The demise of salmon and trout in the western United States has become a conservation crisis of enormous biological, economical, and political significance.
In the mid-1800s approximately 10-16 million anadromous salmon and trout returned to spawn each year in the Columbia River Basin: by the 1980s this number had dwindled to approximately 2.5 million. An estimated 80% of current returning fish result from hatchery production. Thus, return of naturally produced anadromous salmon and trout is less than 5% of historic numbers. According to Williams et al. (1992), approximately 75% of historical populations in the Columbia River Basin are either extinct, at high or moderate risk of extinction, or of special concern.
The complex life history of anadromous salmon and trout, combined with their tendency to home to natal streams after several years in the ocean, has produced a complex assortment of many genetically and ecologically distinct local populations within each salmon species.
The earliest fossil definitely attributed to Salmonidae is Eosalmo driftwoodensis from Eocene deposits (40-50 million years ago) in western Canada.
Issues in conservation:
(1)Long range migrations make protection of the habitat of anadromous salmon extremely difficult. The protection of salmon requires a combination of habitat measures that take into account the complex life history of these fish.
(2)Natural hybridization is much more common in freshwater fishes than in other vertebrates... The widespread introductions of salmonid species to waters outside their native range has made hybridization and introgression major conservation problems for native trout.
(3)Commercial exploitation of salmon further complicates their conservation... An adequate number of individuals for each local reproductive population is needed to ensure persistence...The result of regulating fishing on a stock basis and ignoring the reproductive units that together constitute a stock is the disappearance or extirpation of some demes.
(4)Hatchery propagation of salmon and trout provides a variety of indirect and direct threats to wild populations. Indirect: the real causes of salmon decline were ignored while the public was assured that hatcheries could maintain productivity. Direct: hatcheries have had harmful effects on wild fish through a variety of genetic and environmental mechanisms.
(5)Legal agreements often greatly restrict actions that can be taken for management and conservation of salmon... examples: agreements with Native American tribes, international fishing agreements, international hydro-electric agreements, national fishery management acts, various state laws etc.
The preponderance of evidence for local adaptation suggests that conservation should focus on small units (locally adapted spawning populations).
The failure of most stock transfers of anadromous salmonids to produce self-sustaining natural populations suggests that if we do not conserve local populations, we will not be able to reestablish them within years or even decades.
Although the number of hatchery fish has increased substantially in recent years, total salmon abundance has generally not. Thus, in many cases there has simply been a replacement of wild fish with hatchery fish.
Efforts to alter the hatchery environment to more closely mimic natural conditions may reduce the magnitude of selective changes but cannot escape them altogether.
Supplementation issues:
A number of issues remain to be resolved before it can be determined whether supplementation is a viable conservation strategy for salmon.
We are not aware of a single empirical example in which supplementation has been successfully used as a temporary strategy to permanently increase abundance of naturally spawning populations of Pacific salmon.
Ryman and Laikre (1991) pointed out that supplementation can lead to reductions in effective population size of the hatchery/wild system as a whole through augmenting only part of the gene pool of a population (i.e. that part represented by broodstock taken into the hatchery).
The single most important consideration was whether increases in abundance attained by supplementation were sustained after it was terminated. This is determined primarily by whether factors responsible for the original declines are adequately addressed.
The legacy of a century of viewing salmon hatcheries as a means of mitigating losses to natural production (rather than fixing the problems that caused the declines) has resulted in a somewhat schizophrenic view of many supplementation programs. The stated objectives of supplementation programs typically include production of excess fish for harvest as well as increases in natural populations... it is clear the pursuit of the former may compromise efforts to conserve wild populations.
The intended duration of supplementation programs is often not clearly articulated.
NMFS has determined that under the ESA, salmon hatcheries are not a substitute for conservation of natural populations, and that recovery will be evaluated in terms of viability of the natural populations rather than the number of hatchery fish.