ABSTRACT
Adverse ecological effects on wild fish resulting from releases of hatchery-reared fish are increasingly being scrutinized and balanced against benefits afforded by hatchery programs. To improve understanding of the potential ecological effects of hatchery steelhead trout (anadromous form of Oncorhynchus mykiss) on wild trout (resident Oncorhynchus species) populations, we studied releases of 23,000 to 38,000 hatchery-reared steelhead trout smolts into a Yakima River, Washington, watershed from 1991 through 1994. In this chapter we synthesize results from many aspects of these studies as they relate to minimizing ecological risks to wild trout. We snorkeled in control and treatment streams to observe behavioral interactions between hatchery steelhead trout and wild salmonids. Movement of residual hatchery steelhead trout was examined using traps, and direct underwater observation and relative abundances of hatchery and wild fish were estimated by electrofishing. Instream enclosures were used to determine whether residual hatchery steelhead trout impacted growth of wild rainbow trout or spring chinook salmon (O. tshawytscha). Potential for adverse impacts resulting from ecological interactions among wild salmonids and hatchery steelhead trout was greatest when (1) hatchery fish did not emigrate quickly; (2) water temperatures were over 8°C; (3) hatchery fish were the same species as the wild salmonids; (4) hatchery fish were larger than the wild salmonids; (5) habitat and/or food were limiting; and (6) numbers of fish released was over about 30,000. Ecological interactions with wild salmonids could be reduced or minimized by releasing (1) only actively migrating smolts (no residuals); (2) hatchery fish that are smaller than wild fish; (3) the minimum number necessary to meet management objectives; (4) fish that are less likely to engage wild fish in agonistic encounters; (5) when water temperatures are relatively cold (less than 8°C); (6) in areas where wild salmonid populations are absent; and possibly (7) in areas where habitat diversity is complex. Management actions that encourage angler harvest of hatchery steelhead trout residuals while protecting coexisting wild species may also help minimize negative ecological impacts. Implementing these strategies may reduce the number of returning hatchery-origin adults of the target group but will help reduce risk to the sustainability of wild fish populations.
