AGE AND GROWTH ESTIMATION OF THE EUROPEAN EEL (ANGUILLA-ANGUILLA L) IN CONTINENTAL WATERS - METHODOLOGY, VALIDATION, APPLICATION IN MEDITERRANEAN AREA AND COMPARISONS IN EUROPE

The individual age of eels in the continental waters is still difficul t to evaluate. The otolithometry is the only method used. A short desc ription of the preparation techniques and otolith observations is give n, with their advantages and disadvantages : whole observation, grindi ng, slices, burning-cracking, dyeing, scanning electron microscopy, ch emical analysis and image analysis. All these methods are based on the interpretation of seasonal marks on the otoliths. In spite of the ext reme diversity of the methods, the choice of one universal and reliabl e is difficult. Applications have to be validated for each population of this species which has a strong adaptative plasticity. There is onl y few validation experiments of the age estimations of eels in the lit erature (knowledge of the timing of the mark deposition on otoliths). Such experiments are presented for two Mediterranean populations : a d irect validation with tetracycline labelling in a natural pond in Cama rgue, and a semi-direct validation with the observation of the timing apparition of otolith marks in lagoons. These experiments permitted to determine a precise rythm of mark deposition and to identify which ma rks can be used to estimate the age. A large opaque zone appears durin g the spring, a large hyaline zone deposits in summer and a structural stainable discontinuity is led down each winter. These marks can be u sefull for the age estimations of eels in the Mediterranean area. Ther e is a great variability of the growth marks, of the rythms deposition s and of the otolith morphometry. This variability is shown for indivi duals of the same or different populations. These results conduct to t he think that there is some ''otolith ecotypes'' : the different ecoty pes are in relation with the environment and seems to depend on the sa linity and probably on the trophic level; it shows also the individual history. This variability influences directly the individual growth. Growths in Mediterranean environments such as lagoons were measured an d compared to those of the literature. The growth in brackish water en vironment is greater than those in freshwater environments.