Comparing long-term mortality of individual invasive silver carp (Hypophthalmichthys molitrix) between two large rivers

Few effective methods for controlling invasive species exist.Within invaded fish assemblages, commercial harvest is a management option.Silver carp (Hypophthalmichthys molitrix) populations are dense throughout much of the Mississippi River basin and are harvested for control and commercial use.Quantifying mortality rates of silver carp and comparing them to other studies will guide expanding removal efforts throughout the watershed.Silver carp total annual mortality was compared between the Illinois River which is heavily harvested and the Wabash River which is not.The contribution of fishing mortality to total mortality should be apparent in the Illinois River if it exceeds natural mortality.Detection and survival probabilities were determined using a longitudinal Cormack-Jolly-Seber model of acoustically tagged adult silver carp during 2011 through 2018 in both rivers, which is a unique approach only possible with long-term data from individual fishes.Proportion detection probability of individuals was consistently > 0.8 among years in both rivers.Survivorship analysis translated to total annual mortality (mean 95% confidence interval) for the tagged silver carp in the Wabash River being 0.03 0.02 across all years.This estimate was considerably lower than in the Illinois River where total mortality ranged from 0.23 0.07 in 2017 to 0.78 0.14 in 2014.Despite the high total annual mortality of silver carp in the Illinois River, densities of these fish remain high there, likely due to high reproduction of survivors and immigration from the connected Mississippi River basin.This study serves as a method for evaluating the impacts of large-scale removal efforts.But the relative impact of density-dependent intraspecific feedbacks such as improved reproduction and reduced intraspecific competition of survivors in heavily invaded ecosystems needs to be understood.