ABSTRACT
Genetic population structure of naturally spawning wild and hatchery summer-and winter-run steelhead in the Kalama River is described using analyses of allozyme data from 16 polymorphic loci collected from adult fish representing 6 hatchery summer-run, 6 wild winter-run, 6 hatchery winter-run, and 5 wild summer-run broods. The data suggests some degree of persistent reproductive isolation among all four spawner groups in the Kalama River. The existence of relatively discrete spawning populations in the basin suggested the potential for using a genetic approach to estimate smolt production from each spawner group. A mixed stock production analysis using the adult data as a baseline and data obtained from two collections of outmigrating naturally produced smolts is presented with smolt production from the two outmigrant cohorts partitioned among the four spawner groups. The results demonstrate the utility of mixed stock production analysis for estimating smolt production of sympatric population and also show limited reproductive success of hatchery fish spawning naturally. The latter corroborates findings from earlier work by the Kalama Research Team.
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The primary objective of Kalama research to date has been to assess the relative reproductive performance and contribution of hatchery and wild steelhead spawning in the wild. For the purposes of this report, wild fish are defined as naturally produced fish, regardless of ancestry, and hatchery fish are those spawned and reared for some portion of their life in the hatchery environment.
Earlier Kalama work focused on evaluating reproductive competence of highly domesticated, non-locally derived stocks of hatchery fish using a genetic mark approach. Hatchery fish, of both summer and winter-run races, were selectively bred so that smolt cohorts released in the basin had high frequencies of otherwise uncommon alleles.
When those smolts returned as adults, they were screened for the marker alleles and then allowed upstream to spawn among themselves or with wild fish. An increase in the frequency of the marker alleles in the subsequent naturally produced generation was an indication that the hatchery fish did successfully reproduce. The magnitude of the increase was an index of how successfully they reproduced relative to the wild fish. Natural production by hatchery fish was found to be much lower (on a per spawner basis) than that of wild fish: recent analyses indicated that reproductive success of hatchery fish (to the adult stage) averaged approximately 16% and 8% of that of wild Kalama steelhead for summer-and winter-run fish respectively. Findings from recent analyses agree qualitatively with previous published work.
The earlier genetic mark work with the summer-run steelhead suggested that, in some years, even with low relative reproductive success, as much as 40% of the ãwildä adults on the spawning grounds had at least one hatchery parent- simply because of the sheer-numbers of hatchery spawners in the previous generation. The question, as yet unanswered, is how can the genetic differences persist in the face of such large potentials for introgression.
Hatchery spawners produced far fewer smolts than expected given the abundance of hatchery spawners on the spawning grounds. Smolts captured in 1994 were almost entirely attributed to production by wild fish: mean production by hatchery summer-and winter-run spawners combined was less than three percent. The proportions of offspring of wild fish closely matched the proportions of wild spawners 2 years earlier when the majority of the 1994 smolts were spawned.
Most of the smolts were again produced by wild spawners and, again, hatchery winter-run fish were essentially non-reproductive (to the smolt stage). However, 30% of the total smolt cohort was attributed to production by hatchery summer-run fish spawning naturally. Still, relative, reproductive success of summer-run hatchery fish is considerably lower than that of wild fish since, in 1996, hatchery summer-run were the most abundant spawners ö nearly twice as numerous as the wild summer-run.
Despite several generations of relatively high potential for genetic introgression, wild summer and winter steelhead in the Kalama maintain a substantial degree of genetic distinction from their hatchery stock counterparts.
The domesticated, non-locally derived hatchery steelhead strains used in the Kalama Basin are less reproductively competent in that basin than the indigenous wild strains.