9404 - Laserfiche WebLink

Home Browse Search

condensed into a capture history (i.e., CH) matrix (Burnham et al., 1987; Lebreton et al., 1991). Fifty-seven matrices were derived (three camps over 19 trips). Closed population estimates.---Population estimates were generated from each CH-matrix under assumption that the three stream reaches contained closed populations. This was appropriate given the brief sampling period at each camp (see Otis et al., 1978), and because only adults were censused. Closure was tested by examining numbers of individuals tagged within one reach then recaptured within a second reach during the same trip. Nine different closed-population estimates were derived from each CH- matrix using an updated (30 Dec. 1991) version of the computer program CAPTURE (G. C. White, D. R. Anderson, K. P. Burnham, and D. L. Otis, Los Alamos Natl. Lab., 1982, unpubl.). Models and assumptions are explained in Otis et al. (1978), Pollock et al. (1990), and Nichols (1992). The single best-fitting population model, as indicated by goodness-of-fit tests and comparisons between competing models, was retained. In this first analysis, population estimates were made relative to one another by dividing each by length of reach (in km). ANCOVA (Proc GLM; SAS, 1985) then contrasted relative population estimates by reach, using fishing effort as a covariate. In a second analysis, tag/recaptures were evaluated for the entire LCR (rather than by reach). Here, 19 CH-matrices were generated, one for each month of study. Again, the single best- fitting population model was retained. ANOVA was used to test the 19 estimates against those summed by reach for each month. The hypothesis under test is that monthly estimates are not significantly different from those summed by month over reaches. Finally, a third analysis collapsed all tag/recaptures into a single CH-matrix (i.e., each column of the CH-matrix represented a single month). Here, five best-fitting estimates were retained. However, assumptions of closure may be violated in this analysis by movements of G. cypha into/from the mainstem Colorado River over the 19-month study interval, and by recruitment of juvenile chubs into the adult population. Thus, while this analysis is a logical culmination of population estimates (a) by reach, (b) by month summed over reach, and (c) solely by month, results are heuristic rather than practical. RESULTS Fishing effort and unadjusted population estimates.---Fishing effort differed significantly among reaches (F=6.40; P < 0.0035; Proc GLM; SAS, 1985), with effort at Salt Canyon greater than that at Confluence (Sidak's multiple range test; SAS, 1985). Efforts at Salt and Powell Canyon reaches were statistically similar, however. Population estimates (normalized by river km) 4