The freshwater mussel (Bivalvia: Unionidae) assemblage of the Mississagi River, Ontario: a stronghold for the federally Endangered Hickorynut (Obovaria olivaria)

Abstract

Federally Endangered Hickorynut (Obovaria olivaria) was historically known to occupy a short section of the Mississagi River. Whether this distribution represented the true range of the species or simply reflected a paucity of directed mussel sampling was unclear. The Mississagi River was systematically surveyed in 2023 to evaluate the freshwater mussel assemblage in the watershed, focussing on Hickorynut. The river was divided into 21 contiguous reaches, the first of which was 1 km long, followed by 20, 2 km reaches from the Red Rock Falls Generating Station downstream to the mouth at Lake Huron. Each reach was surveyed at least once using SCUBA; additional surveys were conducted in some reaches if warranted (e.g., historical presence of Hickorynut). In total, 35 sites were surveyed (26 SCUBA, nine snorkel) and 7590 mussels of seven species, including 141 Hickorynut were found. A single Lake Floater (Pyganodon lacustris) was found and genetically verified, representing the third confirmed detection of the species in Canada. Eastern Elliptio (Elliptio complanata) was the most common species found, occurring at all sites. Our results clarify the distribution of mussels throughout this stretch of the Mississagi River and extend the known distribution of Hickorynut ~22 km upstream of its previously known range. These findings will aid in the conservation and management of freshwater mussels in Canada and inform future assessments of atrisk mussel species.