The effects of x-irradiation, diethylstilbestrol, and size at time of release on the early sexual maturation of coho salmon Oncorhynchus kisutch Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ws859j95d

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  • Precocious male coho are of limited economic value and control of their numbers returning to the hatchery or the increased survival of full-sized adults would be beneficial. Factors influencing early sexual maturation and attempts to suppress this maturation of males through the incorporation of a hormone in the diet, treatment of parr with X-irradiation and control of the size at time of release by manipulation of the food were investigated. The investigation was carried out at the Fish Commission of Oregon’s Big Creek Hatchery from June 1964 to January 1967 using the 1963 and 1964 broods of coho. A portion of the treated animals of the 1963 brood was retained through adult maturation in fresh water at Big Creek and a similar group was held in salt water at Bowmans Bay, Washington. Fish in one production pond received diethyistilbestrol incorporated into the Oregon Moist Pellet diet at 200 [mu]g/g from November 1964 through April 1965. Juvenile coho that received the various exposures of X-irradiation were taken to Corvallis for treatment and then returned to Big Creek. An experiment involving manipulation of diet to achieve two distinct sizes of smolts was initiated in April 1965 and carried through to March 1966, at which time the smolts were released. Juvenile coho salmon that received the hormone in their diet produced only half as many precocious males as the control when reared in fresh and saltwater ponds and only one quarter as many precocious males when released to the ocean. The adult survival for the two groups was equal in the ponds although only 30% of the hormone group was recovered at the hatchery as compared to the control. No long term effects on the viability of the gametes was noted but the mean length of fish in the hormone group was significantly smaller than that of the control fish. X-irradiation of parr did not inhibit early sexual maturation. No long term effects on the viability of gametes was noted. X-irradiation is not a practical tool for suppression of gonad development because juvenile coho have a low tolerance. Early sexual maturity is positively related to size of smolt at time of release. The release of larger smolts resulted in a 12-fold greater return of precocious male coho, and almost a twofold greater return of adults. From the results future suggestions for investigation are outlined. They include: further investigation of chemical suppression of the pituitary; various feeding regimes in an attempt to control growth; manipulation of hatchery procedures to produce smolts that would be migrants in their first year of life and combinations of the above.
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