Researchers uncover a whale of a mystery
Orcas three times more likely to die in even years
Southern resident killer whales are three times more likely to die in even years than odd years, a pattern that cannot be explained by the widely accepted threats to their health.
Successful orca births are also 50-per-cent less frequent in even years than odd years over the past two decades, according to a study by Washington state researchers.
Factors such as shipping noise, toxin accumulation and the scarcity of prey such as chinook salmon cannot account for this mysterious effect, said lead researcher Greg Ruggerone.
Since 1998, 61 southern residents have died in years ending in an even number, while just 17 died in odd years, according to data from the Center for Whale Research.
Thirteen newborn and four yearling calves died in even years, while just five newborns died in odd years.
This pattern suggests there is a potent and previously unknown threat to the health of the endangered community of orcas that summer in the Salish Sea, between Campbell River and Seattle.
The study’s authors, who specialize in pink salmon research, twigged to the pattern while looking at graphs of whale mortality on the Center for Whale Research web page.
“It took about one minute to see this pattern,” said Ruggerone. “When we looked at the birth data, that fell into place as well.”
Until the birth of a calf earlier this month, the southern residents had not had a successful birth in three years and in 2018 three members died, leaving just 74 whales.
The authors believe that hyper-abundant pink salmon may be the X factor, because their numbers surge every other year due to their short two-year life cycle.
A series of studies over the past decade has revealed that the dramatic growth in the number of pinks is strongly associated with the decline of some sockeye and chinook runs and even the breeding success of certain sea birds.
The number of sockeye, pink and chum in the North Pacific is higher than it has ever been and pinks are by far the most numerous, according to studies in 2010 and 2018.
Pinks appear to be climate-change winners, thriving even as waters in the North Pacific Ocean and spawning rivers get warmer and they are out-competing other salmon species for food in the open ocean.
“People don’t realize what having so many (pinks) in the ocean all eating does to other species,” said Andrew Trites, head of UBC’s Marine Mammal Research Unit. “What does that leave for others to eat?”
Up to 15 million pinks return to the Fraser River through the Salish Sea in odd years at roughly the same time as summer and early fall chinook runs, when they outnumber chinook about 50 to one.
How pink salmon affect the health of the southern residents — directly or indirectly — is a mystery. But there are clues.
“Our leading hypothesis is that (hyper-abundant) pink salmon interfere with the southern residents’ foraging efficiency when they are attacking chinook,” he said. “Perhaps they are interfering with the whales’ echolocation.”
The nutritional deficit is assumed to weaken the whales, resulting in higher mortality, miscarriages and newborn mortality the following year.
Starving whales burn into their fat reserves after a lean summer, which would flood their bodies with accumulated toxins and interfere with reproduction, he suggested.