ADAPTIVE CLUSTER SAMPLING OF ST. CROIX RIVER FRESHWATER MUSSELS AT FRANCONIA, MINNESOTA

Daniel J. Hornbach, Mark C. Hove, and Ben D. Dickinson, Macalester College, Department of Biology

Estimating population size of endangered or threatened species is often difficult due to low organism density. Typically a large number of samples must be collected to estimate population size. Another approach is to utilize adaptive cluster sampling, which increases sampling efficiency for organisms that are spatially clustered. Adaptive cluster sampling involves selecting sites randomly, but collecting additional samples from neighboring cells when a target species is encountered.

We developed a modified adaptive cluster sampling technique to estimate population size of federally endangered Quadrula fragosa and Lampsilis higginsii, and Wisconsin state endangered (Minnesota threatened) Cyclonaias tuberculata in the St. Croix River. Previous studies have shown that some of these species are most commonly found in areas of high mussel density. We initiated the study by randomly selecting twenty 50-m2 cells from a 2 km river reach near Franconia, Minnesota. We sampled three 1-m2 quadrats from each cell and if at least 4 mussels were found in one quadrat we labeled the cell "high (mussel) density." Of the twenty cells sampled, ten were high density. We initiated adaptive cluster sampling in these ten high-density cells. We randomly selected ten 1-m2 quadrats within each high density cell, and collected substrate samples, measured hydrologic parameters, and counted and identified living and dead mussels. When a target species was found we sampled four adjacent quadrats. Neighboring cells were sampled until no additional target species were observed.

We collected target species from three of ten 50-m2 high -density cells. These included 5 Cyclonaias tuberculata, 2 Quadrula fragosa and 2 Lampsilis higginsii. Four of these individuals were collected from one adaptive sampling location. We are processing data to estimate population sizes and habitat characteristics for these three species. We plan to examine population size and habitat characteristics of these species at Interstate State Park during the summer of 2006, and determine the efficacy of this sampling technique in a river reach with higher mussel density.

Suggested reading

Smith, D. R., R. F. Villella, and D. P. Lemarie. 2003. Application of adaptive cluster sampling to low-density populations of freshwater mussels. Environmental and Ecological Statistics 10: 7-15.