Cost-effective monitoring of lakes newly infested with zebra mussels
This project will develop recommendations for underwater survey methods to estimate zebra mussel population abundance and distribution. Having better survey methods will guide and improve treatment options and pre- and post-treatment monitoring.
Currently, when a lake is newly infested with zebra mussels, it’s difficult to know the extent of the population. The earliest stages of a lake colonization are difficult to monitor because abundance is low, mussels are sparsely distributed, and their small size makes them hard to find and count. These survey methods are needed by agencies, watershed districts, and lake managers who are confronted with a new infestation.
SCUBA divers will use line-transect sampling to create, test, and standardize sampling methods to quantify zebra mussel populations. They will work in ten to twenty newly infested lakes throughout Minnesota over two years. Simulation modeling will also be used to evaluate the efficiency of alternative survey designs and to provide recommendations regarding appropriate sampling effort.
Implementing these recommendations will result in standardized data which will help guide zebra mussel treatments and evaluate their effectiveness statewide and beyond.
As of January 2019, field surveys were complete and three different survey techniques were implemented: double-observer surveys with distance sampling, double-observer surveys without distance sampling, and quadrat counts. Two approaches for analyzing the data were developed: a straightforward approach that can be implemented with existing open-source software and a more refined approach that can be used to explore the effect of covariates (such as plant presence) on detection probabilities and zebra mussel density. Both methods produced density estimates that were 3 times larger than the observed densities (uncorrected for detection). These results demonstrate the importance of estimating and adjusting for detection probabilities <1 rather than relying on observed counts when comparing densities over time or space.
In summer 2018, the field team has visited 30 lakes and sampled roughly half. Sampling was split up into phases: the first two phases were used to quickly assess relative abundance and spatial distribution of mussels in a set of candidate lakes without attempting to estimate detection probabilities or correct for imperfect detection. The third phase was used to more rigorously compare alternative survey methods useful for estimating abundance (i.e., correcting, as necessary, for mussels not observed in the surveyed area) in a small number of low density lakes.
Results from the 2017 field season have been analyzed and a manuscript is underway. Students from Carleton College completed a simulation study to explore the efficiency of different survey designs using simulations. Their results support the use of distance sampling for estimating density of zebra mussels in lakes, but point to the need for increased sampling effort to reduce uncertainty associated with density estimates.
Zebra mussel survey design
Watch this video to learn more about survey design, data entry, and observer methods.