Effect of sampling effort on the efficiency of the timed search method for sampling freshwater mussel communities

Qualitative sampling is recommended over quantitative sampling when the objective of a mussel survey is to find rare or endangered species. Although advice on how to conduct quadrat surveys is available, there is little information on the influence of sampling effort on the effectiveness of timed searches. We conducted timed searches for mussels at 28 sites on 5 rivers in southwestern Ontario using 4.5 person-hours (p-h) of sampling effort/site. The survey period was divided into 3 equal time intervals, and the number of species and individual mussels collected within each interval were recorded. Increasing the sampling effort dramatically improved the detection of rare species. Sampling for 1.5 p-h, which is the level of effort commonly used in timed search surveys, would have resulted in <1/2 of the rare species being found and would have underestimated species richness by an average of 37%. Seventy percent of encounters with 3 federally endangered species took place during the 2(nd) and 3(rd) time intervals. Increasing the sampling effort from 1.5 to 4.5 p-h/site resulted in ail but 2 of the 28 species being found at more sites. Even 4.5 p-h of effort may be insufficient for detecting all species at all sites or estimating species richness for a river. Researchers who rely on qualitative surveys to determine the presence of rare and endangered mussel species should consider the amount of time they spend at their sites because sampling effort can significantly affect their ability to detect these species.