EFFECTIVE POPULATION-SIZE IN WINTER-RUN CHINOOK SALMON

Winter-run chinook salmon from the Sacramento River, California, is fe derally listed as endangered Since 1989 there has been a program to au gment the natural population by capturing adults, artificially spawnin g them, raising the young, and releasing the smelt. Here we estimate t he effective population size of these captive-raised fish, the natural run, and the combination of both groups over the three-year period fr om 1991 to 1993. We find that the most appropriate estimate of the eff ective population size of the captive-raised progeny is a variance est imate of effective population size standardized so that the number of released smelts returning to spawn was the same as the number of spawn ers used to produce the smelts originally. We have generated 10,000 ra ndom samples to simulate returns from these released progeny. The esti mates of the effective population sizes in 1991, 1992, and 1993 were o nly 7.02, 19.07, and 7.74, respectively. We then determined limits on the effective population size of the natural run based on 0.1 and 0.33 3 of the run-size estimates. Using estimates of the captive proportion of the run, the minimum estimates of the effective population size of the overall run for the three years were 21.9, 127.3, and 39.0, and t he maximum estimates were 61.6, 401.0, and 108.7. It does not appear t hat the hatchery program has reduced the overall effective population size. The run sizes in each year are extremely low, however, and it is possible that fish will be lost from this run in one of the years in the immediate future, making reestablishment of a healthy run even mor e difficult.