Using Sonar Habitat Mapping and GIS Analyses to Identify Freshwater Mussel Habitat and Estimate Population Size of the Endangered Fat Threeridge in the Apalachicola River, Florida

Freshwater mussels are widely imperiled and vital to aquatic ecosystems, yet many species inhabit large, turbid rivers that are challenging to study. We used a low-cost, side scan sonar mapping approach to define freshwater mussel habitat at an intermediate (meso) scale throughout a meandering portion of the Apalachicola River. Significant differences in mussel counts were found between mesohabitat classes, and consistent, unexpected numbers of mussels were found also in deep water habitats. Counts of the most abundant mussel encountered, Amblema neislerii, were paired with environmental variables to develop both a presence/absence and count model. Average density per mesohabitat class and model results were used to estimate the population size of A. neislerii. We estimated ~3 - 5 million A. neislerii residing in the 700 ha study reach in areas of the channel commonly exhibiting a smooth/plane bedform.  In addition, a high degree of mesohabitat persistence was observed by analyzing sonar imagery collected before and after flood events.  Low-cost, side scan sonar helped define suitable habitat for freshwater mussels in this river, and facilitated a landscape-level population estimate of A. neislerii.