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Puget Sound Study: Pinniped Predation On Juvenile Salmon Making Salmon Recovery More Difficult
Posted on Friday, June 23, 2017 (PST)

A complicated weave of protected species – both fish and mammals – in Puget Sound highlights the issues fish and wildlife managers face in recovery choices.


Since 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.


In 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.


Like 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.


“Predator 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.”


According 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.


Steller sea lions in 2015 consumed 104,000 adult chinook salmon, while California sea lions consumed about 55,700 adults.


Most 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.


All 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).


When translated to returning adults, the numbers are impressive.


“Our 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.”


That’s 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.


The 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.


“Although 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 study concludes.


“Estimates 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,


Co-authors 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, Seattle.


Chasco 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.”


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