Influence of Unequal Catchability on Population Estimates Using the Lincoln Index and the Removal Method Applied to Electro-Fishing

Abstract

The accuracy of the removal method was tested using 2 types of pulsing DC electro-fishing gear on juvenile trout populations of known size. The experiments resulted in underestimations ranging between 11.1-22.0% of the total population size after 4 removal fishings. Marking experiments revealed unequal catchability among individuals, partly, at least, explaining the observed underestimations. To clarify the effect of unequal catchability, the capture-recapture and removal methods, applying the Lincoln Index and Moran''s estimator, were tested by Monte Carlo simulation for various beta distributed catch probabilities. All simulations yielded considerable negative bias, proportional to the population size and rapidly increasing with a decreasing mean and an increasing standard deviation of the catch probability distribution.