EFFECTIVE POPULATION SIZE IN WINTER-RUN CHINOOK SALMON

Hedrick, P.W., D. Hedgecock, and S. Hamelberg. 1995. Pages 615-624, Conservation Biology. Vol 9, No. 3. June 1995.

Winter-run Chinook salmon from the drainage of the Sacramento River, California, was listed as endangered by the state of California in 1989 and federally listed as threatened in 1990 and endangered in 1994 under the Endangered Species Act.  Because the estimated annual run number dropped from an average estimate of 86,509 in the period 1967-1969 to low numbers in the late 1980s, such as 533 in 1989, a program was started in 1989 to capture adults, artificially spawn them, raise the young in the Coleman National Fish Hatchery, and then directly release these progeny to augment the natural population.  There is, in addition to this direct supplementation, an effort begun in 1991 to rear a portion of these captive-bred progeny to adulthood for use as future brood stock.

A primary concern in the conservation of endangered species is to keep the population size at a level that avoids as much as possible the effects of inbreeding depression  and maintains enough genetic variation both to keep the present fitness high and the future adaptive potential substantial.

Supportive breeding from a captive population to augment a natural population may in some cases actually result in a decrease in the overall effective population size.  If the effective populations size of the captive-raised (hatchery population) individuals is small, but, because of low mortality in the young raised in captivity, they contribute a large proportion to the population, then the captive breeding program may actually make the overall effective population size less than if there were no augmentation (supplementation) program.  The greatest danger of such a reduction in overall effective population size occurs when the effective population size of the natural portion of the population is small, the contribution from the captive population is large, and the effective population size of the captive population is small.

Because of unequal numbers of the sexes in the captured spawners and variable timing of ripening of these spawners, some fish contributed more than the average while other made no contribution to the captive progeny pool.

Releasing hatchery-raised fish without trying to equalize family contribution may result in selection favoring genotypes that have high fitness in the hatchery.

To avoid the long-term loss of genetic variation...the effective population size should be larger than 500.  But in a recent reevaluation of this calculation based on the proportion of the variation that is potentially adaptive, Lande (1995) has suggested that an effective size of 5000 may be more appropriate to retain genetic variation.

We do not feel that a hatchery program by itself (or in combination with the captive broodstock program) is sufficient to prevent extinction of winter-run Chinook.  These programs are significant in that they may be potentially useful in maintaining the natural run in the short term and provide additional time until environmental changes are made to protect winter-run Chinook in the wild.  In fact, of most immediate concern is that the natural population continues to survive and increase as result of the implementation of the various environmental changes being recommended.  Comments by Frazer (1992) and Meffe (1992) point out the dangers of relying on programs such as this hatchery program without rectifying the environmental problems that have led to near extinction of a species.

Captive breeding programs have been estimated for a number of endangered species, particularly birds and mammals.  It was hoped that these programs would provide additional animals for use in reintroduction programs either to establish new wild populations or to augment remnant wild populations.  In many cases, the survival and reproduction of these released animals has not been as successful as had been hoped initially, and further examination of problems encountered with the release of each species has been necessary.  It is obvious to us that the maintenance of the natural environment to maintain a natural population may be the best and most economical approach to preserving many endangered species and that captive breeding programs, including hatcheries, should not be seen as a substitute for rehabilitating the environment.