An evaluation of rearing densities to improve growth and survival of hatchery spring Chinook salmon

Abstract

We evaluated growth and survival of spring Chinook salmon Oncorhynchus tshawytscha reared at varying densities at Warm Springs National Fish Hatchery, Oregon. For three consecutive brood years, density treatments consisted of low, medium, and high groups in 57.8-m3 raceways with approximately 16,000, 24,000, and 32,000 fish/raceway, respectively. Fish were volitionally released in both the autumn and spring to mimic the downstream migration timing of the endemic wild spring Chinook salmon stock. Just prior to the autumn release, the rearing density estimate was 4.24 kg/m3 for the low-density group, 6.27 kg/m3 for the medium-density group, and 8.42 kg/m3 for the high-density group. While weight gain did not differ among density treatments (P = 0.72), significant differences were found in median fork length (P<0.001) for fish reared at different densities. Fish reared at high density exhibited the highest on-hatchery mortality rate during two brood years; however, differences in mortality rate among densities were not significant (P = 0.20). In one brood year, adult recovery rates appeared to support the hypothesis that lower initial densities improved postrelease survival (P<0.01). All rearing densities utilized in this evaluation were relatively low and may partially explain why more differences were not readily apparent among density groups. In addition, the volitional release was a confounding factor in our study because we were unable to quantify the number of fish released in the autumn.