Cybium, Revue Internationale d’Ichtyologie
Copyright © 2003 Société Française d’Ichtyologie

(1) Laboratoire de sclérochronologie des Animaux aquatiques, Centre IRD de Bretagne, BP 70, 29280 Plouzané, FRANCE.

Résumé. - Les otolithes (sagitta) de larves de Cubiceps pauciradiatus récoltées au cours de neuf campagnes océanographiques réalisées dans le golfe de Guinée ont permis d’estimer leur âge et leur croissance. Après avoir présenté une technique simple d’extraction des otolithes, la formation journalière de microstructures a été démontrée sur 47 larves en suivant l’évolution de l’accroissement marginal au cours d’un cycle de 24 heures. Le nombre de microstructures présentes sur les otolithes a permis de déterminer l’âge de 95 larves et d’établir la relation de croissance Lt = 1,415 e^0,131t liant l’âge à la longueur standard de l’individu.

Abstract. - Age and growth of Cubiceps pauciradiatus larvae (Nomeidae) from the eastern tropical Atlantic Ocean.

Otoliths (sagitta) from Cubiceps pauciradiatus larvae collected during nine oceanographic cruises conducted in the Gulf of Guinea were used to estimate the age and growth of this species. The otoliths were extracted from larvae under a binocular microscope equipped with a polarized light and using an optic fibre mounted in a hand chuck. Cleaned and dried otoliths were glued on a microscope glass slide in a drop of clear epoxy resin, then dried in an oven (30°C). When necessary, the upper part of the resin drop was lightly ground with a 1200 µm wet sand paper and polished with 1 µm aluminium powder. Otoliths were observed under a light microscope. Microstructures were visible and each one, corresponding to one increment, was composed of 2 different layers: the incremental zone (L-zone), which is rich in calcium carbonate and the discontinuous zone (D-zone) richer in protein and poorer in calcium.
To validate the age (1 increment = 1 day) we used the marginal increment method, which consists of measuring the variation of the marginal increment (L-zone) during a 24 hours cycle. For this age validation, 47 otoliths with known hour of catch were used. Our results indicate that observed microstructures are daily increments. They also prove that the D-zone was formed rapidly, during the daytime (between 09.00 to 11.00 AM), whereas the L-zone was formed during the afternoon and at night.
Otolith reading was repeated 2 times under a light microscope (x 1000), by the same reader. The number of otolith increments permitted to age 95 larvae and to establish the growth relationship Lt = 1.415 e^0.131t between age and standard length. The observed growth rates were 0.25 mm/day for the first days of life and 0.95 mm/day for larvae 18-20 days old. These rates are comparable to those proposed by several authors about other short-living mesopelagic fishes.

Key words. - Nomeidae - Cubiceps pauciradiatus - ASE - Guinea Gulf - Larvae - Otoliths - Age - Growth.