the past 20 years, endangered Southern Resident Killer Whales (Orcas) in the
Salish Sea have had more births and fewer deaths in odd years than in even
years, according to a recent study.
biennial pattern coincides with an “exceptional abundance” of pink salmon in odd
years, leading scientists to wonder if pink salmon abundance in the Sea during
years when Orca birth rates are low and their death rate is higher are
1998 to 2017, mortality of newborn and older whales was 3.6 times higher (61
versus 17 whales) and successful births 50 percent lower (16 versus 32 whales)
in even years than in odd years. During that 20-year period, the population of
the Southern Resident Killer Whales declined from 92 to only 76 whales.
the recent 20 year period of population decline, mortality was 3.1 times higher
in even years than it was during the earlier 22 year period (1976-1997) of
population increase, whereas mortality in recent odd years was 43 percent
lower, according to the study’s lead author, Dr. Greg Ruggerone, research
scientist at Natural Resources Consultants in Seattle.
to the State of Washington’s Southern Resident Killer Whale Task Force, the
leading reasons for the whales’ decline has been the low abundance of its key
prey species (chinook salmon), toxic contaminants and vessel noise. (See CBB,
November 20, 2018, “Orca Recovery Task Force Recommendations Include
Considering Removal Of Lower Snake Dams,” http://www.cbbulletin.com/441811.aspx.)
those factors alone do not explain the biennial pattern, Ruggerone said.
leading hypothesis is that the exceptional abundance of pink salmon returning
to the Salish Sea in odd years (avg. 17.8 million versus 0.4 million in even
years) interferes with foraging of the killer whales that target chinook salmon
during spring through early fall,” Ruggerone said. “We present evidence that
the causal mechanism is indirectly linked to pink salmon (O. gorbuscha), which
exhibit a unique and extreme biennial pattern of abundance and interact
strongly with other species in marine ecosystems in the North Pacific.”
biennial pattern began almost exactly when the Southern Resident population
began its decline and when pink salmon escaping fisheries in the Salish Sea
more than doubled, he added.
said that diet studies show that Southern Resident Killer Whales almost never
eat pink salmon, but the abundance of pink salmon in odd years could interfere
with both chinook salmon and the whales:
Pink salmon return to the Salish Sea from approximately mid-July to early
September, which is about the same time as summer-run chinook and the early
portion of fall run chinook salmon.
Pink salmon migrate along the same migration route as chinook salmon (e.g. west
side of San Juan Island and into Boundary Pass as they return to the Fraser
River), which is the core foraging area of the Southern Residents.
Pink salmon migrate along surface waters whereas chinook are deeper.
SRKW can distinguish between pink salmon and the larger chinook salmon via
echolocation, we suspect that the high density of pink salmon (about 50 pinks
to one chinook) reduces foraging efficiency, especially in years when chinook
abundance is relatively low,” Ruggerone said.
the period of Southern Resident decline, the number of pink salmon escaping
fisheries in this region more than doubled (because fishing effort was reduced
to conserve chinook and sockeye salmon), he said.
mortality occurred primarily in even years because other studies show a one
year lag between low food abundance and mortality--this lag is expected in such
a large mammal,” he said.
studies by Ruggerone and others have documented the large impact of pink salmon
on the North Pacific ecosystem, including zooplankton, phytoplankton, other
species of salmon, and seabirds. (See CBB, April 13, 2018, “Carrying Capacity:
High Numbers Of Pink, Chum Salmon In North Pacific May Be Hurting Chinook,” http://www.cbbulletin.com/440518.aspx)
salmon returning from the North Pacific have never been more abundant than they
were during 2005-2015 (since detailed records began in 1925),” Ruggerone said,
adding that pink salmon account for nearly 70 percent of all adult Pacific
scientists do have an alternative hypothesis: the high mortality in even years
since 1998 is the normal response to low chinook salmon abundance during this
recent period of decline, and reduced mortality in odd years is because pink
salmon somehow enhance SRKW foraging in odd years.
do not have a mechanism that explains this hypothesis, but we offer it so that
it may be considered and evaluated in future analyses,” Ruggerone said.
study, “Unprecedented biennial pattern of birth and mortality in an endangered
apex predator, the southern resident killer whale, in the eastern North Pacific
Ocean,” was published online Jan. 3, 2019 in the Marine Ecology Progress Series
co-authors are Dr. Alan Springer, professor emeritus, University of Alaska,
Leon Shaul, fisheries biologist III, Alaska Department of Fish and Game, and
Gus van Vliet, retired scientist.
hypothesis conflicts with current thought about Southern Resident decline,
which is due to a lack of chinook salmon. However, Ruggerone said, the biennial
pattern adds complexity. Still, this complexity also offers keys to better
understanding the decline of Southern Residents and so provides additional
ideas for enhancing their recovery.
types of studies are needed to test the opposing hypotheses, he said. The first
is a field study to evaluate the foraging efficiency of Southern Residents
during odd versus even years. The second study is to model Southern Resident
mortality and births in relation to chinook salmon, pink salmon, and other
have begun the modeling effort but progress is slow because we have no funding
for this effort,” Ruggerone said. “We have begun developing actions needed to
improve whale foraging, depending on the outcome of these studies.”
CBB, September 28, 2018, “Orca Task Force Recommendations Include Focus On
Salmon Runs; Non-Native Game Fish To ‘Predatory,’” http://www.cbbulletin.com/441561.aspx
CBB, Sept. 14, 2018, “NOAA Fisheries Studying Nighttime Behavior Of Endangered
Killer Whales As Part Of Action Plan” http://www.cbbulletin.com/441483.aspx
May 11, 2018, “Puget Sound Boaters Asked To Observe ‘No-Go’ Zone To Protect
Foraging Orcas,” http://www.cbbulletin.com/440697.aspx
March 16, 218, “Washington Governor Signs Executive Order To Protect Orcas,
Chinook Salmon” http://www.cbbulletin.com/440354.aspx
CBB, Jan. 15, 2016, “Study: Chinook Salmon Make Up 80 Percent Of Diet For
ESA-Listed Killer Whales In Pacific Northwest” http://www.cbbulletin.com/435857.aspx