The use of diets supplemented with live food lo increase the postrelease foraging ability of hatchery-reared fall chinook salmon Oncorhynchus tshawytscha was investigated. Replicate groups of fry were reared in six 2.4-m-diarneter circular tanks and fed one of two diets. Fish in three tanks received a commercially available pelletized diet; fish in the other three tanks were given the opportunity to forage on natural live prey (mysids, mosquito larvae, chironomid larvae, and daphnia) prior lo their daily ration of pellets. Foraging by individual lish was observed in special 200-L tanks, and fish conditioned to live food fed on twice as many familiar prey (chironomids) and novel prey (mayfly larvae) as unconditioned fish fed on. This suggests that diets supplemented with live food can be used to increase the postrelease foraging effectiveness of hatchery-reared chinook salmon.

ii EXECUTIVE SUMMARYThis report covers the 1996-1998 Natural Rearing Enhancement System (NATURES) research for increasing hatchery salmon postrelease survival and producing fish with more wildlike behavior, physiology, and morphology prior to release. Experiments were conducted evaluating automatic subsurface feeders; natural diets; exercise systems; seminatural raceway habitat enriched with cover, structure, and substrate; and predator avoidance conditioning for hatchery salmonids. Automatic subsurface feed delivery systems did not affect chinook salmon depth distribution or vulnerability to avian predators. Live-food diets only marginally improved the ability of chinook salmon to capture prey in stream enclosures. A prototype exercise system that can be retrofitted to raceways was developed, however, initial testing indicated that severe amounts of exercise may increase inculture mortality. Rearing chinook salmon in seminatural raceway habitat with gravel substrate, woody debris structure, and overhead cover improved coloration and postrelease survival without impacting in-culture health or survival. Steelhead fry reared in enriched environments with structure, cover, and point source feeders dominated and outcompeted conventionally reared fish. Exposing chinook salmon to caged predators increased their postrelease survival. Chinook salmon showed an antipredator response to chemical stimuli from injured conspecifics and exhibited acquired predator recognition following exposure to paired predator-prey stimuli. The report also includes the 1997 Natural Rearing System Workshop proceedings.NATURES type research is called for in the Columbia Basin Fish and Wildlife Program (sections 7.2D.1-3) and the ESA Snake River Salmon Recovery Plan (sections 4.4.c-d). NATURES can be used to help restore depleted stocks within the Columbia River Basin. Conservation programs may benefit from increased postrelease survival to produce more recruits to the spawning population. Production programs can use NATURES to reduce their impact on wild-reared fish by releasing fewer smolts to negatively interact with wild fish, while maintaining their recruitment levels to the fishery.iii CONTENTS PagePreface v Over the history of the project, the effects of seminatural raceway habitats, automated underwater feeders, exercise current velocities, live food diets, and predator avoidance training have been investigated. The findings of these studies are reported here and in an earlier contract report ). The first section of the current report summarizes the earlier research results. The remainder of the report focuses on research that has been conducted between 1996 and 1998. This includes studies on the effect of seminatural raceway habitat, automated underwater feeders, live food diets, predator avoidance training, and exercise on salmon and steelhead trout. IntroductionTraditionally, salmon (Oncorhynchus spp.) are reared in barren concrete raceways that lack natural substrate, in-stream structure, or overhead cover. The fish are fed in an u...

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