Sex Determination in Freshwater Eels and Management Options for Manipulation of Sex

Olfactory sensitivity to bile fluid and bile salts in the European eel (Anguilla anguilla), goldfish (Carassius auratus) and Mozambique tilapia (Oreochromis mossambicus) suggests a `broad range' sensitivity not confined to those produced by conspecifics alone

“…The different bile composition between males and females could be related to their different diets. In the case of the eel [a fish with a clearly different growth strategy between the sexes as females can be 20 times bigger than the males (Davey and Jellyman, 2005)] males have a preference for invertebrates and small fish whereas females prefer bigger fish (Jellyman, 2001). However, during their migration from the rivers and lakes of Europe and North Africa to their spawning grounds in the Sargasso Sea, eels do not feed (Tesch, 2003;van Ginneken et al, 2005).…”

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

“…In addition, larger female size does not necessarily mean higher growth rates but could result from a longer lifespan and later age at maturity. 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).…”

“…For example, differences in aquatic productivity along the Atlantic coast of Canada can be an order of magnitude higher in brackish estuaries than adjacent freshwater systems (Gagnon 2002). The ubiquitous evidence pointing to density as the primary determinant of sex in American eel (Krueger and Oliveira 1999;Oliveira and McCleave 2000;Tesch 2003) is only an indication of the role that aquatic productivity has on habitat selection, growth rates, and the subsequent gender determination resulting from fast early life growth (Mittwoch 1996;Davey and Jellyman 2005). Evidence indicates that slow growth prior to sexual differentiation favours the production of females in European eel (Holmgren and Mosegaard 1996;Holmgren et al 1997).…”

“…Thus, density will be correlated with sex ratio as long as density is correlated with productivity. High densities without high habitat productivity, and without the opportunity to move to lower density habitats (e.g., an isolated pond), will only produce stunted individuals (e.g., Paulovits and Viró 1986) and will not produce the high early life growth rates during the freshwaterbrackish phase deemed necessary for the production of males (Holmgren and Mosegaard 1996) or the larger individuals (generally females) found in less dense habitats (Davey and Jellyman 2005). …”

“…American eel remaining at high densities in highly productive habitat and those moving to less productive habitats achieve similar fitness through life history trade-offs. In females, there is a clearly defined trade-off between survival and fecundity (Davey and Jellyman 2005). As a result of the ideal free distribution, estuarine American eel in temperate latitudes grow faster than freshwater individuals, but size and age at maturity is lower and shorter in estuaries than freshwater systems (e.g., Helfman et al 1984).…”