The Age, Growth and Emergence of Larval Fish in The Upper Mississippi River

The Upper Mississippi River (UMR) contains diverse habitat structure, providing important nursery habitat for larval fish in backwater areas. However, how larval fish use these habitats and the environmental cues that contribute to their abundance and emergence is not well understood. Therefore, we evaluated environmental variables that contribute to larval fish emergence and habitats that influence their abundance. We sampled for larval fish from May-September, 2018 in Pools 17-19 in low-velocity habitats at water temps greater than 17°C using larval light traps (8 per night, n= 280 traps total). Each larval fish was identified to family and further resolved to species using genetic barcoding. Age was determined from daily otolith increments and used to back-calculate hatch date. Date of species-level emergence and abundance were modeled in relation to habitat structure, water temperature, and discharge. The average age for all fish was fifteen days and the average length was 8 millimeters. Hatch dates of Clupeidae and Catostomidae occurred in April-May, while Cyprinidae and Centrarchidae had continuous hatch dates April-August. Temperature and family had a statistically significant impact on the growth rate of the fish (millimeters per day). Information gained will contribute to a better understanding of this vulnerable life stage in a large, dynamic river system.