Relations between physical habitat and American eel abundance in five river basins in Maryland

Although the American eel Anguilla rostrata occurs in a variety of habitats over large geographic areas, little is known regarding the specific habitat relations that regulate eel distribution and abundance in freshwater streams. We evaluated the importance of 17 physical habitat, chemical, and biological variables in predicting eel density in five major river basins in Maryland. Because artificial structures impede eel migration in all five basins, only sites determined to be on unblocked streams or downstream of structures that significantly restrict eel passage were used in the analysis. Stepwise regression identified a model consisting of four variables-velocity-depth diversity, the log-transformed distance (km) to the Chesapeake Bay, the log-transformed density of noneel fishes, and the distance to a semipassable or impassable structure-as the best predictor of eel densities. When applied to a random subset of data not used for model development, the model correctly predicted 44.4% of eel densities and had a high mean prediction error ( 1.86 eels/75 m(2)) compared with the mean eel density for all sites included in model development (3.33 eels/75 m(2)). However, the Pearson correlation coefficient between the observed and predicted eel densities (r = 0.64; P < 0.05) indicated that the model was generally successful in predicting trends in observed densities. The study results are consistent with previous research suggesting the lack of significant eel-stream habitat associations, although the presence of a variety of velocity-depth regimes may support higher eel densities. Higher eel densities near artificial structures known to affect upstream movement suggest that eels are accumulating downstream of these structures because of impeded migration.