(29C) Population Size

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An extract from Willow Creek Drainage Fishery Inventory, Regional Fishery Management Investigaions, Project No. F-71-R-8

"Tributary Evaluation
Population Assessments
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Estimates from the maximum likelihood method were calculated with the aid of computer program modified from Platts, et al. (1983) for the double density version Osborne 1 computer. The program provides point estimate, 95% confidence limits, and test for constant catchability using chi-square goodness of fit. Calculations for the two pass and mark recapture estimators were made using programs written for the Hewlett-Packard (HP) Model 29C calculator (Appendices 1 and 2). These programs remove bias and calculate point estimate and 95% confidence limits. All confidence limits expressed in this report are at the 95% level.
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Mainstem Evaluation
Population Assessments
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We estimated population densities on one section of lower Willow Creek, one headwater section of Willow Creek, and one headwater section of Grays Lake Outlet. A generator-powered Coffelt shocking unit mounted in a drift boat was utilized to collect fish in lower Willow Creek. In order to estimate the population of game fish, successive trips were made through a 4.8km section located between Kepps Crossing and Clowards Crossing, marking or tagging fish on the initial run and making recaptures the following day. Nongame fish were numerous but no attempt was made to capture them due to time constraints. We used the Peterson mark recapture estimator (Everhart et al 1976) programmed into a Hewlett-Packard Model 29C calculator to obtain point estimates and 95% confidence limits of the population size. Total length of all gamefish captured was measured to the nearest millimeter and we took weights from a representative sample of the fish in order to calculate a condition factor.
To estimate population size in the headwater section of Willow Creek we used the generator-powered backpack shocker to collect fish on two successive passes and then calculated the estimated population size using the HP 29C calculator program described earlier. All trout captured were measured, tagged and released. Nongame fish were captured on the first pass to obtain length frequency information and to obtain a rough estimate of species composition.
A Peterson single mark recapture estimate was made on a 550m segment of Grays Lake Outlet above the barrier falls. We captured-fish with the backpack shocker, measured length and weight, jaw tagged them and released them. Fry and nongame species were counted and measured but no attempt was made to estimate the numbers present. The estimated number of age 1+ and older fish were calculated on the HP 29C calculator program.
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APPENDIX
Appendix 1. Calculator program for Hewlett-Packard model 29C used in estimating population size from two successive passes pg. 86
Appendix 2. Calculator program for Hewlett-Packard model 29C used to estimate population size for the mark-recapture method pg. 89

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