Idaho has been trying to unravel the secrets of hatchery and wild salmon interactions in nature. Since hatchery salmon do not survive as well as wild salmon, it is important to fix this problem. Fortunately, Idaho still has some wild chinook to work with so they can investigate this problem. The study noted that the Lemhi River, where this study takes place, has fewer wild salmon than it use to. The number of wild juveniles in the Lemhi ranged from 300,000 to 1.2 million from 1963 to 1974, but today the juvenile chinook abundance is estimated to be just 25,000. The following accounts are taken from this study. The findings are consistent with what other researchers have found when comparing hatchery to wild fish performance in nature.
· In general hatchery fish tended to move out of the artificial stream sections in higher numbers, to be more active (less reclusive), to use less cover habitat, and to be more aggressive than the natural chinook salmon.
· We noted that the hatchery fish were more mobile than the natural fish. A higher proportion of the natural chinook salmon exhibited cover-seeking behavior. Natural fish were found near instream cover and the hatchery fish were more in the open.
· We noted a behavioral shift in habitat used by the natural chinook salmon in the fall when combined with increasing numbers of hatchery fish. When the natural chinook salmon were alone in the stream, the majority of the fish were found on the pool bottom near the cobble substrate. But when the natural fish were combined with low numbers of hatchery fish, more natural fish were found in the pool water column with a few on the pool bottom. As more hatchery fish were added to the stream, the distribution of the natural fish cam to resemble that of the hatchery fish.
· Hillman and Mullen reported in another study, that as the hatchery fish moved downstream the natural chinook salmon would leave their usual stations at the shallow river margins and join the hatchery fish at the center of the river near the surface. Thus, in the presence of the grater numbers of hatchery fish, the natural chinook would mimic the behavior of the hatchery fish. It was further noted that in leaving the refuge of the marginal waters, the natural chinook salmon became targets of selective feeding by resident trout.
· The introduction of hatchery fish can displace the natural salmon from their preferred habitat. The authors cite four studies where this was also observed.
· The aggressiveness of the hatchery fish may have important implications for their survival. A fish which spends more time and energy in aggressive behavior will have less energy for food gathering. And, while the aggressive fish may procure a superior feeding position, the gains in food energy may not necessarily compensate for the energy expended. In short, overt aggressiveness may not be cost effective in terms of the food budget and can reduce the survival of the fish. It may also make fish more vulnerable to predation.
· Thus, it is possible that a hatchery supplementation program may inadvertently replace the target natural population with a population having a lower survival and reproductive potential.
This study joins other conducted with salmon and trout out the perils of adding hatchery fish to natural populations. By definition, supplementation of natural populations of trout or salmon is aimed at increasing the abundance of fish. But if all that is accomplished is the displacement of the natural fish with an inferior hatchery fish, then the hatchery program becomes one of the causes of native fish decline and extinction.