TRENDS IN VITAL-RATES OF THE EUROPEAN EEL - EVIDENCE FOR DENSITY-DEPENDENCE

We used a population model to investigate possible density-dependent p rocesses in the European eel (Anguilla anguilla L.). We derived this m odel to include fundamental features of this catadromous fish that hav e been historically neglected, such as high plasticity in individual g rowth, length-dependent sexual maturation, sexual dimorphism, and skew ed sex distributions. Populations were sampled three times in the Vall i di Comacchio lagoons during 1974 through 1989, a period during which both commercial yield and density declined. A bootstrap procedure per mitted us to estimate demographic parameters and evaluate their uncert ainty. With the model we could estimate eel recruitment to lagoons and the density of prereproductive eels per length and age class, especia lly 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 femal e body size, as well as age and length of both sexes at sexual maturit y, increased. Density-dependent processes, such as competition for lim ited food and space, and trade-offs between reproductive success and s urvival provide a parsimonious explanation (though not one we could te st) for these shifts in population parameters.