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estimates based on other techniques (Shaul and Clark, in press; Johnson, in press). For GSI to provide reliable information for the wide range of mixed-stock fisheries of Pacific salmonids, baseline data must be available for a large number of potential source populations covering a broad geographic area. Such data have been gathered over the last decade, and currently the body of genetic data on Pacific salmonids probably exceeds that for any other organisms except Drosophila and man. In the past several years a number of agencies (National Marine Fisheries Service, Washington State Department of Fish- eries, Fish and Wildlife Service, University of California at Davis, Canada Department of Fisheries and Oceans) have used GSI to analyze mixed-stock fisheries for chinook, chum (0. keta), pink (0. gorbllscha), and sockeye (0. nerka) salmon (reviewed by Shaklee and Phelps, in press). Figure 1 illus- rates how results obtained by GSl can be used to describe seasonal changes in the stock composition of a fishery. For sockeye salmon, use of a combination of genetic and non- genetic markers (freshwater age, scale patterns, parasites) has proved to be the most powerful means of analyzing mixed-stock fisheries (Wood et al. 1989), and the same may prove to be true for coho salmon (0. kislltch) as well. GSI analyses have led to a more precise and comprehen- sive picture of stock structuring than can be provided by conventional tagging techniques. Nevertheless, the full potential of GSI as a management tool has only begun to be realized-perhaps a reflection of the time lag that typically separates a technological development and its practical implementation. The proven power of the approach, in combination with the accumulation of successful manage- ment applications, projects an increased role for GSI in the near future. Fraser River Basin Test Fishery 1987 60 40 20 60 Percent 40 Contribution 20 o Hatchery Monitoring and Evaluation In the hatcher~ environment, the rate of genetic change in a stock can be greatly increased as a result of inbreeding or artificial selection (intentional or unintentional). Some degree of adaptation to rearing conditions may be desirable, particularly for salmon reared in net pens or as a permanently cultured stock ip mitigation hatcheries. Rapid genetic changes due to cmance, however, cannot be expected to be beneficial. We agtee with Allendorf and Ryman (1987) that the goal of hatch~ries designed to provide fish for release into the wild should be to preserve as nearly as possible the genetic makeup of the ancestral stock. The rate of random genetic change depends on the parameter effectite population size (N,.). There are a variety of opinions regarding the minimum acceptable value of N,. (see Simberloff 1~88); almost certainly, there is no single number that would apply to all species. Most geneticists, however, would probably agree that an N,. of a few hundred per generation isl necessary to avoid long-term deleterious effects from inbreeding and genetic drift. Waples (in pressl) showed that in P~cific salmon, N, per generation is equiva- lent to the effective number of breeders per year (N,) times the average age! at reproduction (= generation length). Although many h~tcheries regularly spawn several hundred or more adults ea~h year, skewed sex ratios in the spawners, unequal fertilization rates of different individuals in mass spawnings, and, high v.ariance in survival rates among families all may cause N,. to be smaller than the number that actually spawn. In the last decade, fisheries managers have been increasingly aware of practices that can result in genetic changes, and guidelines have been proposed to minimize them (el g., Meffe 1986; Simon et al. 1986; Allendorf Upper Fraser Fraser River Fraser May June July-Aug Sept-Oct Figure 1. GSI estimates of contributions of chinook salmon stocks to a 1987 test fishery ~ear the mouth of the Fraser River (Canada). Dramatic changes in the stock composition of the fishery over a period of several months ~re apparent. This type of information can be used to adjust fishing pressure to target abundant stocks and protect less abundant ones. I Genetic data were compiled by the National Marine Fisheries Service using procedures described by Aebersold et at. (1987); the baselihe and fishery samples were provided by the Canada Department of Fisheries and Oceans and the Washington Department of Fisheries, September - October 1990 21