ABSTRACT:
A number of published studies have shown genetic differences between hatchery and wild anadromous Pacific salmon (Oncorhynchus spp.) Nevertheless, none of these studies has provided compelling evidence that artificial propagation poses a genetic threat to conservation of naturally spawning populations. Hence constructive debate and consensus on how to limit deleterious genetic effects from artificial propagation have been limited or ineffectual, often because participants don’t agree that a problem exists. When the published studies and three studies in progress are considered collectively, however, they provide strong evidence for a problem - evidence that the fitness for natural spawning and rearing can be rapidly and substantially reduced by artificial propagation. This issue takes on great importance in the Pacific Northwest where supplementation of wild salmon populations with hatchery fish has been identified as an important tool for restoring these populations. Recognition of negative aspects may lead to restricted use of supplementation, and better conservation, better evaluation, and greater benefits when supplementation is used.
QUOTES FORM TEXT:
While artificial propagation may temporarily solve demographic problems, it may cause genetic and other problems which might increase the risk of extinction once supplementation ends.
Here we focus on just one of the negative effects from artificial propagation- genetic changes that reduce population fitness for natural rearing. Such changes reduce the productivity and viability of a population for natural rearing; however, such changes have not been universally accepted as real or highly relevant to management of Pacific salmon Substantial genetic change and corresponding loss of fitness for natural rearing have seemed unlikely for populations experiencing natural conditions during most of their life cycle. Apparent loss of fitness in hatchery populations of resident trout for rearing in natural systems was demonstrated more than a third of a century ago, and was consistent with the genetic change generally expected from domestication of captive populations.
The existing data suggest that populations are genetically changed by hatchery programs. All observed differences between hatchery salmonids and wild salmonids suggest that hatchery fish are genetically inferior to wild fish for natural rearing.
All five of the studies in natural streams suggest the same conclusion - hatchery programs for steelhead or stream type chinook salmon (i.e. programs holding fish in the hatchery for one year or longer) genetically change the population and thereby reduce survival for natural rearing. Not only are these studies consistent among themselves, but they also are consistent with the results from the eight studies summarized in the preceding section, which included additional species and work on tow continents. In view of this consistency and the apparent lack of contradictory results from other studies, one conclusion seems obvious - substantial genetic change in fitness results from traditional artificial propagation of anadromous salmonids held in captivity for one-quarter or more of their life.
The above conclusions imply that 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 typical population proposed for supplementation presumably is one of low productivity which is substantially below carrying capacity. Continued supplementation of such a population may reduce its productivity so that the population becomes dependent on supplementation and cannot replace itself otherwise. Additional effects include the possible loss of local adaptations from genetic homogenization when the supplemented population consists of subpopulations, and inadvertent reductions in effective population size. Managers and clients must decide whether the risk of degradation from supplementation is acceptable, and if so, for how many of the naturally spawning populations.
A simple model suggests that the actual production from a supplementation program for steelhead will increase initially, but then may stabilize at less than one-half of the level expected without consideration of genetic change.
The two studies comparing HH and WW in streams and in hatcheries provide firm evidence that the genetic change from artificial propagation, in large part, results from natural selection for fish that are well-adapted to the hatchery environment - i.e., from domestication selection.
Apparently domestication selection often is intense. The fitness of stream type chinook salmon from natural rearing was diminished after four generations of culture, despite continuous gene flow from the wild population (on average, wild fish comprised 38% of the hatchery broodstock). The fitness of steelhead for natural rearing was diminished after only two generations in the hatchery. Presumably substantial change occurs in the first generation; however, the appropriate experiment is only now being initiated.
Substantial declines in fitness for natural spawning and rearing clearly are undesirable both for conservation and the economic efficiency of supplementation.
RESPONSES TO AVOID NEGATIVE CONSEQUENCES OF ARTIFICIAL PROPAGATION:
· Use only wild fish as broodstock in the hatchery
· Explicitly include genetic effects when estimating benefit-cost ratios for deciding whether to supplement
· Restrict the number of hatchery fish added to a wild population
· Restrict the number of wild populations to be supplemented
· Modify artificial rearing environments (selective regimes) to be more similar to the natural environment
· Modify artificial propagation to reduce mortality or differential reproductive success
The only responses known to substantially reduce the problem of hatchery supplementation are restricting the number of hatchery fish (including the option of no supplementation) and restricting the number of populations supplemented (i.e., designating a substantial proportion of the viable wild populations to remain free of supplementation).