Detectability Affects The Performance of Survey Methods - A Comparison of Sampling Methods of Freshwater Mussels in Central Texas
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Designing effective surveys for freshwater mussels (Unionidae) is a challenge, because they are spatially clustered and often found in low densities. The objective of this study was to examine how the effectiveness of three different survey methods (timed searches, transect method, and adaptive cluster method) varied between different habitats at six sites in the San Saba, Guadalupe, and San Antonio Rivers in Central Texas. Species richness, the total number of mussels per search effort, species composition and size distribution obtained with different survey methods were compared between sites. Timed searches were generally the most effective method in detecting species especially when densities were low (≤ 0.2 individuals per m2) or mussels were highly clustered. The adaptive cluster method, however, was as effective as timed searches in detecting species when densities were moderate or higher (>2 ind. per m2) and detected more species than timed searches at a site at which habitat conditions hindered searches. The performance of adaptive cluster in respect to number of mussels found per unit search effort seemed to be enhanced by sandy substrate facilitating the detection of mussels, and timed searches were less effective at sites at which habitat conditions hindered the detectability of mussels. Differences in detectability of mussels was not only associated with habitat conditions, but also with the size of mussels, their behavior and morphology. Timed searches detected a larger proportion of larger mussels that tended to be less burrowed and that had shells with more sculpturing compared to quantitative methods. In addition, surveyors with more search experience detected a larger number of mussels. Our results suggest that to design effective surveys variation in detectability of mussels must be considered which depends on local habitat conditions, experience of surveyor, behavior, size and morphology of mussels.