In response to the Northwest Power and Conservation Council’s April 25, 2016 request, the Independent Scientific Review Panel evaluated the finding from the Idaho Supplementation Studies Project Completion Report 1991-2014. This completion report was drafted under project 1989-098-00, Idaho Supplementation Studies (ISS), by staff from the Idaho Department of Fish and Game (IDFG), Nez Perce Tribe (NPT), and Shoshone-Bannock Tribe (SBT), in collaboration with the US Fish and Wildlife Service. The ISS was designed to measure the population effects of intentional supplementation on the abundance and productivity (per capita production of progeny) on Chinook salmon, Oncorhynchus tshawytscha, during and after supplementation.
Because the ISS project is complete and the report is final, the ISRP’s comments focus on the appropriateness of the analytical methods used, significance of the study’s findings, and scientific aspects of management implications. The ISRP also suggests additional opportunities to use ISS data for potential ad hoc analyses and examines implications for the Council’s Fish and Wildlife Program and the Council’s Research Plan, which is currently in revision.
The ISRP found the ISS to be a very important and valuable study. The questions addressed by the study are pivotal for salmonid restoration and recovery. The study showed how supplementation influenced natural populations in two major Snake River subbasins, the Salmon and Clearwater. The study’s extensive geographic scope, use of treatment and reference populations, long duration, comprehensive field data, and analytical approaches have provided managers and policy makers with insights and recommendations on how supplementation should occur and be evaluated throughout the Basin.
Some key ISS contributions, among many, that the ISRP would like to highlight include:
- A trend of increasing abundance in both supplemented and reference streams occurred during the course of the study. However, unlike abundance, productivity decreased during the study period in both supplemented and reference streams.
- Fixed factors—such as the ratios of supplementation and non-treatment hatchery females to natural-origin females, number and types of hatchery juveniles released, and occurrence of endemic fish in supplementation broodstocks—were important predictors of abundance during the freshwater portion of the life cycle. Random factors that were outside of the Program’s ability to adjust—e.g., hydrosystem and ocean conditions—largely controlled adult returns and outweighed any supplementation effects.
- Due to the significant effects of random factors, reference stream are needed to monitor the effectiveness of supplementation programs.
- Results from this study support the idea of controlling the types and abundance of fish allowed to escape and spawn in supplemented populations. In both the Salmon and Clearwater subbasins, natural origin females were more productive than supplementation females who, in turn, were more productive than non-treatment hatchery females. Using management tools such as selective fisheries and weirs along with regulating the numbers, types, and locations of where hatchery juveniles are released to influence the composition of spawning populations is a forward looking strategy.
- Differences in broodstock composition between the two subbasins showed the value of integrating locally adapted fish into supplementation broodstocks.
- At least in this study, no lasting reductions in abundance or productivity were detected for Chinook salmon. This is an important management finding because it implies that supplementation can occur in a population without incurring lasting reductions in fitness.
- The ISS program has developed a valuable dataset that spans over two decades. The ISRP strongly recommends continuation of the monitoring and evaluation effort.
- Increases in abundance from a supplementation project will not continue if factors originally limiting a population are not addressed. When the project first began, over 23 years ago, a commonly held belief was that the Salmon and Clearwater subbasins would be capable of producing and supporting greater numbers of spring Chinook if more adults could be introduced into these areas. As the study progressed, this perception changed. In many of the streams examined, it was apparent that salmonid populations were already at or near capacity as density dependence was observed. The study showed that in order for supplementation to be effective, an understanding of the factors that limit a population was needed. Once limiting factors become known, it may be possible to increase capacity and abundance.
The management recommendations included in the study’s final report are thoughtful and should be considered when future supplementation programs are being developed and implemented. Results from the ISS are also certain to stimulate discussions on broader policy questions related to supplementation actions. For instance, when should supplementation be initiated or stopped? How many resources—e.g., money, fish, effort—should be allocated to supplementation, and when does supplementation contribute to recovery and mitigation goals? Although the ISS was not designed to answer such wide-ranging policy questions, results from the study would certainly be germane to policy level discussions about the continued and future use of supplementation as a fisheries management and recovery tool.
Please see the full ISRP report and the ISS’s Completion Report for detailed findings.