Mark W. Chilcote

Differential Reproductive Success of Hatchery and Wild Summer-Run Steelhead under Natural Conditions

Abstract We compared the relative reproductive success of naturally spawning, summer-run hatchery and wild steelhead trout Salmo gairdneri by electrophoretic examination of juveniles for a specific genetic marker. The success of hatchery fish in producing smolt offspring was only 28% of that for wild fish. We also found that 62% of the naturally produced summer-run smolts were offspring of hatchery spawners. Their dominance occurred because hatchery spawners, within the watershed we examined, effectively outnumbered wild spawners by at least 4.5 to 1. We suggest that, under such conditions, the genetic integrity of wild populations may be threatened. We also suggest that hatchery fish may be an important component of the spawner-to-smolt recruitment relationship for summer-run steelhead.

Electrophoretic comparison of the reproductive success of naturally spawning transplanted and wild steelhead trout through the returning adult stage

Abstract A previous electrophoretic assessment of the natural reproductive success of sympatric transplanted hatchery and wild summer-run steelhead trout Oncorhynchus mykiss (formerly Salmo gairdneri) populations was extended to include returns to the adult life history stage. The mean percentage of offspring from naturally spawning hatchery steelhead decreased at successive life history stages from a potential of 85–87% at the egg stage to 42% at the adult stage. In addition, reproductive success of naturally spawning hatchery steelhead compared to wild steelhead decreased from 0.750–0.788 at the subyearling stage to 0.108–0.129 at the adult stage. In freshwater, the period of greatest differential mortality for offspring of hatchery and wild steelhead occurred from the subyearling to smolt stage, suggesting that influences such as predation and competition affected survival of hatchery offspring to a greater extent than did environmental and ecological effects directly associated with differences in parental spawning time. Differential mortality of hatchery offspring also occurred during the smolt to adult phase, and was of a magnitude similar to that for the egg to subyearling phase. Poorer survival for naturally produced offspring of hatchery fish could have been due to long-term artificial and domestication selection in the hatchery population, as well as maladaptation of the transplanted hatchery stock in the recipient stream.

A Genetic Comparison of Sympatric Populations of Summer and Winter Steelheads

Abstract The 1973 and 1974 year classes of adult, wild summer and winter steelheads (Salmo gairdneri) were genetically compared at five polymorphic enzyme loci: alpha-glycerophosphate dehydrogenase-1; lactate dehydrogenase-4; malate dehydrogenase-3,4; superoxide dimutase-1; and phosphohexose isomerase-3. Overall genetic differences between summer and winter steelhead groups were not significantly greater than genetic differences observed for 1973 and 1974 year classes compared within each group. These findings, in conjunction with field observations of known summer-winter steelhead spawning pairs, suggests that summer and winter steelheads in the Kalama River are not at present reproductively isolated.

Movement and Survival of Presmolt Steelhead in a Tributary and the Main Stem of a Washington River

Abstract Presmolt steelhead (Salmo gairdneri) were marked in Gobar Creek from 1981 to 1984 and movements between this creek and the main stem of the Kalama River in Washington were examined. Some of these migratory parr travelled a considerable distance down the river, whereas others remained in the tributary. We found that migrant parr were capable of surviving to the smolt stage. Most migrant parr remained in downstream areas prior to smolting, with few returning upstream and emigrating as smolts from Gobar Creek. Of all juvenile steelhead from Gobar Creek that survived to become smolts, most emigrated from the creek to downstream areas as parr and subsequently outmigrated from these downstream areas the following year. The freshwater survival of a majority of the steelhead spawned in tributaries may depend largely on the availability of suitable rearing environments in downriver areas.

Occurrence of Anadromous Brown Trout in Two Lower Columbia River Tributaries

Abstract In the Pacific northwest, brown trout (Salmo trutta) are found in selected streams and lakes east of the Cascade Mountain Range but have not been reported heretofore from coastal streams or tributaries of the lower Columbia River. Two brown trout were captured—one in 1979 at Kalama Falls salmon hatchery and the other in 1984 in Herrington Creek, a tributary of the South Fork Toutle River in Washington. Both fish probably came from previous plantings elsewhere. Viable anadromous populations via upriver or lake plantings have not yet been established in the lower Columbia River tributaries but could become so in the future.