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UBC Theses and Dissertations

Year class strength and catchability of mountain lake brook trout De Gisi, Joseph S.


Increased gillnet catch per effort (cpe) of juvenile salmonids occurred following intense exploitation of the adult population, for several studies conducted in mountain and arctic small lakes. Higher cpe may reflect increased catchability or greater numbers, so behavioral or numerical responses cannot be inferred from changes in cpe alone. I used age structured estimation methods, and gillnet depletion data from 1986 to 1992 for seven Sierra Nevada small lake brook trout populations, to reconstruct year class strength and prerecruit (age 1) gillnet catchability prior to and during the experimental removals. I made Walters-Collie (1988) estimates of year class strength for the seven study lakes across the years of the removals. The within-year depletions and available models consistently underpredicted the number of fish remaining in the lake, so estimates did not use the within-year structure of the data. Ageing error correction provided little change in the estimated strength of cohorts produced during the mid- to late 1980's. Estimates showed an inverse relationship between year class strength and adult population size, for cohorts from 1984 to 1990. Prerecruit q[carat] also appeared inversely related to adult population density for most lake populations. This may have been either a direct effect of adult density, or indirectly mediated through the effect of adult density on prerecruit length at age. Year and cohort-specific adult q[carat]'s showed little evidence for density dependence in adult q[carat]. The vulnerable proportion of the adult population appeared insensitive to population density. I developed a modification of the WC fitting to adjust for between-lake variation in encounter probability which estimated a relative activity parameter, k[carat] . Relative to q[carat], variation in k[carat] was reduced and showed little apparent between-lake density dependence.

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