BRIEF COMMUNICATIONS Timing and rate of sexual maturation of European eel in brackish and freshwater environments

The European eel Anguilla anguilla (L.) is a catadromous fish: individuals grow and feed in brackish and fresh waters as yellow eels until sexual maturation, when they metamorphose to the silver stage and leave continental waters to reach their spawning grounds in the Sargasso Sea. Sexual maturation of European eels, as well as sex differentiation, depend upon body size rather than age, with fastgrowing individuals maturing earlier than slower ones (Vøllestad & Jonsson, 1988; De Leo & Gatto, 1995). Sex differentiation and maturation, along with the differences in the growth paths of males and females, play a key role in determining the size and sex structure of A. anguilla populations. European eels show indeed delayed sex differentiation and clear sexual dimorphism, with females attaining larger body size and starting sexual maturation later than males (Vøllestad, 1992; Krueger & Oliveira, 1999). Accordingly, the population size structure of the spawning stock is clearly bimodal. The differences in maturation between males and females affect the timing of migration, alter the sex ratio and influence the effective output of spawners of European eel populations. Identifying the triggers for metamorphosis and understanding the way environmental factors affect sex maturation of European eels is one of the preliminary steps needed to develop reliable demographic models which, in turn, can provide