A single salmon may represent a mere morsel to a killer whale, but it is still worth sharing with the family, according to a federal Fisheries study of B.C.’s northern resident orcas.

“We watched these animals take salmon, break them apart and then share them with one another,” said Brianna Wright, a federal fisheries technician with the Pacific Biological Station in Nanaimo.

“That increases your overall fitness because your DNA is shared with everyone and is being passed on. You’re helping your relatives to reproduce and survive.”

Speaking at a weekend marine mammal symposium at the University of B.C., Wright noted that neither sex of the killer whales leaves the matriarchical line into which they are born. “It’s highly unusual,” she said. “They’re one of the most … stay-with-family groups of animals that exist.”

Individuals mate outside the family group, using acoustic recognition to avoid inbreeding.

The federal study of more than 600 “prey-sharing events” by northern resident killer whales from 2002 to 2014 showed that salmon is more likely to be shared with the closest kin.

Almost 90 per cent of shared catches are with individuals within the “matriline”. Adult males share less often, presumably because their offspring are not within their matriline. But they do feed their mothers, even when they are past their reproductive years, because mothere continue to feed them.

“To share with an animal that can’t reproduce is not a selective behaviour,” Wright said. “Why would you do it? It appears to be an altruist behaviour for the benefit of the mother, but it’s really for the son so she stays near and keeps feeding him.”

Mothers give greater preference to males due to their increased opportunity to spread their genes, researchers suspect. Sharing with daughters also drops off over the years, ending at about age 10 as they become sexually mature.

“When a daughter had her first calf, the mother stopped sharing with her,” Wright said. “It’s probably not a neglectful thing. The daughter now has offspring and she’s sharing with them and less with the mom.”

Resident killer whales eat only fish — preferably chinook salmon — whereas transient killer whales only eat mammals such as harbour seals and porpoises.

Also at the symposium, Paul Cottrell, coordinator of the federal marine mammal response program, noted that six humpback whales along with five fin, one sperm and one grey whale were found dead on the B.C. coast this year compared with about 40 whale strandings in Alaska.

Three of the fin whales — a threatened species in B.C. and the second largest animal on earth, after the blue whale — were found dead on the same beach near Bella Bella on the central coast in November.

“It was almost surreal, all in one small bay,” Cottrell said in an interview. “There was no obvious cause of death. It’s one that’s really puzzling.”

Necropsy test results are pending on the animals. Factors in whale standings typically include ship strikes, toxins, disease, and entanglement in fishing gear.

lpynn@vancouversun.com

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