Flooding in the Shiawassee Flats and Implications for Fish Recruitment in Saginaw Bay

Abstract

River floodplains can provide migratory fish species with spawning, nursery and foraging areas; but are among the world’s most threatened ecosystems due to pervasive land drainage, mainstream dredging, channel straightening and levee construction. The availability of food and habitat in estuary wetland areas varies with the frequency, timing, extent and duration of flood pulses. The relationship between floodplain dynamics and biological productivity is further complicated in temperate areas where mismatches between life history staging and flooding are frequent. The floodplains of the lower Saginaw River Watershed have been substantially altered by development with unknown impacts on migratory fishes in Saginaw River and the Saginaw Bay of Lake Huron. As anthropogenic changes continue to alter surface and Great Lakes hydrology it is important that we better understand how lower river and coastal fisheries are affected by floodplain connectivity. I examined the dynamics and annual trends in spring flooding of the Shiawassee Flats area, a large floodplain wetland of the Saginaw River, and the corresponding abundance of juvenile migratory game and forage fish in Saginaw Bay during a twenty- year period (1996 to 2015). Annual spring and summer flood inundation was highly variable in timing, duration and extent. Across the 20 year period, catch rates for juvenile walleye and yellow perch were not significantly correlated with inundation or other hydrologic variables. Juvenile rainbow smelt catch rates were correlated with the duration of inundation while juvenile gizzard shad catch rates were correlated with flood extent and earlier seasonal flooding. When the analysis was restricted to the period following the recovery of natural walleye recruitment in the Bay (2006 to 2015), juvenile walleye, rainbow smelt, and white bass densities were correlated with longer lasting floodplain inundation, while juvenile yellow perch numbers were associated with earlier flooding. Juvenile spotttail shiner were correlated with later, longer floods. Overall, I found that some statistical links exist between floodplain inundation in the Shiawassee Flats and juvenile populations of key migratory species in Saginaw Bay. However, the volatile ecological history of the Bay fish community, together with the limited size of my data series, suggest observed correlations between YOY catches and flooding in the Flats region are preliminary at best and further exploration is required.