Estimating the density and distribution of invasive populations is critical for management and control efforts but can be a challenge in nascent infestations when population densities are low. Statistically-valid sampling designs that account for imperfect detection of individuals are needed to estimate densities across time and space. Survey methods that yield reliable estimates allow managers to determine how invader biomass affects ecosystem services and evaluate population trends and effectiveness of control measures. We investigated the use of distance sampling via self-contained underwater breathing apparatus divers to determine densities of invasive Zebra Mussels (Dreissena polymorpha) in 2 recently-invaded lakes in central Minnesota. This framework allows divers to cover the large areas necessary in low-density, recent infestations. We estimated that a diver could detect between 5 and 41% of the mussels present in the surveyed area, depending on the specific diver and on whether the lake bottom was vegetated. Presence of vegetation had a greater influence on detection probability than diver identity. We also found our sampling design did not meet a key assumption of conventional distance sampling: that detection along the transect line is perfect. Therefore, accurate density estimates required a double-observer approach. These results highlight the importance of accounting for detectability when comparing estimates over time or across lakes, particularly when different observers conduct surveys. However, further evaluation is needed to determine if changes in field sampling techniques can meet the assumptions behind conventional distance sampling for freshwater mussels. We also suggest that the efficiency of distance sampling should be compared to alternatives such as quadrat sampling across a range of mussel densities.
Bibliographical noteFunding Information:
This study was funded by the Minnesota Aquatic Invasive Species Research Center with funding from the Minnesota Environmental and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources. JF received partial support from the Minnesota Agricultural Experimental Station and the McKnight Foundation. We would like to thank Kylie Cattoor and Keegan Lund of the Minnesota Department of Natural Resources for assisting us with the data collection on Lake Burgan and for productive discussions, Steve McComas of Blue Water Science for his insights on surveying Zebra Mussels, and Rich Rezanka and Tom Jones with the Minnesota Department of Natural Resources for advice on our initial attempts to design this study. We thank David L. Smith and 2 anonymous reviewers for their comments that have greatly improved the quality of this manuscript. The data and code for analysis used in this study are available on the University of Minnesota’s data repository at https:// conservancy.umn.edu/handle/11299/201572.
- Freshwater mussels
- Sampling methods