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4. I I I I I I I I I I I I I I I I I I I The pooled fish/area method was used to calculate catch rates for seining. This method is simply the total number of a fish divided by the sum of area seined. The advantage of this method is that it considers all area sampled and it is the same method used in previous investigations. The disadvantage is that it does not allow for a determination of statistical variability. The geometric mean of the sample CPE's was used to compare densities of age-O squawfish. This method is being used by the Service's Database Manager on ISMP data (Personal communication, C. McAda, February 24, 1992). It involves a log transformation of CPE's, primarily because catch data for Colorado squawfish in backwaters are highly variable and not normally distributed. The geometric mean calculation method was as follows: First, a CPE for age-O squawfish was computed for each seine haul (age-O/l00m2). Then a '1' was added to the CPE of each seine haul to account for zero catches. This adjusted CPE was then transformed to a natural logarithm. The transformed values were averaged, by reach and year, and the antilog of the average minus 'I' yielded the geometric mean. Standard deviation was computed from log-transformed values, and the antilog taken to provide bounds around the geometric mean. The computation is expressed as: GMaE = antilog [(lIn) E In (CPE + 1)] - 1 where: GMaE = CPE = n = geometric mean catch per effort catch per effort of age-O squawfish (age-O/l00m2) total number of seine hauls (with and without target fish) The advantage to this approach is that it reduces variation by making the variance independent of the mean. However, log transformations tend to weight higher CPE's less and geometric me3n CPE's do not necessarily reflect actual fish densities. 3.2.3 Trappiol! Clear plastic light traps of the quatrefoil design were used in the Lake Powell inflow to capture larval fish. The traps were manufactured by Ed Tyberghein (Southern Concepts) and are similar to the quatrefoil trap described by Aoyd et al. (1984) and modified by Mueller et aL (1993). Tyberghein's light trap is 305 mID tall. 250 mm in diameter with a slit width of 23 mm. The design incorporates a styrofoam top that allows the trap fioat and a 2.25 V bulb as a light source. Power is provided by two D-cell batteries and output can be maintained for 30-40 hours. Light traps were set at dusk in slackwater habitats and allowed to fish all nighL Traps were pulled before dawn since daylight allowed the fish 10 move back out of the traps. All fish captured in light traps were pre-sorted in the field. Fish large enough to identify were counted and recorded on the data sheeL FISh too small to identify in the field were preserved. Catch rates for larval light traps are presented as number of fish per 100 hours of sampling (fish/100 hr). 3.3 STOMACH CONTENT ANALYSIS Stomach analysis were performed on most predatory species caught elecuofishing and seining. Only predators over 125 mID TL were examined since Colorado squawtish were approximately 10-11 months old and 45-75 mm 11.. Stomach contents were preliminarily identified in the field when possible and preserved in 70% ethanol Fmal identification of stomach contents was performed by BIO/WEST.