Spatiotemporal Variability in Spring Spawning Dynamics of Northern Lake Michigan Migratory Fishes

Dozens of fish species undertake reproductive migrations from the Great Lakes into tributary rivers, yet the composition and timing of these migrations are poorly documented.  To characterize patterns in phenology, demography, and assemblage of spring spawners in Lake Michigan, we captured fish in- and out-migrating over two years in six tributaries. We found that Longnose Suckers (Catostomus catostomus) and White Suckers (Catostomus commersoni) were the dominant spring migrants, followed by Northern Pike (Esox lucius) and Steelhead (Oncorhynchus mykiss). All of these species entered streams within a short temporal (~10 days) and thermal (6-8^oC) window, but the duration of migrations were more protracted than expected.  Interannual arrival timing of spawning migrants varied by up to 4 weeks and was significantly correlated with temperature and discharge, although the strength of the relationship varied by species and stream. The demography of migrating suckers suggests size and sex-selective mortality may be occurring on larger males, which could have important implications for egg fertilization rates and recruitment.  Our detailed records of the phenology of spring migratory fishes provide insight into the reproductive biology of Great Lakes fishes, and offers additional context for conservation efforts regarding ecosystem subsidies, non-native species, critical habitat protection, and climate change.