TEMPORAL, SPATIAL, AND BIOLOGICAL VARIATION OF NEMATODE EPIDEMIOLOGY IN AMERICAN EELS

American eels (Anguilla rostrata) are infected by the non-native parasitic nematode Anguillicoloides crassus, which can cause severe swim bladder damage. We investigated epidemiology of A. crassus to better understand its population-level effects on American eels. Nematode prevalence, abundance, and intensity and swim bladder damage were quantified in glass eels, elvers, and yellow eels from the lower Chesapeake Bay and related to season of capture, river system, and total length. Age-variant force-of-infection and disease-associated mortality were estimated using a three-state irreversible disease model, which assumes recovery is not possible. Results showed glass eels have very low infection prevalence and severity compared with elvers and yellow eels. Nematode abundance varied by season, river, and eel length, whereas swim bladder damage varied by season and eel length. Nematode abundance and swim bladder damage were weakly positively correlated. Force-of-infection, based on swim bladder damage, peaked at age 2, and disease-positive eels had an estimated lower annual survival probability of 0.76 compared with disease-negative eels. Full understanding of American eel population dynamics will require broader knowledge of cryptic disease-associated mortality throughout North America.