Scientific Studies & Reports
The PWS Wild Salmon Protection Act is grounded in science to be an incredible tool for saving our wild salmon.
A global synthesis of 51 years of peer-reviewed studies on the interaction between hatchery and wild salmon, conducted by researchers from the United States and Canada, determined that human interventions intended to enhance natural processes have generally produced detrimental rather than beneficial outcomes.
Large-scale hatchery programs, while designed to supplement fisheries, can undermine wild salmon by reducing genetic diversity, weakening reproductive success, and creating competition for food in the open ocean.
This page compiles a selection of peer-reviewed studies and technical reports documenting hatchery–wild salmon interactions. These findings provide the scientific foundation for the Act’s enforceable limits on hatchery-origin straying, mandatory monitoring, and stock-of-concern designations.
Recent Studies, Reports, and Other Scientific Literature
Hatchery expansion isn’t delivering proportional returns.
“Releases of coho salmon … increased steadily … from fewer than 1 million fish in 1980 to nearly 24 million fish in 2016. … However, during the most recent 5-year period (2012–2017), the average harvest of hatchery fish increased only slightly (5%) … despite the near doubling of hatchery production.” Shedd et al. (2022).
The Act's cap on hatchery-origin spawners (pHOS) avoid flooding systems with fish that don’t improve yields but instead hurt wild stocks.
When too many hatchery fish stray into natural streams, they can overwhelm wild populations, diluting wild salmon genetic diversity and resilience.
“While wild salmon populations are diverse and resilient, it is crucial to consider how management strategies might modulate mechanisms underlying pHOS and temporal segregation to mitigate for risks of introgression and homogenization.” May et al. (2024).
The Act sets enforceable pHOS limits to prevent homogenization of wild stocks.
Samuel A. May et al. (2024)
Salmon hatchery strays can demographically boost wild populations at the cost of diversity: quantitative genetic modeling of Alaska pink salmon
A multi-generational genetic modeling study showing that while hatchery-origin strays can increase spawner abundance in the short term, they erode long-term genetic diversity and adaptability of wild pink salmon populations.
McMillan, J.R. et al. (2023)
A global synthesis of peer-reviewed research on the effects of hatchery salmonids on wild salmonids. McMillan, J.R., Morrison, B., Chambers, N., Ruggerone, G.T., Bernatchez, L., Stanford, J., & Neville, H.M. (2023). Fisheries Management and Ecology.
A global review of 206 peer-reviewed studies (1970–2021) evaluating how hatchery salmonids affect wild salmonids across genetic, ecological, fishery, and disease pathways.
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83% of studies found adverse or minimally adverse effects on wild salmonids.
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Genetic risks (loss of diversity, introgression, population homogenization) were the most common adverse findings.
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Reported benefits were rare (3%) and mostly limited to short-term, intensive recovery programs for highly depleted stocks.
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Evidence also points to density-dependent effects in the ocean where large hatchery releases can depress wild survival and growth.
Kyle Shedd et al. (2022)
Reduced relative fitness in hatchery‐origin Pink Salmon in two streams in Prince William Sound, Alaska. Evolutionary Applications, 15(3), 429–446
Field-based study in PWS showing that hatchery-origin pink salmon have lower reproductive success compared to natural-origin salmon, raising long-term concerns for wild population viability.
Shaul, L.D., et al. (2019)
Coho Salmon Hatchery Production and Survival Trends in Southeast Alaska, 1980–2017 (2019) (unpublished Alaska Dep’t of Fish & Game Technical Report)
Long-term analysis showing that despite hatchery coho production in Southeast Alaska more than doubling since the 1980s, harvest contributions have stagnated while survival rates of hatchery fish declined relative to wild stocks. Findings indicate that hatchery fish often replace, rather than add to, wild harvests and may undermine the sustained yield of wild salmon.
Tillotson, M.D. et al. (2019)
Artificial selection on reproductive timing in hatchery salmon drives a phenological shift and potential maladaptation to climate change. Evolutionary Applications, 12(7), 1344–1359.
Long-term analysis of a hatchery-supplemented sockeye population shows that hatchery practices select for earlier spawning, advancing median spawning dates by weeks. Because timing is highly heritable, this hatchery-driven shift can maladapt fish to local conditions (e.g., warmer water, prey mismatch), reducing productivity and resilience. Findings underscore that hatchery selection can erode the adaptive diversity wild populations need under climate change.
Jalili Kolavani, N., & Mather, C. (2025)
Regulating a “fish out of place”: A global assessment of farmed salmon escape policies and frameworks
Jalili Kolavani, N., & Mather, C. (2025). Marine Policy, 173, 106572.
A comparative policy review of 14 top salmon-farming regions assessing regulations to prevent and respond to farmed salmon escapes from open net-pens.
Key findings include:
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All 14 regions now have escape policies, but their effectiveness is limited by gaps and inconsistent enforcement.
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Escapes are inevitable with open net-pen infrastructure; monitoring and reporting don’t prevent escapes on their own.
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Sanctions are often weak; stronger regimes (e.g., Norway, Chile) pair stricter penalties with technical standards; Washington State went further by ending Atlantic salmon net-pens in state waters after a major escape.
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Tools like triploid (sterile) fish can reduce genetic introgression risk, but don’t stop escapes or other ecological effects (disease transmission, competition).
Sabrina Larson (2015)
STUDENT THESIS: Policy, Management, and Production Strategies to Reduce the Risks of Hatchery-Wild Interactions in Alaska Salmon Hatcheries (Dec. 2015) (M.S. thesis, Alaska Pacific University).
Larsen’s thesis examines Alaska’s salmon hatchery programs, linking policy and hatchery practices to biological risks from hatchery–wild interactions. She evaluates maternal (female-level) effects and pressure-shock duration in a Chinook triploidy program at the William Jack Hernandez Sport Fish Hatchery, finding that female-specific variability and incubation position meaningfully influence survival and triploid outcomes. Evidence suggests that longer pressure shocks can improve triploidization rates, but survival trade-offs, logistical constraints, and ploidy-testing challenges (e.g., freeze–thaw effects on DAPI fluorescence) complicate reliable implementation. Overall, Larsen argues that while triploids can reduce hybridization risks, large-scale deployment in Alaska’s nonprofit hatchery sector is likely impractical due to cost, consistency, and performance concerns.
Hatcheries drive wild salmon to spawn weeks earlier than natural timing, a heritable shift that can erode genetic diversity and reduce resilience under climate change.
“Since its initiation in 1991, the hatchery has, on average, selected for earlier spawning, and, depending on trait heritability, could have advanced spawning by 1–3 weeks over this period. Artificial selection for early spawning may be maladaptive… potentially causing mismatch with favorable conditions and reducing the population’s productivity.” Tillotson et. al. (2019).
By limiting hatchery strays, the Act safeguards wild salmon genetics and resilience.