The Effect of Moderately Increased and Variable Raceway Flow Rates on Juvenile Physiology, Survival, and Adult Return of Hatchery-Reared Chinook Salmon

Abstract

Increasing hatchery raceway flow to a rate similar to that seen in nature exercises salmon in the expectation that improving swimming ability will result in better survival. However, insufficient water prevents most hatcheries from fully simulating natural stream currents. We examined the effect of moderate and seasonally variable flow rates (0.25–0.75 body lengths/s) on growth and physiology of juvenile Chinook salmon Oncorhynchus tshawytscha and their ability to withstand acute stress, survive downstream migration, and return as adults. Changes in salmon growth, condition, and hematocrit followed similar patterns and did not substantially vary between treatments or cohorts. Patterns of change in plasma glucose levels were also similar for each treatment. Hepatosomatic index was higher in the 1994 cohort than in the 1995 cohort and in the exercised salmon. Liver glycogen levels were higher in the 1995 cohort. Plasma glucose and cortisol levels increased after stress and were greater in the control. Hematocrit decreased following stress but did not differ between cohorts or treatments. Hepatosomatic index decreased following stress and was higher in the 1994 cohort. Liver glycogen levels did not change following stress or between treatments but were greater in the 1995 cohort. Downstream survival did not vary between cohorts or treatments. Migration time did not vary between treatments, but the 1995 cohort migrated more quickly. Harvest and stray rates were very low and did not vary between cohorts or treatments. Hatchery return, total return, and total survival rates did not vary between treatments, but the 1995 cohort had greater survival. A greater percentage of the exercised salmon returned at age 3 than was the case for the control salmon. Our results provide little evidence that rearing Chinook salmon under a moderately increased and seasonally variable flow regime confers any benefit to the salmon over that derived from a steady, low flow rate.