Demographics and Parasitism of American Eels in the Chesapeake Bay, USA

The Chesapeake Bay supports the largest U. S. harvest of American eels Anguilla rostrata, yet little is known about the underlying demographics and production rates that sustain these harvests. Chesapeake Bay subestuaries (Sassafras, Chester, Choptank, Patuxent, Potomac, and James rivers) are expected to provide productive but varying growth habitats for yellow-stage American eels due to differences in land use, prey availability, and salinity. We compared length, age, growth, mortality, condition, and health (prevalence and incidence of parasitism by the swim bladder nematode Anguillicola crassus) of yellow American eels (n = 850) in the six principal subestuaries. As has been observed for other systems, female American eels in the Chesapeake Bay were larger, older, and heavier and had higher growth rates than male, intersexual, or undifferentiated American eels. Prevalence of male and intersexual individuals in the upper bay region was higher than that in the lower bay and higher than that reported for estuaries in South Carolina, Quebec, and the Hudson River, New York. American eel demographic attributes (gender, length, weight, condition, age, growth, and parasitism) differed substantially among the subestuaries. Individual growth rates ranged from 26.7 to 149.3 mm/year; the Choptank River had the highest mean growth rate (72.7 mm/year), and the Chester River had the lowest (60.2 mm/year). Estimated instantaneous loss rates (0.52-1.01 per year) did not vary systematically among subestuaries. Prevalence of parasitized American eels ranged from 17.8% to 72.0% and was higher in the upper bay subestuaries than in the lower bay. Swim bladder damage and Anguillicola crassus presence were not associated with American eel age or growth rate. In summary, female prevalence, growth rates, and condition were lower and parasite prevalence and intensity were higher in the less-saline upper bay than in the lower bay, suggesting fundamental differences in the productivity and spawning contributions between these two regions.