Regional variation and the effect of lake:river area on sex distribution of American eels

Abstract

Silver phase American eels, Anguilla rostrata, were collected while migrating from five rivers in Maine, U.S.A. Sex ratios varied from 49 to 98% male for these rivers and had a range of 46% over a 30 km distance between the mouths of three rivers. The proportion of male eels was inversely related to the amount of lacustrine habitat in the five drainage areas (r= 0·95, P=0·014). A combination of these sex ratios and published data from two Nova Scotia rivers showed large variation in the proportion of male eels within 1 of latitude. Thus, the hypothesis from the literature that the distribution of the sexes is dependent upon distance of larval transport was not supported. Eels migrating from lacustrine habitats within a river were predominately female, while eels migrating from fluvial habitats were predominately male, regardless of upstream distance. Apparently river habitat influences the distribution of the sexes and may play a role in sex determination. 2001 The Fisheries Society of the British Isles

A synthesis of the ecological processes influencing variation in life history and movement patterns of American eel: towards a global assessment

“…Exceptions to the above patterns are represented by higher proportions of males reported in northern subpopulations (e.g., Vladykov 1966; Krueger and Oliveira 1997;Jessop et al 2006) including freshwater systems (Facey and Helfman 1985;Krueger and Oliveira 1999), high variation in sex ratios over small geographic areas (e.g., Krueger and Oliveira 1999;Oliveira et al 2001), predominance of males in freshwater habitats (e.g., Winn et al 1975;Naismith and Knights 1990), and female predominance in brackish water (e.g., Hansen and Eversole 1984;Naismith and Knights 1990;see Fig. 3 for a summary of these relationships).…”

“…High male densities in coastal habitats are more evident in temperate latitudes (Vladykov 1966;Helfman et al 1987;Krueger and Oliveira 1999). The proportion of males has been found to be inversely related to the amount of lacustrine habitat within temperate drainages (Oliveira et al 2001), a relationship that may be influenced by the relative productivity of lacustrine and riverine environments (see below section on ''Aquatic productivity'').…”

“…3 for a summary of these relationships). Male dominated rivers have been also found at mid-Atlantic latitudes (Krueger and Oliveira 1997) suggesting that sex ratios may not be an exclusive function of latitudinal clines (Oliveira et al 2001).…”

“…In addition to the genetic evidence, highly skewed sex ratios of adult American eel (Oliveira et al 2001) strengthen the case for panmixis since different proportions of males and females are produced in different regions and only a panmictic population or quasi-panmictic assemblage of populations would enable the viability and persistence of multiple locations exhibiting extreme effective population sizes or the absence of males (e.g., in Lake Ontario and the upper St. Lawrence River ;Hurley 1972;Jessop 1987;Fournier and Caron 2005). Additional evidence of panmixis in American eel comes in the form of synchronous declines over portions of the species range and synchronous declines in glass eel size at arrival to the coast (Sullivan et al 2006).…”

“…Arguably, existing evidence supports the hypothesis that eels evolved a migratory behaviour in response to the relative habitat productivity prevailing in their geographical range (Edeline 2007)-temperate eels living at higher latitudes with higher oceanic productivity exhibit a greater probability of remaining in lower reaches in brackish water than eels living at lower latitudes ) with higher freshwater productivity. But other factors must be involved since general patterns defining the variation in life history traits and movement patterns as functions of latitude (Helfman et al 1987;Tesch 2003) or ocean-freshwater productivity gradients (Jessop et al 2004) show some inconsistencies across the entire geographical range of the species (e.g., Hansen and Eversole 1984;Krueger and Oliveira 1999;Oliveira et al 2001). Since panmixis would preclude the segregation of genotypes, local differences in life history patterns would be most likely phenotypic in origin (Jessop et al 2004) and triggered by biotic and abiotic factors associated with local habitats.…”