Evidence of niche similarity between cutthroat trout (Oncorhynchus clarkii) and brook trout (Salvelinus fontinalis) : implications for displacement of native cutthroat trout by nonnative brook trout
Shepard, Bradley Bernard
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To evaluate whether nonnative brook trout, Salvelinus fontinalis, and native westslope cutthroat trout, Oncorhynchus clarkii lewisi, occupied a similar niche I developed and evaluated finite population correction factor (FPC) methods for estimating fish biomass in small streams (< 5 m wide). These new FPC methods take advantage of the fact that relatively high proportions of the total population are captured and can be measured and weighed during removal population estimation. Biomass estimates for these FPC methods had much smaller coefficients of variation than the traditional method for both field and simulated data. Coverage by 95% confidence intervals for the FPC methods were much closer to the 95% nominal level than for the traditional method, especially when capture probabilities were higher than 0.5. Using simulated data, I found that removal population estimates deviated significantly from true population sizes, but that these deviations clustered near zero when the ratio of captured fish to the estimated number was 0.7 or higher. Six to eleven multi-pass electrofishing efforts successfully eradicated nonnative brook trout from 1.7 to 3.0-km treatment reaches of four streams. Brook trout were eradicated to conserve native westslope cutthroat trout and evaluate competitive influences of brook trout on westslope cutthroat trout. Eradication success was related to stream size, distribution and abundance of brook trout, years of treatment, number of treatments per year, amount of cover, cover reduction efforts, and beaver ponds. Total trout biomasses significantly increased in all three streams after brook trout were eradicated, indicating that brook trout and cutthroat trout probably have similar niches and that interference competition may be occurring. Densities of juvenile and adult cutthroat trout were significantly (P < 0.05) and negatively affected by densities of juvenile and adult brook trout. I did not find a difference between cutthroat trout and brook trout density effects on body condition of cutthroat trout. I found evidence for size-asymmetric competition in one stream, but not in another. Interspecific competition between brook trout and cutthroat trout appeared to be as strong as intraspecific competition within cutthroat trout, providing insight into one mechanism by which brook trout might displace cutthroat trout.