Balancing Fisheries Management and Water Uses for Impounded River Systems

The Potential Importance of Reservoirs in the Western United States for the Recovery of Endangered Populations of Anadromous Steelhead

Frank P. Thrower, John E. Joyce, Adrian G. Celewycz, and Patrick W. Malecha

doi: https://doi.org/10.47886/9781934874066.ch21

Abstract.—As a consequence of ubiquitous reservoirs impounded on Pacific Northwest streams having native runs of steelhead Oncorhynchus mykiss, hundreds of isolated populations of formerly anadromous fish were unintentionally created. Despite extensive stocking of resident hatchery fish, it is likely that when conditions were favorable, many populations adapted to their new environments and remain viable to this day. Severe population declines of anadromous fish have occurred in recent decades. If these isolated resident populations retain ancestral anadromous adaptations, they have value as potential reservoirs for anadromous restoration after issues leading to decline have been addressed. We produced five broods of F₂ offspring from pure lines of anadromous steelhead and resident rainbow trout (nonanadromous O. mykiss; descendants of a previous stocking 70 years before from the same anadromous steelhead stock), plus reciprocal hybrid lines to determine if captive populations of F₁ offspring of fish thus isolated for decades can still produce smolts that adapt to seawater and successfully return to spawn as ocean-ranched adults. Although the anadromous line produced significantly more smolts than the resident line, the marine survival of smolts was related to genetic origin more than to smolting history of the parents. Smolting rates and marine survival of the F₂ progeny were similar to, or higher, than those of the F₁ progeny, indicating that completely captive broodstock, derived from isolated populations, could be an important component of a recovery plan for endangered stocks of anadromous rainbow trout.