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Long-Term Idaho Salmon Supplementation Study Delivers Mixed Results; Not a Stand-Alone Recovery Tool by Staff Columbia Basin Bulletin, January 12, 2018

Supplementation is Not a Stand-Alone Recovery Tool

A newly published study finds that hatchery supplementation after 22 years in two Idaho drainages, increased chinook salmon abundance at some life stages, but the effects did not persist after supplementation of hatchery stock ceased and had no apparent influence on productivity.

The study, according to its authors, represents one of the largest manipulative experiments ever undertaken in the fisheries field. It found that, while supplementation can be a useful tool, the underlying causes of population declines need to be addressed.

It is a distillation of a previous report that was reviewed in 2016 by the Independent Scientific Review Panel (see CBB, August 12, 2016, Science Review Of Idaho Salmon Supplementation Study Discusses ‘Pivotal' Questions).

In that review, the ISRP said that "The questions addressed by the study are pivotal for salmonid restoration and recovery. 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."

The study of supplemented chinook salmon in the Salmon and Clearwater sub-basins involved 27 streams. Some13 of the streams were supplemented and 14 were reference streams.

The ISRP review, "Review of the Idaho Supplementation Studies Project Completion Report 1991-2014," can be found on the Northwest Power and Conservation Council website.

The study defines supplementation as "the use of artificial propagation to maintain or increase natural abundance while maintaining the long-term productivity of the target population."

"Here we focus on the relevant results and their implications, while sparing the reader most of the program's background information and history" of the previous report, said David Venditti, senior fisheries research biologist with the Idaho Department of Fish and Game.

He said the evaluation is of the "demographic effects of supplementation broadly at the basin level and more intensively using just the subset of supplemented populations with weirs. Our intention is that managers can use these results to estimate the benefits they could expect from supplementation and to provide guidance on implementing future programs."

Apart from the wide scope of the study, two things set it apart from other supplementation studies, he said: it includes multiple reference streams to help measure the treatment effects and supplementation was actually stopped in order to assess the techniques' long-term effects.

"A few studies have done one or the other but none, to the best of our knowledge, have done both," he said. "And as such, despite being over two decades old, the study design remains cutting edge."

"Effects of hatchery supplementation on abundance and productivity of natural-origin Chinook salmon: two decades of evaluation and implications for conservation programs," was published online Dec. 4, 2017, in the Canadian Journal of Fisheries and Aquatic Sciences.

Venditti's co-authors are Ryan Kinzer, research scientist with the Nez Perce Department of Fisheries Resource Management; Kim Apperson, regional fisheries biologist, Matt Belnap, regional fisheries manager, Matt Corsi, principal fisheries biologist, Bruce Barnett, data coordinator, and Timothy Copeland, program coordinator, all with IDFG; and Kurt Tardy, fisheries biologist with the Shoshone-Bannock Tribes Fisheries Department.

Among the conclusions, according to Venditti, are:

• Supplementation increased the abundance of natural-origin parr and smolts to Lower Granite Dam, but had a smaller effect on adult returns.

• The overall effect was greater in the Salmon basin, where supplementation stocks were integrated with the natural population than in the Clearwater basin where they were not.

• However, the supplementation effect did not persist into the post-supplementation phase. After supplementation ceased, abundance and productivity in supplemented and reference populations returned to approximately their pre-supplementation relationships, suggesting that the long-term effects of supplementation were neutral.

• Abundance increases were most effectively generated by increasing the number of natural origin females in the spawning population, while integrated supplementation females were slightly better or about the same as females from the segregated hatchery stock.

"Based on these findings, we provide guidance for conservation programs," the study says. "Supplementation alone is not a panacea because it does not correct limiting factors, which must be addressed to achieve population levels capable of sustaining ecological function and harvest."

Venditti said the authors' intentions were for fisheries managers to use the study results to estimate benefits they can expect from a supplementation program.

For managing wild populations, the first priority should be to increase natural-origin abundance through other means, such as habitat restoration, he said. If supplementation is necessary, a brood stock integrated with the natural population would be preferred, but if sufficient wild fish are not available, localized or nearby hatchery stocks could provide benefit if used judiciously.

Similarly, for supplementation stocks, he continued, maintaining the link between the hatchery and natural populations should be a priority.

Also, when natural origin escapement is moderate to high, consider using supplementation fish to seed under-utilized or newly restored habitat to bring new production areas online and reduce spawning density in core habitats.

"The decision to supplement is at least partially a policy decision based on perceived risks versus potential benefits," he said. "We identify several areas where supplementation programs can be beneficial, so it should continue to be one part of the region's integrated management strategy.

The study shows that supplementation can lead to a short-term stabilization or even a slight increase in abundance with little or no reduction to productivity, he added.

"But managers need to realize that salmon supplementation should be intended to get populations over the hump, while the underlying mechanisms affecting the population are addressed. Our clearest, positive effects were observed in the freshwater life stages, indicating that the hatchery/supplementation fish reproduced successfully," he said.

Species not affected by the out-of-basin factors salmon encounter, such as sturgeon and burbot, could also benefit from this type of intervention, Venditti added.