A method for quantifying hand-net samples of stream invertebrates

Abstract

Quantitative sampling using a hand-net can be accomplished by taking three successive catches of invertebrates from the same point on the streambed. This is a form of removal sampling. By plotting the decline in number of individuals in each catch against the total previously caught, the total population at the sampling point can be estimated. From this, the probability of capture in a single catch (p) can be calculated. For Agapetus, other trichopteran, leptophlebiid, caenid and gripopterygid larvae from a site on a tributary of the Acheron River in southern Australia, p varied from 0.66 to 0.81. Additional data for a species of Gammarus from the Credit River in Ontario gave a p value of 0.67. In three successive catches the overall probability of capture exceeded 95% for all taxa, indicating that with this degree of effort most individuals present were caught.