Pinniped Predation on Salmonids in the Washington Portions of the Salish Sea and Outer Coast

Populations of harbor seals, Steller sea lions, and California sea lions (hereafter ‘pinnipeds’) have increased substantially in the Salish Sea and coastal waters of Washington State following implementation of the US Marine Mammal Protection Act (MMPA) in 1972. During this period, many populations of Pacific salmon in Washington waters, which are at depressed levels and several of which are federally listed under the US Endangered Species Act, have declined in abundance or have failed to recover and continued to exist at low abundance. Because pinnipeds are abundant and widely known to be predators of both juvenile and adult Pacific salmon, these marine predators have been implicated as a primary factor contributing to continued depressed populations of salmon in Washington State.

The Washington Department of Fish and Wildlife asked the Washington State Academy of Sciences to examine the scientific basis for the concern that recovery of salmon populations in Washington State’s Salish Sea and outer coastal waters has been impeded by pinniped predation. This report is a summary of the WSAS committee’s findings, following critical review of the existing literature on this topic and from information provided by scientists, managers, tribal representatives, and other participants in workshops.

The report is organized to provide a review of existing evidence about pinniped and salmonid populations in the Salish Sea, pinniped predation on salmonids, and the impacts of pinniped predation on salmon recovery. Key findings are summarized below.

Pinniped and Salmonid Populations in the Salish Sea

The most common pinnipeds inhabiting the Washington State Salish Sea and outer coastal waters are harbor seals, California sea lions, and Steller sea lions, and all have increased in number substantially since the MMPA was enacted by the US Congress in 1972. Although this report makes no attempt to evaluate whether current population levels are at carrying capacity in Washington State, we acknowledge that historical human harvests of pinnipeds likely kept pinniped population sizes smaller than those currently observed. Unfortunately, pre-MMPA population data are not available to provide reliable baseline comparison levels for present day abundance of pinnipeds.

Salmonids are an important economic and cultural resource for Washington State. Populations of wild Chinook salmon, coho salmon, sockeye salmon, and steelhead have decreased from the 1970s to the present.

Trophic interactions involving pinnipeds and salmonids in the Salish Sea and Outer Coast are numerous and complex. Pinnipeds and salmonids exist within a greater ecological context and food web; many of those trophic interactions have the potential to either mediate the impact of pinniped predation on salmonids or affect the outcome of management actions aimed at reducing pinniped predation on salmon. The increase in pinniped populations in Washington State since the enactment of the MMPA has likely influenced the structure of the entire ecosystem. Given the large number of trophic links between pinnipeds and salmonids and the potential for direct and indirect ecological interactions, it is impossible to predict with certainty the outcomes for salmon and the rest of the food web under scenarios where the pinniped population size is changed.

Pinniped Diet Composition

It is widely understood that harbor seals, Steller sea lions, and California sea lions are predators of all age classes of Pacific salmon. Detailed studies of stomach contents of dead pinnipeds and examination of pinniped scat also suggest substantial variation across locations within Washington State waters and over time with respect to species and age-classes of salmon and the proportion of the pinniped diet they constitute. In specific locations and during certain seasons, pinniped predation on salmon is intense and reduces the number of adult salmon reaching spawning grounds. Diet studies also show that pinnipeds have a broad diet composition, including a variety of species beyond salmonids that include species that are both predators of salmon (e.g., Pacific hake) and prey of salmon (e.g., Pacific herring).

Several modeling approaches have been used in recent years to estimate the number of salmon consumed by pinnipeds in Washington State waters. While all make simplifying assumptions about the nature of pinniped-salmon interactions, all demonstrate that the number of salmon eaten by pinnipeds currently is substantial and has increased steadily since the passage of the MMPA, paralleling pinniped population increases. Although these reconstructions are useful for understanding general trends in predation intensity in Washington State waters, they do not necessarily provide an accurate reflection of specific predation rates on individual salmon populations, nor do they allow an explicit determination of whether pinniped predation is depressing salmon abundances at either the ecosystem level or on individual salmon stocks.

Rate of Pinniped Predation on Salmonids

Due to low abundance of threatened salmonids and large numbers of pinnipeds, even minimal predation can strongly impact salmonid stocks. Rates of pinniped predation on salmonids vary spatially, seasonally, and intra-annually, and by sex of the pinniped.

Certain aspects of the natural and constructed environment can affect pinniped predation on salmon by causing salmon to congregate in certain areas, influencing salmon migration and anti-predation behaviors, or increasing pinniped predation behavior. Salmon aggregate in the marine environment at habitat features that increase biological productivity. Artificial structures can interfere with salmon migration behavior and cause increased vulnerability to predation by compromising or reducing the effectiveness of salmonid anti-predator behaviors.

Several lines of evidence suggest that some individual pinnipeds act as salmon ‘specialists’, preying heavily on salmonids. It is thought that the population-level generalist diet of harbor seals in the Salish Sea and Steller sea lions along the Outer Coast is actually comprised of a mixture of individual specialists. Importantly, pinnipeds can learn successful foraging habits and change their foraging behavior based on knowledge transmitted by others of their species. Research suggests that the size, behavior, and origin of salmon may also play a role in determining which salmon are consumed by pinnipeds.

There is some evidence of prey buffering, the hypothesis that pinniped consumption of salmonids would be reduced by the increased presence of alternative prey species such as herring. However, it is not known if feeding on abundant alternative prey would increase pinniped populations, potentially intensifying the impact of pinniped predation on salmonids.

Impacts of Pinniped Predation on Salmon Recovery

The evidence summarized above is consistent with the hypothesis that pinniped predation is a plausible explanation for reduced abundance of salmon in Washington State waters and lack of salmon recovery following efforts to protect them. However, this evidence does not support a definitive conclusion that pinnipeds are a primary cause of the lack of salmonid population recovery in these ecosystems. Among the most important sources of uncertainty are: 1) whether pinniped predation appreciably adds to the mortality of salmon or whether pinnipeds are simply killing individuals that would otherwise die before maturing to adulthood (i.e., ‘compensatory’ mortality), 2) the role of alternative prey (e.g., herring) in either increasing pinniped populations and thus predation rates on salmon or decreasing predation by providing alternative food sources, and 3) whether the indirect effect of pinniped predation on salmon predators such as Pacific hake offsets the direct impact of pinniped predation on salmonids.

Scientific research to improve our understanding of pinniped - salmon ecological interactions (e.g., further characterization of behavior and diet) will resolve some of the uncertainties in our understanding of the role of pinnipeds as predators in salmon food webs. Development and refinement of models to synthesize field observations will be essential in interpreting emerging information from new and ongoing field studies. These focused studies will continue to build the body of knowledge about species interactions. These approaches, however, are not likely to lead to robust conclusions about the role of pinniped predation in the depression of Washington State salmon populations. Providing concrete answers to the question that motivated the WDFW request for this report – Are pinnipeds currently impeding the recovery of salmon? – will require robust adaptive management approaches that experimentally change pinniped populations at spatial and temporal scales that can meaningfully impact the ecosystem.

Strategic and appropriately scaled adaptive management of pinniped populations is key to resolving these uncertainties but will require carefully constructed lethal removals and intensive monitoring of salmon. Other approaches are unlikely to lead to fundamentally new insights. Importantly, however, current uncertainties about the salmon-pinniped system should not be perceived as an obstacle to adaptive management, but rather should motivate well-crafted experimental approaches funded with adequate resources.

Such experiments might involve changing the MMPA to allow applications from researchers from Tribal, State, or Federal governments for research permits of the MMPA for more geographically focused manipulations of local pinniped behaviors or abundances or the encouragement of treaty-protected tribal harvests of pinnipeds. Because the MMPA currently imposes severe constraints on the potential scope of such experiments, meaningful management action within the waters of coastal Washington or the Salish Sea is unlikely in the absence of legislative changes to the Act. However, maintaining the status quo of management actions without a more thorough understanding of the role of pinnipeds in this ecosystem could further depress salmon populations that play an important ecological, social, and economic role in the inner and outer coastal ecosystems of Washington State.