January 18, 2019
By Gene Johnson, The Associated Press
SEATTLE -- Over the years, scientists have identified dams, pollution
and vessel noise as causes of the troubling decline of the Pacific
Northwest's resident killer whales. Now, they may have found a new and
more surprising culprit: pink salmon.
Four salmon researchers were perusing data on the website of the Center
for Whale Research, which studies the orcas, several months ago when
they noticed a startling trend: that for the past two decades,
significantly more of the whales have died in even-numbered years than
in odd years.
In a newly published paper, they speculate that the pattern is related
to pink salmon, which return to the Salish Sea between Washington state
and Canada in enormous numbers every other year -- though they're not
sure how. They suspect that the huge runs of pink salmon, which have
boomed under conservation efforts and changes in ocean conditions in the
past two decades, might interfere with the whales' ability to hunt their
preferred prey, Chinook salmon.
Given the dire plight of the orcas, which officials say are on the brink
of extinction, the researchers decided to publicize their discovery
without waiting to investigate its causes.
"The main point was getting out to the public word about this biennial
pattern so people can start thinking about this important, completely
unexpected factor in the decline of these whales," said one of the
authors, Greg Ruggerone. "It's important to better understand what's
occurring here because that could help facilitate recovery actions."
Ruggerone, president of Seattle-based Natural Resources Consultants and
former chairman of the Columbia River Independent Scientific Advisory
Board, and the other authors -- Alan Springer of the University of
Alaska at Fairbanks, Leon Shaul of the Alaska Department of Fish and
Game, and independent researcher Gus van Vliet of Auke Bay, Alaska --
have previously studied how pink salmon compete for prey with other
As news stories chronicled the struggles of the orcas last year -- one
whale carried her dead calf on her head for 17 days in an apparent
effort to revive it -- the four biologists looked at data on the Center
for Whale Research's site. Thanks to their previous research, it took
them only a few minutes to recognize a trend that had escaped the
attention of other scientists.
"We know that some are good years for the whales and some are bad years,
but we hadn't put it together that it was a biennial trend," said Ken
Balcomb, the centre's founding director, one of the foremost experts on
the so-called Southern Resident killer whales.
Further analyzing the data, the researchers found that from 1998 to
2017, as the population of whales decreased from 92 to 76, more than 3.5
times as many newborn and older whales died during even years -- 61,
versus 17 in odd years. During that period, there were 32 successful
births during odd years, but only 16 during even years.
That biennial pattern did not exist during a prior 22-year period from
1976 to 1997, when the whale population was recovering from efforts to
capture orcas for aquarium display, the researchers said.
But in 1998, salmon harvests were curtailed amid efforts to boost runs
decimated by overfishing, pollution and habitat loss. A strong change in
ocean conditions occurred around the same time, benefiting pink salmon
especially by increasing the abundance of zooplankton, which make up
much of the pink salmon's diet.
The combined effect of the ocean changes and fishing restrictions has
greatly benefited the pinks, which are by far most numerous salmon
species in the North Pacific. When they return to the Salish Sea, there
are about 50 for each of the bigger, fattier Chinook. Nearly all pinks
return to their natal streams in odd years, completing their two-year
life cycle, unlike other salmon, which stay in the ocean longer.
Meanwhile, Chinook populations have continued to struggle -- the dearth
of Chinook is considered the most severe threat to the orcas -- and many
scientists say they will continue to do so unless four dams on the Lower
Snake River are breached. The researchers speculate that the blossoming
numbers of pinks in the Salish Sea during odd-numbered years have
interfered with the echolocation the orcas use to hunt increasingly
sparse Chinook. The orcas almost never eat pink salmon.
Because the whales are such large mammals, the theory goes, the stress
caused by the pinks in odd years would not affect their mortality rates
and reproductive rates until the following year -- and that's why more
die in even years.
Another possibility is that presence of pinks means less food for the
Chinook -- and thus less food for the orcas, Ruggerone said.
The researchers also put forth a contrary hypothesis: that the presence
of pinks somehow enhances the orcas' hunting, improving their survival
in odd-numbered years -- though they say they have no reason to believe
that's the case.