Recovering populations of killer whales, sea
lions and harbor seals on the West Coast have dramatically increased their
consumption of chinook salmon in the last 40 years, which may now exceed the
combined harvest by commercial and recreational fisheries, a new study finds.
While the recovery of marine mammals
represents a conservation success, it creates complex tradeoffs for managers
also charged with protecting the salmon they prey on, the study concludes. The
U.S. Marine Mammal Protection Act of 1972 protects all marine mammals,
including whales and pinnipeds (seals and sea lions) within the waters of the
United States. The Endangered Species Act protects nine West Coast populations
of chinook salmon.
The study https://www.nature.com/articles/s41598-017-14984-8 was
published last week in the journal Scientific Reports. The findings resulted
from a collaboration of federal, state and tribal scientists in the Pacific
Northwest, including Oregon State University and NOAA Fisheries. The research
was designed in part to understand the pressures on chinook salmon consumed by
southern resident killer whales, which in contrast to other killer whale
populations are endangered and show few signs of recovery.
Southern residents spend much of the year in
the inland waters of Washington and consume about the same volume of salmon
today as they did 40 years ago, the study found. The study suggests that, at
least in recent years, competition with other marine mammals may be more of a
problem for southern residents than competition with human fisheries.
"We have been successful at restoring and
improving the population status of protected marine mammals," said Brandon
Chasco, a doctoral candidate at Oregon State University and lead author of the
study. "But now we have the potential for protected seals and sea lions to
be competing with protected killer whales, and all of which consume protected
The study used models to estimate marine
mammal consumption of chinook salmon based on several assumptions about their
diet and the size and weight of salmon. The researchers estimate that from 1975
to 2015, the yearly biomass of chinook salmon consumed by pinnipeds (sea lions
and harbor seals) and killer whales increased from 6,100 to 15,200 metric tons,
and from five to 31.5 million individual salmon.
Over the same time span, they found that
annual fisheries harvest decreased from 16,400 to 9,600 metric tons, and from
3.6 million to 2.1 million individuals.
Overall, several growing populations of
resident killer whales in Canada and southeast Alaska are estimated to consume
the largest biomass of chinook salmon, but harbor seals consume the largest
number of individuals, including juvenile chinook salmon, according to the
Salmon recovery programs underway up and down
the West Coast have boosted numbers of wild salmon, the research found. However,
increased predation by recovering marine mammals may be offsetting reductions
in recreational and commercial harvests, and "masking the success of
coast-wide recovery efforts," the scientists wrote.
Isaac Kaplan, a research fishery biologist at
NOAA Fisheries' Northwest Fisheries Science Center and a coauthor on the study,
said the researchers quantified only one of many challenges to chinook salmon
“The better we understand the different
obstacles to salmon recovery, the better we can account for them as we plan and
carry out recovery programs," Kaplan said. "Recovery efforts must
account for all of these challenges, and we're providing more details about one
important part of that picture."
The Columbia River has previously been
identified as an area with high marine mammal consumption of salmon,
specifically by seals and sea lions in the estuary. The researchers found that
in 2015 in the Columbia River, harbor seals on the river consumed 14 metric
tons of chinook salmon, compared to 219 and 227 metric tons consumed by
California and Steller sea lions, respectively.
Considering the consumption of just adult
chinook salmon in 2015, the researchers estimated that harbor seals consumed
1,000 adult chinook salmon, while California sea lions consumed 46,000, and
Steller sea lions consumed 47,000.
"Consumption in the ocean is also a
significant source of mortality, but has been largely unmeasured until
now," said Chasco, a National Marine Fisheries Service-Sea Grant
Population Dynamics Fellow in the Department of Fisheries and Wildlife in OSU's
College of Agricultural Sciences. "Now managers have more information to
work with in balancing these difficult tradeoffs."
Study collaborators included researchers at
NOAA Fisheries' Northwest Fisheries Science Center in Seattle, Smith-Root, a
fisheries conservation technology firm in Vancouver, Washington; Western
Washington University; Makah Fisheries Management in Neah Bay, Washington; the
Washington Department of Fish and Wildlife; and the North Gulf Oceanic Society
in Homer, Alaska.
The Pacific Salmon Commission funded the