SEATTLE – Hungry young orcas grow up to be stunted orcas, new research shows, revealing that salmon run downturns can have lifelong effects.
The findings, published last month in the peer-reviewed scientific journal Endangered Species Research, were based on aerial photos taken by drone of whales in both the southern and northern resident orca populations.
The photos document just how closely the health of resident killer whales is tied to the abundance of their favorite prey: big chinook salmon.
Younger whales born since the 1980s in both the northern and southern populations of salmon-eating resident orcas are shorter in length than older whales that grew up when chinook runs were more abundant, the photos revealed.
It was a significant difference: The stunted whales growing up in lean times were on average nearly half a meter shorter than older adults, according to the paper published by authors from SR3: Sealife Response, Rehabilitation and Research, the National Oceanic and Atmospheric Administration, Vancouver Aquarium and Southall Environmental Associates Inc.
The findings suggest the effects of hunger not only can be lethal, taking out calves and adults, but also can have long-term consequences for the condition of the whales that survive, said John Durban, author and senior scientist with Southall Environmental Associates, or SEA. "It was shocking; some of these effects are pretty big," Durban said. "The average difference in size is a couple of feet."
The stunted whales actually were the lucky ones; some of the others born and growing up in the same time frame didn't make it at all. A spike in killer-whale deaths tracked closely with a West Coast-wide crash in chinook abundance in the 1990s, according to a 2009 paper published in Biology Letters, by lead author John K.B. Ford, working then for Canada's Department of Fisheries and Oceans.
While that relationship is changing statistically, scientists know lack of adequate prey is affecting southern whales' survival. So are boat noise and pollution. Scientists also are looking at inbreeding and disease as contributors to the decline of the southern orcas.
Northern and Alaskan resident killer whales have grown significantly in population even as the southern residents have declined. The northern and Alaskan residents have several advantages over the southern orcas.
They have access to a wider variety of fish runs and cleaner, quieter water. They also get first crack at fish of Washington origin. Many of the chinook born in Washington rivers head north to British Columbia and the Gulf of Alaska, where they are caught by fishermen and orcas before the southern residents ever get a chance at them.
The southern orcas were listed as an endangered species in 2005. Their population goes up and down, but recently has continued to decline and is now at a 30-year low. There are only 73 left.
Meanwhile, chinook also are shrinking. Jan Ohlberger of the School of Aquatic and Fishery Sciences at the University of Washington and other authors have documented a pattern of smaller body size in chinook over the past 40 years. Fish today measure up to 9% shorter than in the past. Chinook have shrunk in Alaska. They shrank all down the coast of Washington and clear to California.
Killer whales preferentially target large chinook because they deliver more calories for the hunting effort. The shrunken chinook mean orcas have to work harder to get enough to eat.