Early maturing males in a partially migratory population of anadromous and resident rainbow trout Oncorhynchus mykiss: influences of individual condition and stream temperature

Graduation date: 2009Alternative male phenotypes in salmonine fishes arise from individuals that mature as either larger and older anadromous marine-migrants or as smaller and younger freshwater residents. Variability in age and size of males at maturity is hypothesized to be preceded by early differences in growth in size and lipid storage. Water temperature is one factor expected to influence growth and lipids. To better understand the processes influencing the expression of alternative male phenotypes, I examined the influence of growth in length (fork length) and whole body lipid content on freshwater maturing males in a mixed population of anadromous and resident rainbow trout (Oncorhynchus mykiss). I conducted this study in the John Day River basin in northeast Oregon where both anadromous and freshwater resident rainbow trout coexist. During the summer of 2007 I collected 168 age-1+ O. mykiss (80♀/88♂) nine months prior to the spawning season from thirty different streams. Water temperatures were recorded hourly in these streams\ud from September 2007 through August 2008. Based on the temperature data I identified eight streams with contrasting thermal regimes and delineated those into two groups (warm and cold) to examine associations with growth and lipid storage. I determined sex and state of maturity visually and with the aid of microscopy. Length was measured in the field, age was estimated via otoliths and scales, and whole body lipid content was determined with the diethyl ether extraction method. Results indicated that larger males with higher lipid levels had a greater probability of maturing as a resident at age-1+ than smaller males with lower lipid levels. Among males, 40% were maturing and 80% of those fish had a length greater than 99 mm and whole body lipid content greater than 4% compared to only 19% of the immature males. Comparisons of streams with contrasting thermal regimes indicated that water temperature was associated with growth and lipid content in different ways. Growth was greater in the warm streams while whole body lipid content was higher in the cool streams. My results provide further support for the theoretical expectation that differences in individual condition precede male life history development and suggest that water temperature has a strong influence on the measures of condition commonly used to predict resident male maturity. However, my results also suggest that relationships between individual condition, maturation, and environmental variables (e.g., temperature) are complex and may represent influences of physiological, developmental, and evolutionary tradeoffs. This represents a first step towards understanding how individual condition and environment interact to influence the expression of alternative male phenotypes in rainbow trout