Efficacy of Feed Deprivation During Juvenile Rearing to Reduce Precocious Maturation of Age 1+ Male Spring Chinook Salmon Smolts
High feeding and growth, relative to wild fish, can induce substantial proportions of hatchery reared male spring Chinook Salmon juveniles to mature precociously as age-2 minijacks. Released as smolts, these fish do not migrate to the ocean for rearing, and thus do not reach the adult age and size at which they can contribute to harvest mitigation fisheries and natural population supplementation. Initiation of maturation is thought to occur during a critical time period approximately one year prior to spawning. We performed a study with broodyear 2018 juveniles involving total feed deprivation over two different time periods (Early – August 8 through September 11, 2019; and Late – September 11 through October 24, 2019), to determine whether the rate of precocious maturation of the hatchery reared juveniles could be significantly reduced, while also allowing time post-treatment for the fish to achieve an adequate final smolt size through compensatory growth. At termination of the study in July 2020, the probabilities of precocious maturation of male smolts as minijacks in the Early and Late treatment tanks, as determined by logistic regression analyses, averaged 24.7% and 24.2%, respectively, which were significantly lower than the 38.1% rate for continuously fed fish in the Control tanks. Because the fish had to be pooled from their replicate tanks into a single subdivided concrete raceway for overwintering, fish from each tank were stocked into a particular section of the raceway after receiving an identifying adipose and/or pelvic fin clip combination. Unfortunately, there was some level of misidentification of fish to their original rearing tank, due primarily to regrowth of clipped pelvic fins. Errors in identification were such that the actual reduction in minijack rate was likely even larger, in particular for the Early treatment fish, for which the actual minijack rate was likely closer to 20%. In both May and July 2020, the treatment fish were similar in size to Control fish. Further study of the effects of timing and duration of the food deprivation period on precocious maturation rate and growth is recommended in order to develop a protocol for hatchery rearing of spring Chinook Salmon smolts for consideration by regional fisheries and hatchery managers.
Galbreath, P.F., C.A. Stockton, L.R. Medeiros, C.M. Knudsen, H.M. Nuetzel, and A.L. Pierce. 2020. Efficacy of feed deprivation during juvenile rearing to reduce precocious maturation of age 1+ male spring Chinook Salmon smolts. Columbia River Inter-Tribal Fish Commission Technical Report 20-05, Portland, Oregon. 22p.