Long-term assessment of ichthyoplankton in a large North American river system reveals changes in fish community dynamics

Larval fishes are sensitive to abiotic conditions and provide a direct measure of spawning success. The St. Clair – Detroit River System, a Laurentian Great Lakes connecting channel with a history of environmental degradation, has undergone improvements in habitat and water quality since the 1970s. We compared 2006–2015 ichthyoplankton community data with those collected prior to remediation efforts (1977–1978) to identify patterns in spatial and temporal variability. Both assemblages exhibited a predictable phenology, with taxa from the subfamily Coregoninae dominant in early spring followed by families Osmeridae, Percidae, and Moronidae (May–June) and Cyprinidae and Clupeidae (June–August). While higher densities of larval fish were found in the Detroit River, greater taxa richness and Shannon diversity were observed in the St. Clair River. System wide, 14 new taxa were observed in the 2000s study period. In addition, relative densities of two nonnative species, alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax), declined since the 1970s. Increased larval fish richness and decreased densities of nonnative taxa in the 2000s are consistent with improvements to environmental conditions.