Population density, growth and diet of eels, Anguilla anguilla L., in two rivers in eastern England

Abstract. The population density, growth and diets of eels, Anguilla anguilla L., from four sites on two rivers in eastern England were studied over 2 years. Eel population density varied greatly between sites and seasons, few eels being caught during the winter. Mean population densities ranged from 50 to 518 eels/ha and mean biomass from 35 to 210kg/ha. Older eels were more numerous at upstream sites. Growth rates in the two rivers were generally similar, but eels 9–13 years old at upstream sites were significantly larger in terms of length and weight than their downstream counterparts.
Of 868 eels examined, only 425 stomachs contained food, the percentage of empty stomachs being negatively correlated with temperature. Asellus aquaticus was the predominant food item at all sites in all seasons and in the three size classes of eels studied. Megaloptera and Trichoptera were also important. Fish, largely bottom-dwelling species, were found mainly in the largest size-class of eels and in the winter, when few large eels were feeding. Niche breadth expands with increasing size of cel.