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Idaho Supplementation Study: Boosts Chinook Populations, Benefits Don’t Persist When Program Stops
Posted on Friday, May 13, 2016 (PST)

A long-term study of salmon “supplementation” on two Idaho streams – the Clearwater and Salmon rivers – found that the method successfully increased the number of naturally-produced juvenile chinook salmon at Lower Granite Dam, but that there was only small increase in returning adults.


Once supplementation stopped, however, the benefits it may have afforded to increase the numbers of juveniles and adults in the rivers came to an end.


The 23-year study (1991 to 2014) was funded by the Bonneville Power Administration under the guidance of the Northwest Power and Conservation Council’s Fish and Wildlife Program. The study’s authors completed work in December 2015 and presented their findings to the Council this week in Boise, Idaho.


“What we found was that supplementation increased the number of naturally-produced juveniles from natal habitats and smolts to Lower Granite Dam but the increase translated into only modest increases of adults back from the ocean and it was not maintained after supplementation stopped,” said Dr. Timothy Copeland of the Idaho Department of Fish and Game. 


He added that the benefits of supplementation were greater in the Salmon River basin, where supplementation was done with stock integrated with the natural population, than in the Clearwater basin, where the supplementation stock was not integrated with the target population.


The study defines supplementation as “the use of artificial propagation in the attempt to maintain or increase natural production while maintaining the long term fitness of the target population, and keeping the ecological and genetic impacts on non-target populations within specified biological limits.”


The study, “Idaho Supplementation Studies: Project Completion Report 1991 – 2014,” (ISS) can be found on the Council’s website at The study was designed to measure the population effects of a dedicated, intentional supplementation on the abundance and productivity of chinook salmon during and after supplementation.


Co-authors were David Venditti, Kimberly Apperson, Bruce Barnett, Matthew Belnap, Matthew Corsi, Tyler Gross and Lauri Janssen, all from IDFG; Ryan Kinzer and Ryan Santo of the Nez Perce Tribe; and Kurt Tardy and Angelo Teton of  the Shoshone-Bannock Tribes.


The study encompassed 27 streams. Some nine streams in the Clearwater River basin were supplemented and four were retained as reference streams (no supplementation). The Salmon River basin had four supplemented and 10 reference streams.


“The sheer length and breadth of the study make it one of the biggest manipulative experiments ever attempted in the fisheries field,” the study says.


“The ISS had one of the most difficult designs, and is the most expensive and important research we’ve done,” Virgil Moore, Director of IDFG told the Council Tuesday afternoon. “Thank you for sticking with this. Is it black and white? No. No science is.”


Prior to the study, the population of chinook in the Clearwater River basin had been extirpated, the study says, and fisheries managers had introduced hatchery stock using non-endemic stocks. Wild Salmon River chinook were still present and were incorporated into the supplementation broodstock.


According to the Council, it committed to funding the large-scale management experiment in Idaho to investigate the effectiveness of hatchery supplementation as a means to conserve and manage naturally spawning chinook salmon populations. The study was done by IDFG (the lead agency) and with close cooperation from the Nez Perce Tribe, the Shoshone-Bannock Tribes, and the US Fish and Wildlife Service.


The study was divided into phases, the report says. In the four-year long phase 1 (1992-1995) fisheries managers developed supplementation broodstocks and made baseline measurements of abundance and productivity in the supplementation and reference streams. Reference streams were the Lemhi River, Crooked Fork, American River, Slate Creek, West Fork Yankee River and East Fork Salmon River.


In phase 2 (1996-2007), which was 12 years of the study, managers passed returning supplementation adults above weirs to supplement naturally-origin spawners.


In the five-year phase 3 (2008-2012) managers stopped supplementation and measured abundance and freshwater productivity of the supplemented and reference streams to determine the legacy of supplementation.


“The most important and unique aspect of ISS is the evaluation of abundance and productivity after stopping supplementation,” the study says.


Copeland told the Council that relative to reference populations:

--there was increased abundance during supplementation

--the increase was not maintained after supplementation stopped

--there was no significant productivity loss


“We concluded that supplementation resulted in a population boost that did not persist,” the study said.


The study also found that population abundance at all life stages increased with the addition of female spawners, but that varied by the origin of the female and diminished through the life cycle.


Natural-origin females had the greatest effect, supplemented females the next best effect, followed by non-treatment hatchery females (the reference groups.)


The bottom line, he said in his presentation to the Council, was that there are short-term population benefits with supplementation, with a low cost to natural fish productivity, and that supplementation is a valuable tool for managing populations.


Still, Copeland said, by the definition of supplementation as practiced in the Columbia Basin, “supplementation worked.”


“The Idaho Supplementation Studies were initiated to provide the most complete scientific evaluation of supplementation possible,” Copeland said in an email. “The decision to supplement or not is at least partly a policy choice based on values and risk assessment. The goal of the project was to evaluate the demographic effects of supplementation with respect to unsupplemented references, thus giving managers an expectation of the possible benefits, and to provide guidance for implementing supplementation programs in the future.


“We found that supplementation is a useful tool in the manager’s toolbox and it is capable of providing benefits when done right,” Copeland concluded. “So supplementation should continue to be part of the region’s integrated management program.”


Among the management guidelines included in the report are:


--there was a lot of variability inherent among the study streams, which shows that supplementation needs to be customized to the target population/species and adaptively managed, which requires an effective monitoring program.


--Benefits were reduced as population abundances increased, meaning that placing spawners in unused habitats may be more effective when returns are good rather than just releasing extra spawners into the population.


-- The researchers found that the natural fish in the highly introgressed Clearwater populations did better than the hatchery fish spawning in the wild, which shows the strength of natural selection and the wisdom of using stock integrated with the natural population for supplementation.


“Our findings might seem obvious to some but this is the benefit of hindsight,” Copeland said. “When the project was conceived, the benefits of supplementation were not obvious and it took an ambitious study design to get the answers the right way.


“The whole study turned on two things: the large number of reference populations, which gave a reliable yard stick to measure effects, and ceasing supplementation to determine the long-term effects. The ISS study design is still state-of-the-art in the Columbia basin.”


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