From NW Fishletter

[6] NEW PAPER RAISES QUESTIONS OVER FISH FITNESS

The use of hatcheries to supplement wild salmon populations is spreading throughout the Northwest as fish agencies and tribes attempt to restore salmon and steelhead populations listed under the End***ered Species Act. But the ultimate value of this strategy is still hotly debated, and a recent paper on the subject looks at the genetic tradeoffs such efforts can bring about.

Published in a recent issue of the ICES Journal of Marine Science, authors R.R. Reisenbichler and S.P. Rubin say that hatchery supplementation will reduce the productivity of naturally reproducing salmon. Recognition of its negative aspects may lead to restricted use of supplementation strategies, better conservation, and better evaluation.

The authors focus on just one of the negative effects from artificial propagation: genetic ch***es that reduce population fitness or natural propagation. Fitness is a term used to describe those factors important for the survival of the fish throughout their life cycle. Reductions in fitness reduce the productivity and viability of a population for natural rearing, the authors say, but these "potential hazards have not been universally accepted as real or relevant to management of Pacific salmon."

The authors note that two published studies and three in progress have shown that the survival of hatchery fish was less than that for wild fish. When developing survival models, fish management agencies routinely factor in a reduced survival rate for hatchery fish, based on survival information that shows wild fish survive at twice the rate of hatchery fish.

"All five of the studies in natural streams," says the paper, "suggest that same conclusion: hatchery programs that rear steelhead or chinook salmon for one year or longer before release genetically ch***e the population and thereby reduce reproductive success when these fish spawn in natural systems." These results are consistent and confirm eight other studies summarized in their report, the authors indicate.

"In view of this consistency, one conclusion seems obvious: substantial genetic ch***e in fitness results from traditional artificial propagation of anadromous salmonids held in captivity for one-quarter or more of their life," say the authors. "These conclusions imply that (hatchery) supplementation (wherein wild fish interbreed with hatchery fish of reduced fitness) will reduce the productivity of naturally spawning populations, and often may compromise conservation objectives."

The authors note that evaluation of hatchery steelhead showed, with time, that juvenile hatchery fish survival continued to decline compared to wild steelhead, which suggests that the fitness of the next generation would be low even before hatchery and wild fish have a chance to interbreed; and "continuous supplementation should progressively diminish the productivity (adult progeny per female) of the naturally spawning population."

Continued supplementation may reduce productivity of a population to such a degree that it is dependent on supplementation and cannot replace itself. This could defeat the conservation purpose of hatchery supplementation--to increase natural production.

The authors note one chinook salmon population suffered reduction in fitness after four generations in the hatchery, despite continuous gene flow from the wild population, where the wild fish made up 38 percent of the hatchery brood stock. Fitness in steelhead was diminished after only two generations, and numerous studies are showing a substantial ch***e in fitness in the first generation of hatchery rearing.

They recommend taking a number of actions to minimize or slow the loss of fitness in hatchery operations, but they conclude by saying, "The only responses known to substantially reduce the problem are restricting the number of hatchery fish and restricting the number of populations supplemented, i.e., designating a substantial proportion of the viable wild populations to remain completely wild." -B. B.

(Source: Reisenbichler, R.R. and S. P. Rubin. 1999. Genetic ch***es from artificial propagation of Pacific salmon affect the productivity and viability of supplemented populations. ICES Journal of Marine Science, 56:459-466).