Parmi les espèces dotées d’une haute valeur commerciale et présentant des priorités pour la commission générale des pêches pour la Méditerranée (CGPM) ainsi que pour le conseil international pour l’exploration de la mer (CIEM), le merlu (Merluccius merluccius) occupe une place importante. Malgré la richesse de la bibliographie sur cette espèce, des lacunes persistent encore, particulièrement sur les modalités de sa croissance. A partir de l’analyse des accroissements journaliers des otolithes des juvéniles du merlu provenant de la côte nord de la Tunisie, les deux phases de croissance, pélagique puis démersale, ont été décrites. La durée de la phase pélagique a été estimée à 54 jours et, à la fin de cette phase, la longueur totale du merlu est estimée à 8 mm. Durant cette période, le taux de croissance est estimé à 0,159 mm jour-1, tandis que durant la phase démersale, le merlu a une croissance qui augmente fortement avec un taux de croissance moyen de 0,734 mm jour-1. A la fin de la première année, le merlu atteint alors une taille de 28 cm dans les eaux tunisiennes.

Hake (Merluccius merluccius) is widely distributed in the Atlantic Ocean and the Mediterranean Sea. It is one of the most exploited demersal fish. In Tunisia, hake is captured from the north to the south (KHOUFI et al., 2010). Hake is an important species of economic interest and thus the subject of a large literature, but its biology remains insufficiently known, particularly on terms of age and growth estimations, which are essential for stock assessment and management. Otolith microstructure provides a useful tool for age determination of juvenile hake, based on daily increment deposition (ARNERI & MORALES-NIN, 2000; MORALES-NIN et al., 2005; KACHER & AMARA, 2005; BELCARI et al., 2006). Consequently, we studied growth and estimated age, using otolith microstructure, during the first year of the hake's life off the northern coast of Tunisia. Specimens were caught from November 2010 to January 2011 and in March 2011, using bottom trawlers. Each specimen was measured to the nearest lower centimetre. The range of lengths obtained was from 8 to 30.5 cm. Left sagittae otoliths were removed from fishes, cleaned and stored dry. Twenty-three calcified pieces were used for micro-increment analysis. Length (± 1 μm) of the otolith major axis was measured with the TNPC software (numerical treatment of calcified pieces). Sagittae were mounted on glass slides and polished with fine abrasive grit to obtain thin sections. Sections were viewed under transmitted light using a microscope, connected to the image analysis software (TNPC) via a camera. Otolith interpretation was made by estimating the daily growing increments, from the core to the radius, and from the radius to the otolith edge. Specimens were examined double blind and only measurements that agreed in two successive readings were accepted. Two phases were analysed: the pelagic phase, which corresponds to the duration in daily growing increments from nucleus to the edge of accessory growth centre, and the juvenile phase, corresponding to the age of the fish from the nucleus to the edge of the otolith. The duration, rate growth and total length of the fish were estimated, analysed and described for both phases through linear and power regressions. Statistical analyses were carried out using the software R and the text editor Tinn-R. The linear regression between somatic growth and otolith growth was significant (Radius=3.959.Age+16.3; R²=0.683; P<0.001), which is concordant with previous studies in the Mediterranean (GARCIAS-RODRIGUEZ & ESTEBAN, 2002; MORALES-NIN & MORANTA, 2004; BELCARI et al., 2006) and in the Atlantic (OTXOTORENA et al., 2010). The mean pelagic phase duration was estimated at 54 ± 8 days. Compared to other studies in the Mediterranean Sea and the Atlantic, this result is similar to those reported for the Tyrrhenian Sea (BELCARI et al., 2006) and the northwestern Spanish Atlantic (PINEIRO et al., 2008). Hake total length at the end of the pelagic phase was estimated to be 8 mm and, during this period, growth rate was found to be 0.159 ± 0.018 mm.day-1, which was near to the estimated growth rate in the Catalan sea (0.15-0.17 mm.day-1, PALOMERA et al., 2005). After settlement at the bottom, growth rate increased greatly, reaching a mean value of 0.743 ± 0.122 mm.day-1. Consequently, , the mean growth rate during the first year of Merluccius merluccius off northern Tunisia was estimated to be 0.635 ± 0.114 mm.day-1. Our study indicates that, during their first year of life, hakes grow at a faster rate than is commonly accepted. Estimation of hake size at one year is in agreement with the growth model derived from recent tag-recapture data for hake (DE PONTUAL et al., 2006; MELLON-DUVAL et al., 2010).