Comparison of Two Life History Strategies after Impoundment of a Historically Anadromous Stock of Columbia River Redband Trout

Abstract

AbstractIn this study we collected information on abundance, age structure, migration, and exploitation to characterize the population demographics and reproductive characteristics of a historically anadromous Columbia River Redband Trout Oncorhynchus mykiss gairdneri population now isolated in a southwestern Idaho reservoir and limited to resident and adfluvial life histories. We estimated there were 3,905 adfluvial individuals in Mann Creek Reservoir in October 2008 based on a mark–recapture population estimate. The adfluvial population sex ratio of 2.78 females per male captured at a weir, peak spawn timing near the peak of the hydrograph (late April), age at spawning (4–6 years), and growth patterns (slow growth in the stream followed by rapid growth in the reservoir) were all characteristic of an anadromous population. Resident fish abundance was not estimated, but the fish were characterized by relatively slow growth, earlier sexual maturity, and a reverse sex ratio (0.23 females per male) compared with the adfluvial fish. The two life histories (resident and adfluvial) and their differential use by the sexes are consistent with life history theory, which suggests female salmonids maximize fitness by increasing body size and fecundity while males attempt to maximize survival at the expense of growth. The migratory fish in this drainage that could have historically exercised an anadromous life history appear to be exercising the next‐best option, an adfluvial life history, which has relatively similar costs and benefits to the anadromous form as distinct from the stream‐resident form. Future studies should evaluate other similar native populations isolated in reservoir systems because these populations could play a role in recovery of endangered steelhead (anadromous Rainbow Trout) populations in the western USA.