The Length of Stream to Sample with a Towed Electrofishing Unit When Fish Species Richness is Estimated

Meaningful estimates of fish species richness in streams for assessments of environmental quality or community-level ecological analyses can be achieved only if the length of each stream segment sampled approaches or exceeds the length at which the cumulative species number becomes asymptotic. I used data from 10 representative sites on nine southern Wisconsin warm-water streams to determine the length of stream that should be sampled when species richness is estimated with a type of towed electrofishing unit commonly used in the midwestern USA. The estimated length of stream necessary to reach an asymptotic species number varied among sites; it was not related to estimated species richness, and it was only weakly and nonsignificantly correlated (positively) with stream size, Although much shorter distances were sufficient at some sites, a stream length of 35 times the mean stream width (at normal base flow), or a length equal to three complete riffle-pool sequences, ensured that the cumulative number of species captured approached or exceeded an asymptotic level. As indicators of the desired lengths, stream widths were more universal and easier to apply than riffle–pool sequences in southern Wisconsin streams.