An Evaluation of a Fall/Spring Volitional Release Strategy for Hatchery Spring Chinook Salmon

Many salmonid fish hatcheries in the Pacific Northwest attempt to rear and release juveniles under conditions similar to those experienced by wild populations. Warm Springs National Fish Hatchery, a spring Chinook salmon hatchery located on the Warm Springs Indian Reservation in north-central Oregon, employs a release strategy where juveniles can volitionally migrate downstream either as sub-yearlings during a fall period or as yearling smolts the following spring.  We PIT tagged three brood years of hatchery production to compare the juvenile migration behavior, downstream survival, and adult return rates of fish volitionally leaving the hatchery during the fall and spring periods.  The number of fish leaving the hatchery during the fall was highly variable, ranging from 15% to 60% of the hatchery’s total annual release.  Fall sub-yearlings appeared to overwinter in the freshwater before migrating to the ocean the following spring.  Outmigrant survival rates of fall released fish were significantly lower than spring released fish. Release to adult survival rates mirrored the juvenile survival patterns, with fish that volitionally left the hatchery during the spring surviving at 2 to 6 times the rate of fall released fish.  The fall/spring volitional release program at Warm Springs National Fish Hatchery has been successful in maintaining wild migration traits in the hatchery population, however the contribution to adult production from fall released fish appears to be small.  Managers need to weigh the benefits and risks of wild-like rearing and release strategies when determining how best to meet hatchery goals and objectives.