A synthesis of information related to the supplementation of salmon and steelhead stocks with hatchery fish was prepared from a review of the published literature (606 studies). We located few studies where the effects of supplementation (defined as the use of hatchery-propagated fish to augment naturally producing stocks) were directly assessed. However, a large number of related studies contained useful information. We focused on hatchery x wild fish interactions and various ecological and methodological factors that influenced them.
Genetic and ecological effects, and changes in productivity of the native stocks that can result from supplementation remain largely unmeasured. Releases of hatchery fish into areas inhabited by wild stocks can theoretically cause a loss of genetic variation and adapted ness when wild and hatchery fish interbreed, and a reduction in stock size resulting from competitive interactions, increasing predation (including fishing), and the introduction of diseases.
For many stocks of salmon and steelhead under consideration for supplementation, the environments that they migrate through and in which they must spawn and rear no longer exist in a pristine state. Supplementation, if done improperly, can be an added burden for the native stocks attempting to adapt to significant environmental changes.
Based on the principles of population genetics and a limited number of empirical observations, offspring of matings between hatchery x wild spawners would be expected to perform less well on average than pure wild-strain progeny, unless the hatchery fish were indistinguishable from the wild fish. Hybridization can break down complex genetic adaptations to specific environments, and thereby reduce the fitness (ability to survive) of progeny of hatchery x wild matings.
Many fisheries geneticists, therefore, recommend that locally adapted wild fish be used to start and replenish hatchery broodstocks. Management practices that promote genetic or phenotypic divergence between hatchery and wild stocks are discouraged where the hatchery fish are going to be used to supplement wild stocks of fish. Gene flow into non-target wild stocks due to straying should also be minimized to maintain and strengthen the adaptation of stocks to their environment.
The risk of hatchery stocks developing undesirable genetic characteristics increases when small numbers of closely related individuals are used as broodstock, when there is purposeful selection for specific traits, and when out crossing with wild fish does not occur routinely. If the traits responsible for poor performance by hatchery fish have a genetic basis, and hatchery and wild fish subsequently interbreed, the wild gene pool may be diluted or otherwise altered. Potentially negative impacts include the introduction or increase in frequency of undesirable alleles, the disruption of locally adapted gene complexes, and the swamping or homogenization of the indigenous gene pool through substantial and repeated introductions of hatchery fish.
In general, the longer a fish has been held in the hatchery the less likely it will be able to compete successfully with wild fish once released.
The potential for density-dependent effects depends on the abundance and distribution of hatchery and wild fish relative to the carrying capacity of the environment. A few studies have reported lower growth and survival among wild fish following supplementation.
Hatchery salmonids may indirectly increase predation mortality among wild fish. Large concentrations of hatchery fish may attract larger than normal numbers of bird, fish, and human predators.
Disease must be considered in an evaluation of supplementation because it is a major cause of mortality in hatchery fish and the hatchery fish may serve as disease vectors.
The general failure of supplementation to achieve management objectives is evident from the continued decline of wild stocks in some areas despite, and perhaps partly due to, increases in hatchery production.