A complicated weave of protected species – both fish and mammals – in Puget Sound
highlights the issues fish and wildlife managers face in recovery choices.
the Marine Mammal Protection Act was passed in 1972, populations of marine
mammals, such as harbor seals, and Steller and California sea lions have
rebounded. However, with the increase in mammal numbers has come a
corresponding increase in predation on salmon in Northwest waters, according to
a recent study.
a domino effect, predation, along with other factors, is making it more
difficult to restore runs of chinook salmon in Puget Sound, listed as
threatened under the federal Endangered Species Act in 1999, and the decline in
salmon is making recovery more difficult for southern resident killer whales in
the Sound, which were listed as endangered in 2005.
in the Columbia River, sea lions are the primary predator on adult salmon, but
so are killer whales in Puget Sound. As it turns out, though, harbor seals that
eat mostly juvenile chinook salmon may be causing the most damage.
consumption of adults may have less of an impact than predator consumption of
juveniles,” said Brandon Chasco, a graduate student at Oregon State University
and a co-author of the study. “Predator consumption may have masked some of the
benefits of salmon recovery efforts.”
to the study, across all seasons, harbor seals consumed 1.1 million adult and
juvenile chinook salmon in 1970, about 13 times more than the 84,500 salmon
consumed by killer whales. By 2015, harbor seals were consuming about 8.6
million salmon, while killer whales remained at a fairly steady 83,200 salmon
(their population in Puget Sound has remained constant for the last 40 years).
By 2015, seals were consuming greater than 104 times more than the whales.
sea lions in 2015 consumed 104,000 adult chinook salmon, while California sea
lions consumed about 55,700 adults.
of the increased consumption of salmon by pinnipeds has been on juvenile
salmon, according to the study. Consumption of just hatchery smolts by harbor
seals increased from about 1 million in 1970 to 8.5 million in 2015, while
consumption of adult chinook salmon increased from 10,400 to 89,000. In comparison,
consumption of adults by both Steller and California sea lions increased from
1,800 in 1970 to 143,900 in 2015.
pinnipeds, but especially harbor seals, eat juvenile salmon, particularly
during the smolts’ spring migration. The study estimates that during the spring
migration, each seal eats about 5.4 smolts per day. In 1970, the authors
estimate the seals ate about 1.8 percent of migrating smolts, but that
increased in 2015 to 22.4 percent (the latter number is based on an approximate
hatchery smolt release of 40 million fish).
translated to returning adults, the numbers are impressive.
results suggest that the total adult returns within Washington State inland
waters during 2015 would be diminished by 1,000 individuals due to California
sea lions, 1,900 due to Steller sea lions, and 158,700 due to harbor seals,”
the study says. “Summed across all pinnipeds, the total annual potential
mortality increased from 18,800 in 1970 to 161,600 in 2015.”
double the consumption by killer whales. All these numbers are expressed as
adult equivalents. The study says that there is evidence that variations in
births and deaths in the killer whale population is linked to the rise and fall
in the number of salmon available to eat.
numbers of salmon also impact commercial and recreational fishing in Puget
Sound. The commercial catch has declined from about 250,000 adult chinook
salmon in 1980 to 100,000 in 2007, and the recreational catch has declined from
about 150,000 to 50,000.
our assumptions are sensitive to assumptions regarding both marine mammals and
Chinook salmon, the main results consistently suggest that across a broad range
of parameter values, harbor seals and fish-eating ‘resident’ killer whales
account for a large majority of consumption of Chinook salmon biomass, and
harbor seals consume many more Chinook salmon in terms of numbers of fish,” the
of Chinook salmon consumption in Washington State inland waters by four marine
mammal predators from 1970 to 2015” was published online January 9, 2017 in the
Canadian Journal of Fisheries and Aquatic Sciences, http://www.nrcresearchpress.com/doi/abs/10.1139/cjfas-2016-0203#.WUk1vGjyvIU
are Isaac Kaplan, Dawn Noreen, Michael Ford, M. Bradley Hanson and Eric Ward,
all with NOAA Fisheries Northwest Science Center in Seattle; Austen Thomas,
Smith-Root, Vancouver, WA; Alejandro Acevedo-Gutiérrez, Department of Biology,
Western Washington University, Bellingham; Jonathan Scorpion, Makah Fisheries
Management, Neha Bay, WA; Steve Jeffries and Scott Pearson, Washington
Department of Fish and Wildlife; and Kristin Marshall, Cascade Ecology,
said the authors, as well as other scientists from Oregon to Alaska, are “very
close to submitting a study that applies this method to the eight spatial boxes
on the west coast, and the Columbia River is one of them.”