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Study Analyzes Effects of Supplementation
on Natural-Origin Salmon Abundance

by Staff
Columbia Basin Bulletin, May 22, 2015

A hatchery steelhead, returned as an adult will be artificially spawned helping to bring about the next generation of hatchery steelhead. A recent study that compared 12 wild chinook salmon populations that had been the focus of hatchery supplementation programs and 10 populations of salmon that had never been the focus of supplementation programs found none to small benefits in natural salmon abundance.

The study analyzed information from a 25-year period and determined that densities of natural-origin spawning adult salmon in the Snake River Basin that had been the focus of supplementation programs had increased just 0 percent to 8.4 percent relative to the 10 salmon populations that had not been the focus of supplementation.

"To put these numbers in perspective, large-scale factors that affect population size, such as hydropower operations or changing ocean conditions, have a much more substantial effect in roughly 9 out of 10 years," said the study's lead author Mark Scheuerell, research fisheries biologist in the Fish Ecology Division with the NOAA Fisheries Northwest Fisheries Science Center.

"Our analysis indicates that the overall effect of supplementation on natural origin abundance has been modest for Snake River spring/summer chinook salmon," he said. "There may be net positive or negative effects for some specific populations, but on average the effect for most populations appears to be small."

The study, "Analyzing large-scale conservation interventions with Bayesian hierarchical models: a case study of supplementing threatened Pacific salmon," was published online April 27, 2015, in the journal Ecology and Evolution (

In addition to Scheuerell, authors are Eric R. Buhle, Fish Ecology Division, NW Fisheries Science Center, NOAA Fisheries; Brice X. Semmens, Scripps Institute of Oceanography, University of California, San Diego; Michael J. Ford, Conservation Biology Division, NW Fisheries Science Center; Tom Cooney, Conservation Biology Division, NW Fisheries Science Center; and Richard W. Carmichael, Northeast-Central Oregon Research and Monitoring, Oregon Department of Fish and Wildlife, Eastern Oregon University, La Grande, Ore.

Since the mid-1980s, hatcheries have been used increasingly to rebuild wild populations of salmon through supplementation programs that use local wild broodstock to produce juveniles. The hatchery-bred salmon are then allowed to return to the stream as adults and to spawn just as wild fish would.

The study used a modern modeling approach, according to Scheuerell, that is more typically used in fields like finance to examine the effects of advertising on sales or political policy on markets.

"One distinct advantage of using this type of modeling approach is that it allows us to simultaneously estimate the effects of hatchery supplementation at two important spatial scales: the individual population and the entire Evolutionarily Significant Unit (ESU)," he said.

"Importantly, it is the latter scale at which listing and de-listing decisions are ultimately made under the Endangered Species Act, and this method provides us with a much better understanding of ESU-level effect of supplementation."

Even with the power of the model (hierarchical Bayesian model), the study could not address whether ecological factors, such as competition, were more important than genetic factors, such as inbreeding depression, in determining the response to supplementation, the study says.

As the researchers modeled the 22 populations over the course of 25 years, they found "very limited support for a supplementation effect at both the individual population and ESU levels," the report says. In fact, there was a 16 percent to 50 percent chance that supplementation had actually "caused some decrease in densities of wild adults across the ESU."

However, when the researchers excluded two of the twelve populations -- the Minam and the Wenaha rivers had some hatchery-origin adults spawning but had not been "intentionally supplemented" -- supplemented populations of wild fish increased from 1 percent to 13 percent relative to non-supplemented populations.

The variability of some of the remaining populations was large over the 25 years of the study. For example, the East Fork of the Salmon River in Idaho saw a range of a 13 percent decrease in the density of natural-origin adults to a 28 percent increase. The upper mainstem of the Salmon River saw a 17 percent decrease in density to a 16 percent increase in density.

"Thus, although artificial propagation (including supplementation) may be a potentially useful intervention for preventing imminent extinction of specific populations (Neff et al. 2011), supplementation may be largely ineffective as a recovery tool for increasing the density of natural-origin adults within this ESU over the long term," the study said.

There could be several explanations for this, according to the study.

First, other studies have denoted the poor reproductive success of hatchery salmon and they could depress the abundance of wild adults, as well.

Second, the theoretical basis of supplementation assumes that target populations are well below carrying capacity, the study says, and the lackluster performance of supplementation shown in this study could be because populations are closer to carrying capacity than thought.

"If habitat capacity has been reduced due to long-term structural alterations, then supplementation without concomitant habitat restoration will be unlikely to provide strong conservation benefits and may simply result in displacement of natural-origin fish by hatchery fish," the study concludes.

On the other hand, if carrying capacity is reduced because of a loss of nutrients in the stream, then supplementation could provide carcasses and increase carrying capacity.

"Also, it is important to note that even if supplementation does result in a modest abundance increase, there are concerns that long-term use of artificial propagation could reduce genetic fitness, contribute to ecological risks such as competition for resources, and serve as vectors for diseases or parasites," the study says.

Study Analyzes Effects of Supplementation on Natural-Origin Salmon Abundance
Columbia Basin Bulletin, May 22, 2015

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