Evidence for Density-Dependent Changes in Growth, Downstream Movement, and Size of Chinook Salmon Subyearlings in a Large-River Landscape

Abstract

We studied the growth rate, 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 and other federally mandated recovery efforts. The mean growth rate of parr in the river did not decline with increasing abundance, but during the period of higher abundance the timing of dispersal from riverine habitat into the reservoir averaged 17 d earlier and the average size at the time of downstream dispersal was smaller by 10 mm and 1.8 g. Changes in apparent abundance, measured by catch per unit effort, largely explained the time of dispersal, measured by median day of capture, in riverine habitat. The growth rate of smolts in the reservoir declined from an average of 0.6 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 smolts in the reservoir. The median date of passage through the reservoir was 14 d earlier on average, and average smolt size was smaller by 38 mm and 22.0 g, in accordance with density-dependent behavioral changes reflected by decreased smolt growth. Unexpectedly, smolts during the high-abundance period had begun to reexpress 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. Thus, this study shows that efforts to recover native fishes can have detectable effects in large-river landscapes. The outcome of such phenotypic change, which will be an important area of future research, can only be fully judged by examining the effect of the change on population viability and productivity.

Received November 26, 2012; accepted May 13, 2013

ACKNOWLEDGMENTS

We thank our U.S. Fish and Wildlife Service and U.S. Geological Survey colleagues whose efforts contributed to this 20-year study. We are grateful for the wealth of data collected by our coworkers from the Idaho Power Company, Fish Passage Center, Nez Perce Tribe, National Oceanographic and Atmospheric Administration, Smolt Monitoring Program, University of Idaho, University of Washington, U.S. Army Corps of Engineers, and Washington Department of Fish and Wildlife, including S. Downing, P. Groves, D. Marsh, F. Mensik, D. Milks, C. Morrill, P. Verhey, and D. Ross. This study would not have been possible without the participation of personnel of the Pacific States Marine Fisheries Commission, including D. Marvin (through 2010) and N. Tancreto (2011–2013), who helped operate and maintain the Columbia Basin PIT-Tag Information System. Peer review by R. Zabel, D. Rondorf, an anonymous reviewer, and the editorial staff improved this manuscript. This study was funded by the Bonneville Power Administration and administered by D. Docherty and S. Bradbury under project number 199102900. The U.S. Army Corps of Engineers cost-shared the project during 2005–2011, with valuable oversight by S. Dunmire and D. Holecek. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. The findings and conclusions in this article are those of the authors and do not necessarily represent the views of the U.S. Fish and Wildlife Service.