Habitat modelling of native freshwater mussels distinguishes river specific differences in the Detroit and St. Clair rivers of the Laurentian Great Lakes

Abstract

Native freshwater mussels (Bivalvia: Unionidae) were seemingly pushed to extirpation following the establishment of dreissenid mussels in the St. Clair (SCR)–Detroit River (DR) System and the current state of unionids in the main channels of the SCR remains unknown. To assess remnant unionid populations, the DR and SCR were surveyed in 2019 and 2021, respectively, using a mixture of stratified random, historical, potential refuge, and model-selected (SCR sampling only) sites (n = 56 DR sites and 51 SCR sites). Data collected from the DR (2019) were used to create unionid species distribution models in MaxEnt, which produced the model-selected sites for sampling the SCR. A total of 14 live unionids representing 9 species were found among 7 sites in the SCR; however, the DR model projected to the SCR failed to be predictive for unionid presence in the SCR (0 % success). Additional models were created for the DR and SCR using MaxEnt and classification and regression tree (CART) Analysis to elucidate the characteristic differences between the two rivers. MaxEnt modelling showed the highest contributing variable for the DR model was water velocity (m/s) while the highest contributing variable for the SCR model was river distance to the outlet which could be one explanation for why the DR model did not accurately predict unionid presence in the SCR. Additionally, when DR and SCR data were combined in the CART model, the model failed to predict any live unionid sites from the SCR. Variability in model predictions demonstrate the characteristic differences between the DR and SCR. Therefore, model verification for both rivers specific models took place in 2022 (n = 8 DR sites and 6 SCR sites). The DR unionid species distribution model identified 3 additional sites with live unionids (38 % of predicted sites), but the SCR model did not locate any additional live unionid locations (0 % of predicted sites). This research updated our knowledge on native unionid distributions and habitat use in the DR and SCR which could contribute towards unionid conservation in the future.