Variation in population and life history traits of the American eel, Anguilla rostrata, in four rivers in Maine

We examined population traits of yellow American eels from nine sites with similar habitat characteristics in each of four rivers in Maine, U.S.A. Migrating silver eels were also collected to compare sex ratio, age and size at migration among the four rivers. Population density and biomass were not significantly different among rivers with mean ranges of 8.4-21.8 eels 100 m(-2) and 380-1485 g m(-2). Pairwise comparisons of the slopes of weight-length relationships of log transformed data (pooled data: intercept = -6.007, slope = 3.094, r(2) = 0.99, and n = 3116) revealed no significant differences among rivers. Length-age relationships (pooled data: intercept = 87.826, slope = 23.444, r(2) = 0.76, and n = 2325) also showed no statistically significant pairwise differences in slopes among rivers. In all rivers, sexual differentiation was complete by 270 mm total length and age eleven. The sex ratios of migrating silver eels were not correlated with yellow eel sex ratios among the four rivers. Mean age at migration among the four rivers was significantly different for males only, with a range of 1.3 years. Both sexes had some significant differences in size at migration among rivers, but the biological importance of the differences is tenuous (male range: 15 mm, female range: 36 mm). The yellow and silver eel population traits from these four rivers showed little variation when riverine habitat was isolated. Variations in traits appeared to be greater when eels from non-riverine habitats may have been present.