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THE USE OF ADAPTIVE CLUSTER SAMPLING TO ESTIMATE THE POPULATION SIZES OF QUADRULA FRAGOSA AND LAMPSILIS HIGGINSII IN THE ST. CROIX RIVER BETWEEN INTERSTATE PARK AND OSCEOLA, WI
Daniel J. Hornbach, Department of Biology, Macalester College
The winged mapleleaf mussel (Quadrula fragosa) and the Higgins' Eye Pearlymussel (Lampsilis higginsii) are federally endangered species. The stretch of the St. Croix River from Interstate Park to Osceola, WI contains one of only two known reproducing populations of Q. fragosa and is one of only two of the essential habitat areas for L. higginsii that have not been infested with zebra mussels (Dreissena polymorpha).
For proper management of these endangered species it is important to have reasonable estimates of the population size. Stratified adaptive cluster sampling (ACS) has been shown to be effective for sampling rare and clustered populations. Freshwater mussels are sedentary organisms making them prime candidates for ACS, especially when implementing a two-stage design (Smith et al. 2004) Hornbach et al., (1996) found Q. fragosa to be clustered and associated with higher overall mussel density.
We sampled the area between St. Croix Falls, WI and just upstream of Osecola, WI. This region was divided into 3 sites: Interstate, Franconia and Peaslee Lake. We sampled 0.21 km2 at Interstate and estimate there are 4250 Q. fragosa. The density of Q. fragosa at Franconia is much lower than at Interstate and we estimate there are 2400 individuals. No Q. fragosa were found at Peaslee Lake. There is a great deal of error associated with these estimates (coefficient of variation of 50-66%) and we did not sample a prime area for Q. fragosa between Folsom and Blast Islands (0.3 km2 we did not sample because of past intensive sampling and movement of Q. fragosa for propagation purposes). With this prime area included we estimate there are approximately 13,000 Q. fragosa in the area between Interstate Park to Franconia.
The mean L. higginsii density varied from 0.008-0.015 individuals/m2 (coefficient of variation of 66-167%) at the three sites. We estimate a total of about 8,900 individuals in the areas we sampled. If we were to include the area not sampled we estimate a total L. higginsii population of 14,400 individuals in this area.
ACS, while helpful in estimating population size was more effective for Q. fragosa than L. higginsii, because of the more clustered nature Q. fragosa individuals. We recommend using two-stage stratified sampling with simple random sampling rather than ACS for future population estimates.
Smith, D.R., J.A. Brown, and N.C.H. Lo. 2004. Application of Adaptive Cluster Sampling to Biological Populations, pp. 75-122. In Thompson, W.L. Sampling Rare and Elusive Species: Concepts Designs, and Techniques for Estimating Population. Island Press, Washington, DC.
Hornbach, D. J., J. G. March, T. Deneka, N. H. Troelstrup, and J. A. Perry. 1996. Factors influencing the distribution and abundance of the endangered winged mapleleaf Quadrula fragosa in the St. Croix River, Minnesota and Wisconsin. American Midland Naturalist 136(2): 278-286.