Abstract Objective Salmon populations have been declining over the past several decades, and recent studies have found that hatchery-reared fish are returning to spawn as adults at a smaller size and at younger ages. Because of these declines and shifts in size and age at maturation, some spring Chinook Salmon Oncorhynchus tshawytscha hatcheries are struggling to reach their programmatic goals. This is compelling hatchery managers to rethink current rearing protocols. The goal of this investigation was to assess and compare the physiology and performance of fish reared under the standard protocol to those of fish reared under an alternative strategy. Methods A comparative study of juvenile spring Chinook Salmon was conducted at the Round Butte Hatchery/Pelton Ladder facility near Madras, Oregon, across five brood years. The standard rearing practice was compared to an alternative protocol, and we applied a unique experimental design whereby fish density (biomass per unit water volume) between the treatments was similar but the size of the individual fish and the number of fish per rearing vessel varied. The different rearing regimes yielded two treatments: (1) Big–Low: fish reared to a larger body size, with fewer fish per raceway (standard protocol) and (2) Small–High: fish reared to a smaller body size, with more individual fish per raceway (alternative protocol). Fish were evaluated for various prerelease (size, energetics, smolt development, and early male maturation) and postrelease (smolt out-migration travel time, adult returns to Bonneville Dam, and age at return) metrics. Results Prior to release, significant physiological differences between the Big–Low and Small–High treatments were observed for some smolt quality indicators (size, condition factor, and percent solid) but not all indicators (gill Na+, K+-ATPase). Despite these differences, the Small–High treatment did not appear to be physiologically compromised by being reared to a smaller body size with twice the number of fish. Postrelease travel time downstream to Bonneville Dam was faster for the Big–Low treatment. Proportionately, the standard production (Big–Low) treatment produced significantly more younger (age-2 and age-3) adult males, but returns were not significantly different for age-4 males and females compared to the alternative (Small–High) protocol. After adjusting for numbers of fish released, returns for all age-classes were higher in the Small–High treatment, suggesting that fish production goals could potentially be reached by rearing more small fish. Conclusions In this comparative hatchery-scale experiment, fish from the alternative rearing strategy generally outperformed fish from the standard hatchery practice. Given the unpredictability and influence of a variety of postrelease variables on survival, hatcheries may want to consider the trade-offs between altering fish size and/or fish abundance within the constraints of density (total biomass) limits for rearing vessels (tanks, raceways, etc.). This study highlights the benefits of periodically examining standard rearing practices to ensure that they are meeting desired hatchery performance goals and objectives.
Spangenberg et al. (Fri,) studied this question.