Pen Rearing and Imprinting of Fall Chinook Salmon, 1985 Annual Report.

Upriver bright fall chinook salmon (Oncorhynchus tshawytscha) are being reared in a backwater and a pond along John Day Reservoir to evaluate the benefits of rearing fish and releasing them off-station compared to traditional hatchery procedures. Fish reared in net pens at a density/feeding combination judged to be the economic optimum of those used during 1984 rearing trials exhibited good growth and smolt development. Size of fish averaged 112 fish/lb (4.0g/fish), ATPase activities ranged from 16.4 to 29.5 micromoles Pi/mg prot/hr at release and total mortality of fish was low among pens, ranging from 0.3 to 1.1%. Poor growth and smolt development was observed in fish reared in a large barrier net, especially during the initial two weeks after stocking. In addition, mortality of fish in the barrier net was high (49%) in relation to any of the other treatments tested thus far. The combined effects of generally poor condition of fish at stocking, low zooplankton densities during the initial two weeks of rearing, and losses to predation were thought to be the primary causes of the slow growth rates and high mortality. Unfed fish in pens utilized the available natural food base, but zooplankton densities were apparently not sufficient for growth, and may have been marginal for sustenance, especially at higher density. ATPase activities at release were significantly higher in low-density pens than in higher density pens, but development at all densities was retarded when compared with ATPase activities of fed fish. Preliminary cost estimates for producing fish-using the rearing strategies developed in the current pen-rearing study compared favorably with the average costs of rearing salmonids in a Northwest hatchery