The Influence of Size at Release on Performance of Imnaha River Chinook Salmon Hatchery Smolts

Abstract

Ten brood years (BYs 1988–1990 and 1992–1998) of spring–summer Chinook Salmon Oncorhynchus tshawytscha smolts that were reared at Lookingglass Fish Hatchery (Oregon) and released from the Imnaha River Weir and Acclimation Facility were evaluated to determine whether size at release affected juvenile migration survival, smolt-to-adult survival (SAS) rate, smolt-to-adult return (SAR) rate, production efficiency, age composition, straying rate, or harvest rate. Smolts were marked with adipose fin clips and were tagged with coded wire tags (all BYs) and PIT tags (BYs 1992–1998). For BYs 1992–1998, the out-migration survival rate to Lower Granite Dam (LGD) on the Snake River was greater for large smolts (30–38 g) than for small smolts (18–23 g). This juvenile survival difference did not translate to an adult survival difference, as the total (ages 3–5) and adult (ages 4–5) SAR and SAS rates did not differ between large and small smolts. Straying rates were less than 0.02% and harvest rates were less than 0.05% for both treatments, and we found no significant differences between groups. Total production efficiency (number of mature salmon/10 kg of smolts released) was greater for small smolts than for large smolts but not significantly so. Small smolts produced significantly more (~10%) age-5 females than did large smolts. Treatments (smolt size at release) did not differ in sex ratio or the age composition of male returns. Because broodstock availability limited production in 5 of the 10 years, we also compared size at release within standard-density (14.8‒22.2 kg/m³) and low-density (2.2‒8.6 kg/m³) rearing years. At standard density but not at low density, juvenile survival to LGD was significantly greater for large smolts than for small smolts. Adult and total SAR and SAS rates, total production efficiency, and straying rates did not differ between standard-density and low-density rearing years. Harvest rate of the small smolts was significantly greater than that of the large smolts at low density but not at standard density. We found no performance benefit in rearing large Chinook Salmon smolts instead of small smolts.

Received April 27, 2015; accepted December 2, 2015 Published online March 30, 2016

Acknowledgments

We thank Timothy A. Whitesel and Steven J. Parker for initiating these studies; the LFH crew for their dedication and for rearing the study fish; and the numerous surveyors who recovered snouts during spawning ground surveys and from hatchery spawning. We are particularly grateful for the assistance provided by Ron Harrod, Andrew Gibbs, Debra Eddy, Mike Flesher, Ted Sedell, and other staff of the ODFW La Grande Fish Research office. Ian Tattam, Lance Clark, and several anonymous reviewers supplied comments that improved the manuscript. Funding for this study was provided by the U.S. Fish and Wildlife Service’s Lower Snake River Compensation Plan.