Quagga mussels continue offshore expansion in Lake Michigan, but slow in Lake Huron

The impacts of invasive dreissenid mussels on the Great Lakes ecosystem strongly depend on their biomass, distribution, and population trajectories. In order to assess recent population trends of dreissenids in the upper Great Lakes we conducted extensive ponar and benthic imaging system (BIS) surveys throughout Lake Michigan (including Green Bay), and the US portion of Lake Huron in 2018 and 2019. We used the two survey methods to quantify the biomass, density, size-frequency distributions, and small-scale spatial arrangement patterns of Dreissena spp. We show that dreissenid populations are continuing their expansion into deeper, offshore regions of Lakes Michigan since the most recent surveys conducted in 2015. In Lake Michigan, average areal biomass of Dreissena spp. was 25.4 g shell free dry mass (SFDM)/m2 with maximum biomass occurring between 81 and 100 m (76.1 g SFDM m2 ± 49.2). Average areal biomass was lower in US Lake Huron (20.7 g SFDM/m2) with peak biomass between 51 and 75 m (21.5 g SFDM m2 ± 58.9 SD). Size-frequency distribution data shows that recruitment is occurring in the deepest portions of both lakes, and suggests that the expansion of dreissenids to deep waters of the upper Great Lakes is likely to continue. We show good agreement between ponar and BIS data and join others in recommending regularly supplementing ponar sampling with BIS methods in dreissenids surveys to increase survey speed and collect novel types of data about mussel populations.