We used a population model to investigate possible density-dependent processes in the European eel (Anguilla anguilla L.). We derived this model to include fundamental features of this catadromous fish that have been historically neglected, such as high plasticity in individual growth, length-dependent sexual maturation, sexual dimorphism, and skewed sex distributions. Populations were sampled three times in the Valli di Comacchio lagoons during 1974 through 1989, a period during which both commercial yield and density declined. A bootstrap procedure permitted us to estimate demographic parameters and evaluate their uncertainty. With the model we could estimate eel recruitment to lagoons and the density of prereproductive eels per length and age class, especially useful information because these data cannot be collected directly. With the exception of age 1, survival increased as density declined. In addition, as density declined, the proportion of females and female body size, as well as age and length of both sexes at sexual maturity, increased. Density-dependent processes, such as competition for limited food and space, and trade-offs between reproductive success and survival provide a parsimonious explanation (though not one we could test) for these shifts in population parameters