Evaluating the effectiveness of retrofitting water control structures with manatee protection systems to reduce mortality

Abstract

Minimizing human-related manatee mortality is a priority management action for Florida manatee (Trichechus manatus latirostris) conservation and recovery. Manatees encounter navigational locks and water control structures along travel corridors, and fatal encounters can occur from crushing, impingement, or entrapment that subsequently result in drowning. Extensive mitigation efforts have been made to protect manatees. This study evaluates the effectiveness of manatee protection systems at navigational locks and water control structures using negative binomial regression models and Friedman's tests with pairwise comparisons from 1974–2020 mortality data. Mitigation effort was stratified into 4 groups ranging from no mitigation to fully mitigated. Regression models for 223 structure-related manatee deaths showed the expected number of annual mortalities was ≤0.26 for all mitigation groups at each structure type. Friedman's tests indicated a difference in mortalities among mitigation efforts for navigational locks (χ₂² = 10.75, P = 0.004) and water control structures (χ₂² = 16.63, P ≤ 0.001). Mortalities at fully mitigated water control structures were lower than at partially mitigated structures in pairwise testing for both analyses; navigational lock mitigation efforts only differed in Wilcoxon rank sums tests. Combined results show that when current manatee protection systems are functional and protocols are followed, mortality is reduced. While these systems are a conservation achievement, continued re-evaluation of mitigation efforts and investigation into new technologies are needed to ensure the continued reduction or elimination of structure-related manatee mortalities.