How efficient is extraction of stream insect larvae from quantitative benthic samples?

Abstract

The efficiency of extraction of Agapetus larvae (Glossosomatidae : Trichoptera) from quantitative benthic samples was estimated by the use of two approaches: an indirect estimate based on the probability of missing a specimen in two scans of a sample (the two-count method) and a direct estimate derived by re-picking samples. Both approaches gave similar estimates for the probability of missing a specimen of a given size and both showed that this probability increased as size decreased; losses ranged from 6% to 30%. The two-count method thus provides a reliable way to compensate for these losses. Such corrections made little difference to conclusions concerning Agapetus population density, larval growth or secondary production derived from a previous study. However, when no corrections are applied, mortality estimates may be inaccurate if early instars are picked with less efficiency than later ones.