younger, but it was more pronounced among southern residents. Adult male SRKWs with a living mother also caught more prey than those whose mother had died.

Males from both species forage in deeper areas, while southern residents catch prey deeper than the northern whales.

The study authors said the whales must balance competing strategies of conserving energy to minimize their likelihood of starvation, or maximizing energy they get from foraging. For southern residents experiencing scarce or patchy prey resources – due to the depletion of Pacific salmon stocks – the study said mothers with calves may favour conserving energy by diving for prey less frequently and receiving from others more often.

The study comes as the two whale species are heading in different directions. The threatened northern residents had their numbers increase by more than 50 per cent since 2001 while southern residents, now numbered at 73, have shown basically no growth since population censuses began in the ’70s.

SRKWs have also experienced high mortality, including the loss of critical information-bearing post-reproductive matriarchs. The study said the loss of those older “keystone individuals” could be impacting overall behaviour patterns.

The study authors wrote that differences in individual foraging roles across populations may also be the outcome of environmental pressures differentially impacting growth rates. They pointed to southern residents experiencing high levels of human disturbance, such as a disproportionate impact of vessel presence on female foraging compared to males.

READ: B.C. study links killer whale decline to lack of energy-rich fatty salmon

Southern Resident Killer Whales