Feeding Selectivity of the American Eel Anguilla rostrata (LeSueur) in the Upper Delaware River

Abstract

-The population of American eels Anguilla rostrata (LeSueur) residing in the Upper Delaware River was sampled monthly from May through August 1986. Gastric examination of 325 eels captured by electrofishing revealed that 154 (47%) were empty and 171 (53%) contained food items. The specimens examined contained 2992 organisms of 59 taxa (four fish and 55 macroinvertebrate) and three non-organism categories. Macroinvertebrates, predominantly of the Class Insecta, were eaten by 169 eels (99% of feeding eels). The orders Ephemeroptera and Plecoptera occurred most often in eel stomachs (69%). The stonefly Acroneuria was the single most numerically dominant taxon observed in the diet, occurring in 67% of eel stomachs that contained food. Fishes were consumed by 12 eels (7%). The clustering near zero of electivity values for the overall study suggests that eels feed opportunistically on many macroinvertebrate taxa. The values also indicate that five food taxa were significantly (P < 0.05) selected as prey by American eels: Baetidae, Drunella, Perlidae, Hydropsyche and Polycentropodidae. The orders Ephemeroptera, Plecoptera and Trichoptera accounted for 92.7% of the organisms consumed, although they comprised 48. 1% of the available organisms. Ten food taxa were significantly (P < 0.05) avoided as prey by eels: Planaridae, Oligochaeta, Pelecypoda, Gastropoda, Gammaridae, Cheumatopsyche, Lepidostoma, Elmidae larvae, Dipteran pupae and Chironomidae. The insect orders Diptera and Coleoptera and the phylum Gastropoda accounted for 4.8% of the organisms consumed, although they comprised 32% of the available organisms.