Individual Condition and Stream Temperature Influence Early Maturation of Rainbow and Steelhead Trout

Alternative male phenotypes in salmonine fishes arise from individuals that mature as larger and older anadromous marine-migrants or as smaller and younger freshwater residents.  To better understand the processes influencing the expression of these phenotypes we examined the influences of growth in length (fork length) and whole body lipid content in rainbow trout (Oncorhynchus mykiss). Fish were sampled from the John Day River basin in northeast Oregon where both anadromous (“steelhead”) and freshwater resident rainbow trout coexist. Larger males with higher lipid levels had a greater probability of maturing as a resident at age-1+. Among males, 40 % were maturing overall and 80 % of maturing fish had a length greater than 99 mm and whole body lipid content greater than 4 %.  Growth was greater in warmer streams whereas whole body lipid content was higher in cooler streams.  Our results support predictions from life history theory and further suggest that relationships between individual condition, maturation, and environmental variables (e.g., water temperature) are driven by complex physiological, developmental, and evolutionary influences.