Sex, Size, and Gonad Morphology of Silver American Eels Anguilla rostrata

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).…”

“…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).…”

“…Several studies have shown that high somatic growth rates during the first years of continental growth produce mostly males (Holmgren and Mosegaard 1996;Davey and Jellyman 2005). Increasing sex-specific divergence in somatic growth rates with age (e.g., Oliveira and McCleave 2002) has been associated with only female American eel achieving sizes enabling picivory (Krueger and Oliveira 1997;Oliveira and McCleave 2000) with higher yellow and silver growth rates than males exhibiting a generalist feeding behaviour (Helfman et al 1987).…”

“…Since mature males are significantly smaller than females (Krueger and Oliveira 1997) and females take longer to mature than males (Oliveira 1999;Oliveira and McCleave 2000), there is a bimodal size-distribution of migrating silver eel (Davey and Jellyman 2005). Eel exhibit sex-specific life-history strategies, with males growing faster than females during their early years but females reaching greater age and size at maturity and size at yellow-silver metamorphosis.…”

“…Some reports indicating extreme male-biased sex ratios of yellow or silver eel in temperate rivers (e.g., Oliveira 1999;Oliveira et al 2001) are inconclusive since these reports classified eel as male or female based solely on size (Krueger and Oliveira 1997;Oliveira and McCleave 2000). However, male-biased sex ratios have been found in temperate rivers when sex was determined through gonad examination (e.g., Krueger and Oliveira 1999).…”