Chronic Exposure of Chinook Salmon Eggs and Alevins to Gamma Irradiation: Effects on Their Return to Freshwater as Adults

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Abstract
Chinook salmon (Oncorhynchus tshawytscha) eggs and alevins were exposed to various levels of continuous gamma irradiation to determine the level at which the return of these individuals as adults would be affected. Large numbers of eggs (63,000 to 140,000) were used for each treatment, along with an equal number of controls. The dose rate ranged from 0.5 R/day in 1960 to 50 R/day in 1970; dose rate approximately doubled in each of the later years. After rearing for about 90 days, the control and irradiated groups were differentially marked and released for their migration to salt water. On their return 2 to 6 yr later, length, weight, and sex data were determined on each adult salmon. No significant change was noted in adult returns until irradiation levels of 10 R/day and above were used. At these levels (10 R/day, 20 R/day, and 17‐50 R/day) analyses showed retardation in growth and increased mortality of the small salmon in fresh water, decreased numbers of adults returning to spawn, increased age at return, and apparent sterility in the adult males. The dose‐response relationship did not appear to be linear.

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