Abundance trends within the freshwater mussel assemblages of seven major southwestern Ontario rivers

Freshwater mussels are important components of aquatic ecosystems yet represent one of the most imperiled taxa worldwide. The Unionid Monitoring and Biodiversity Observation (UMBO) network was established to quantitatively assess mussel abundance and population trajectories across seven watersheds in southern Ontario, Canada: the Ausable, Bayfield, Grand, Maitland, Saugeen, Sydenham, and Thames rivers. In total, 52,072 individuals representing 33 species, including 13 species with conservation status, were collected from 8,799 quadrats over 25 years. Multi-species models were developed to estimate assemblage-level and species-specific population trajectories within each watershed, with trends calculated for 133 species-within-watersheds populations. Nearly half (62) exhibited significantly positive trajectories, whereas only 12 showed declines. Among species with conservation status, most populations increased (23 of 35), while only 3 declined. Overall, many freshwater mussel populations at long-term index sites in southern Ontario increased in abundance since the onset of monitoring; however, these trends have not been linked to associated improvements in habitat condition or water quality. This work underscores the value of long term monitoring programs for detecting population change in long lived freshwater taxa.