Graduate Thesis Or Dissertation
 

Life-history strategies of rainbow trout (Oncorhynchus mykiss) across a volcanic disturbance gradient at Mount St. Helens, Washington

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  • In this study, I examine the influence of habitat gradients created by the 1980 eruption of Mount St. Helens, Washington USA on a recently established Rainbow Trout (Oncorhynchus mykiss) population in the Spirit Lake basin. My goal was to evaluate the responses of fish in seven streams across a volcanic disturbance gradient that included three major zones: Pyroclastic Flow (PF), Debris Avalanche (DA), and Blowdown Forest (BD). I compared fish habitat association and demography, sex ratios among age classes, and odds ratios of maturation to determine life-history responses. Age-1 maturing males were the most common life-history observed for non-young-of-year males across all fish-bearing streams. The propensity toward an early-maturing strategy seems to be mediated by two factors: one, stream resources limit residency (no fish older than age-2) and two, a strong relationship between length and maturity. Sex ratios for age-1 fish ranged in proportion from 100% male-biased in one of three fish-bearing PF streams, 65% male-biased (though not statistically different from 1:1) in one of two DA streams, to a roughly 1:1 ratio in all other streams. These results suggest that the disturbance gradient in the Spirit Lake basin may shift the ecological costs and benefits for sex-specific flexibility in life-history strategies across the landscape.
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