This project is looking to develop methodology to apply a multibeam sonar swath mapping system to generate a probability map of the likely presence and relative abundance of invasive mussels at a large scale.
Current methods for detection of zebra mussel colonies rely on time consuming and expensive diving surveys, video imaging, or sampling of veligers (larvae) in the water. Survey sampling design would be much more efficient given spatially extensive information on the presence/absence of zebra mussel beds. Such remote sensing would also facilitate early detection and warning in rivers, lakes and reservoirs through routine monitoring, or to follow changes in zebra mussel density (boom-bust cycles). Multibeam sonar surveying methods have the added benefit of bathymetric and substrate classification maps, critical information for linking mussel populations to habitat.
Phase III: Mapping mussel densities and developing protocols
This project is a continuation of a laboratory study, Phase I, and a field study, Phase II, to test the feasibility of detection and mapping of mussels and their supporting substrates from a moving vessel using multibeam sonar. Phase III will apply the methods developed in Phases I and II to 1) create maps of substrate and relative mussel densities in two different water bodies at a large scale (~0.5 km 2 ) and 2) develop user-ready protocols and tools for translation of the research results to practice. As multibeam sonar systems continue to be more readily available, this will allow interested groups to utilize this technology. In lakes and rivers, this methodology will enable the scanning of large areas for 1) the detection of zebra mussel colonies, and 2) the detection of areas likely to support zebra mussel colonies (through habitat mapping) that could be prioritized for other sampling methods. Verification of mapping results will be conducted using a combination of SCUBA surveys, underwater video, and grab samples. This demonstration is necessary to make the step from field model development (Phase II) to verified field methodology. This Phase will also be used to test any remaining uncertainties or limitations identified in Phase II, for example, mixtures of native and invasive mussel species. The final field sites will be selected in coordination with ongoing efforts in the state.
The goal of this research phase is to develop a data-driven methodology to relate acoustic backscatter from multibeam sonar to substrate (sand, gravel, etc.) and relative mussel density (high, medium, low) categories and to identify limitations to the utilization of multibeam sonar for mussel surveys. The research team completed two field campaigns in the Summer of 2021 designed to gather robust datasets along 20 m transects including: a) multibeam sonar data with varying acoustic settings (pulse length and frequency), video documentation of substrate utilizing an underwater remotely operated vehicle (ROV), and c) diver visual surveys of substrate and mussel density at 1 x 1 m resolution. The team visited a zebra mussel infested lake with a wide range of substrate types (White Bear Lake) and a stretch of the St. Croix with high densities of native mussels. For each transect, the data were averaged to a 2 x 2 m grid and assigned a category (sand high-mussel density, or sand low-mussel density for example) and the data were converted to real-world coordinates to relate to the sonar surveys. Mulitbeam sonar data were post-processed to remove erroneous points and vegetation or any points within the water column and the cleaned datasets were converted to 5 x 5 cm backscatter mosaics (gridded data of the backscatter strength for each sonar setting). The data were then compared using a two different data driven models that relate acoustic backscatter to the assigned category for data at each ground truth location (2 x 2 m grid from the diver survey). These models are being used to map relative mussel density and substrate that will serve as the basis for the next phase.
Phase II: Field validation of multibeam sonar zebra mussel detection
The first phase of this project (below) revealed sufficient differences in acoustic response of native and zebra mussels and the supporting sediment. This allowed researchers to develop an empirical approach to zebra mussel detection. Phase II of this project will test the use of this and other acoustic signatures to detect and map zebra mussel beds in the field, incorporating a larger range of variables, such as a greater range of mussel densities and substrate mixtures, water depths, and temperatures.
Phase I: Testing the utility of a swath mapping system
This study will test the utility of swath mapping systems such as multibeam sonar for detecting and quantifying the abundance of invasive mussels at a very large scale. Multibeam sonar can map tens to hundreds of square kilometers of river or lake bed in a single day from a moving vessel. There is a strong likelihood that mussels have a distinct acoustic response (echo) compared to their surrounding substrate. If so, this acoustic signature can be readily used to detect and map zebra mussel beds in any navigable waterway of sufficient water depth. This study will define the methodology needed to detect, distinguish and quantify mussels from a moving vessel by studying backscattering of sound by mussels and common mussel-supporting substrates.
The first phase of this study is designed to utilize multibeam sonar to distinguish among substrate, native mussels, and zebra mussels in a controlled laboratory setting. A second phase is planned to validate and develop methodologies for use in the field.
Current methods for detecting and quantifying zebra mussel populations relies on methods that can be very time-consuming and expensive, such as diving, video imaging, and veliger sampling in the water column. Detecting zebra mussel populations early significantly improves the possibility that quarantines can be put in place and treatment options implemented.