Commercial whaling suited killer whales

The end of commercial whaling may have forced scavenging killer whales to turn on smaller prey, such as otters and seals. And their dietary switch could be in part responsible for the current precarious status of some marine mammals – and the increased observation of killer whales attacking other whales, say a pair of Canadian researchers.

Killer whales have been previously accused of decimating populations of sea otters and stellar sea lions in the Aleutian Islands off Alaska, US. This behaviour seemed to coincide with the moratorium on commercial whaling in 1970.

Biologists thought the killer whales' altered habits were a result of commercial whaling’s decimation of one their primary food sources – other whales. While this may have been a factor, the general consensus overlooked the possibility that whaling vessels had also been offering killer whales some very easy meals for hundreds of years.

This is because whalers often left their catches floating in the ocean, tied close to their vessels, for several hours before hauling them aboard – an easily found ocean-based buffet for the killer whales.

"It's another way of thinking about the relationship between whaling, killer whales and their predation on non-whaling species," says co-author Randall Reeves at Okapi Wildlife Associates in Hudson, Canada.

James Estes at the University of California, Santa Cruz, was among the team that first proposed a connection between killer whales, whaling and depleted marine mammals. He was astounded by the new idea. "I just thought, why didn’t we think of that? It was so obvious. There really was a sudden change in the available whale biomass. There would be none of these carcasses around."

Old logbooks
Reeves and his colleague Hal Whitehead at Dalhousie University in Halifax, Canada, searched through historical literature and ship logbooks dating back to the 1700s to assess the prevalence of killer whale scavenging behaviour.

They uncovered ample evidence of killer whales eating caught baleen whales – especially the tongue, commonly thought of as a killer whale 'delicacy'. They also back-calculated the available live whale biomass for killer whales to eat throughout the 20th century, in addition to the amount of whale carcasses available.

While live whale biomass slowly declined between 1900 and 2000, carcass availability fell dramatically between 1970 and 1980. Large whale carcasses from hunting fell from over 60,000 per year to almost none within the decade.

Food stealers?
Reeves says that killer whales have also been attacking the whales of western North America in the last few decades – most notably grey whales, a population that has fortunately recovered well since 1970. The northern sea otters, after a resounding comeback in the 20th century, however, have been faring poorly since the 70s.

But Reeves is not jumping to put all the blame on killer whales. Conservation biologists have long thought these populations of marine mammals were suffering because the fisheries industry was stealing all their food. Sea changes, pollution and disease may also play a role.

"It is a complicated issue, and I don’t think it will be neatly packaged into one explanation. Ever," says Reeves.

Source: www.newscientist.com