The relationship between growth rate and precocious sexual maturation in rainbow trout (Oncorhynchus mykiss) and coho salmon (O. kisutch)

Rainbow trout {Oncorhynchus mykiss) and coho salmon (0. kisutch) produce males which mature at an abnormally early age. The objectives of this study were to critically describe the role of growth rate in early sexual maturation of male salmonids by monitoring growth rate and precocious sexual development in individually identified fish. Underyearling rainbow trout were individually identified using coded tags. These fish were reared at 9°C and 15°C and growth was monitored twice a month in both groups over a 9 month period starting January 24, 1989. The number of precocious males in the 9°C and 15°C groups were 11% and 18% respectively. Yearling coho salmon were similarly tagged, reared at ambient temperature, and monitored for growth over an 8 month period starting March 22nd, 1989. Ten percent of the coho salmon matured precociously. In both experiments there was a similar relationship between fast growth in early spring and precocious maturation. The growth rate of fish that matured declined in autumn relative to non-maturing individuals. Condition factor (length to weight relationship) was greater among those fish that matured precociously. Plasma growth hormone was monitored in the coho salmon. Most samples fell below the detection limits of the assay (1.5. to 3.0 ng/ml). A strategy to reduce the number of precocious males in cultured salmonids is suggested.Land and Food Systems, Faculty ofGraduat