A SIZE AND AGE-STRUCTURED MODEL OF THE EUROPEAN EEL (ANGUILLA-ANGUILLA L)

The life cycle of the European eel (Anguilla anguilla) presents severa l distinctive features, such as high plasticity in body growth, marked sexual dimorphism, sex ratio strongly skewed in favor of females and sexual maturation largely dependent upon the size of individuals. A de mographic model incorporating all these characteristics is derived on the basis of a multiple classification of individuals by age and size, and variability in individual growth is explicitly included. Existing theory for size-structured stocks is extended to include the dependen ce of sexual maturation on size, while natural mortality is age specif ic. Using 1989 population data from Comacchio lagoons (Italy), we esti mate mortality and metamorphosis rate and abundance in each age- and s ize-class for both yellow and silver eels, crucial information for the management of the Comacchio fishery. The use of a nonparametric techn ique (bootstrapping) yields not only the moments, but also the distrib utions of these estimates. Validation of the model is performed on the data collected in 1990. The approach adopted is very flexible and dif ferent assumptions about survival, sexual maturation, and net selectiv ity can be easily incorporated in the model.