Cooperative Fish and Wildlife Research Units Program: Idaho
Education, Research and Technical Assistance for Managing Our Natural Resources

We studied growth rate (g/d), downstream movement, and size of naturally produced fall Chinook salmon Oncorhynchus tshawytscha subyearlings (Age-0) for 20 years in an 8th order river landscape with regulated riverine upstream rearing areas and an impounded downstream migration corridor. The population transitioned from low to high abundance in association with U.S. Endangered Species Act recovery efforts. Mean growth rate of parr in the river did not decline with increasing abundance; but during the period of higher abundance timing of dispersal from riverine habitat and into the reservoir averaged 17 d earlier, and the average size at the time of downstream dispersal was smaller by 10 mm and 1.7 g. Changes in abundance, measured by catch-per-unit-effort, largely explained time of dispersal measured by median day of capture in riverine habitat. Growth rate of smolts in the reservoir declined from an average of 0.6 g/d to 0.2 g/d between the abundance periods because the reduction in size at reservoir entry was accompanied by a tendency to migrate rather than linger, and by increasing concentrations of fish in the reservoir. Median date of passage through the reservoir became an average of 15 d earlier, and average smolt size became smaller by 36 mm and 21.7 g, in accordance with density-dependent behavioral changes reflected by decreased smolt growth. Unexpectedly, smolts during the high abundance period had begun to re-express the migration timing and size phenotypes observed before the river was impounded when abundance was relatively high. Our findings provide evidence for density-dependent phenotypic change in a large river that was influenced by the expansion of a recovery program. As such, this study showed that efforts to recover native fishes can have detectable effects in large river landscapes. The outcome of such phenotypic change, which is an important area of future research, can only be fully judged by examining the effect of the change on population viability and productivity