Palabras clave: laguna, dorada, crecimiento, reproducción.With a yearly production of about 101,598 t in 2002 (Anonyme, 2004) in Europe and the Mediterranean Sea, cultivation of gilthead seabream has benefited from considerable research effort (Chatain, 1997;Shields, 2001). However, research on its biology in its natural environment is out-ofdate and limited (Lasserre and Labourg, 1974;Lasserre, 1976;Arnal et al., 1976;Suau and López, 1976;Chauvet, 1979;Ferrari and Chieregato, 1981;Wassef and Eisawy, 1985; Rosecchi, 1987), notably with regard to reproduction (Arias, 1980). Recently, Kraljevic and Dulčić (1997) studied its growth in the Mirna estuary in the north of the Adriatic Sea, whereas Pita et al. (2002) studied its dietary pattern within the lagunar system of Ria Formosa in south Portugal. Sex inversion in wild populations is treated from an ultrastructural point of view by Bruslé-Sicard and Fourcault (1997).In Mellah lagoon, the sparidae family is represented by seven species, with gilthead seabream occupying an important place with a production of 12 t in 1999, that is, 98% of sparid fishes. However, the biology of gilthead seabream in this particular environment has never been studied in this lagoon. The lagoon is characterised by the presence of a bordigue (fixed fishing gear) which transforms it into a big basin for extensive aquaculture based on trapping the fish. Thus, this work presents new data on the growth and reproduction of gilthead seabream in the area of extensive lagoon aquaculture. MATERIAL AND METHODSSituated in the extreme east of Algeria (8°20'E, 36°54'N), ''Mellah'' is a lagoon of 865 ha, with an average depth of 3.5 m. A total of 632 gilthead seabream were taken by professional fishermen from July 1997 to June 1998. Some of the fish (65%) were fished with monofilament gillnets that were 3 m high with a stretched mesh size of 3.5 cm. The rest (35%) came from the bordigue, particularly at the time of their migration towards the sea (from October to December).The fish total length was measured, and their age was determined by scale-reading. Five to seven scales were removed from under the left pectoral fin, cleaned and observed at a low magnification (x 32). With the help of an ocular micrometer, the total scale radius (R) and the radii of the different growth rings (R 1 , R 2 ,..., R n ) along a median vertical line were measured. In order to determine when these rings formed, we analysed the monthly variations of the scale marginal increment (MI), with MI = R-R n / R n -R n-1 , where R n and R n-1 are respectively the radius of the last and the next-to-last growth rings.The age-length relationship was backcalculated according to the Lee method (1920). The theoretical size of fishes when the first scales formed was obtained by a regression L t = f (R) based on 100 data pairs (157≤Lt≤579 mm, 1.91≤R≤8.27 mm). Sizesat-age (age-length key) were compared with the results of the backcalculation. These backcalculations were used to estimate the parameters L ∞ , K and to of the Von Bertalanff...

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Climate change has a strong impact on marine ecosystems, including temperate species. Analysing the diversity of thermotolerance levels within species along with their genetic structure enables a better understanding of their potential response to climate change. We performed this integrative study on the Mediterranean octocoral Eunicella cavolini, with samples from different depths and by means of a common garden experiment. This species does not host photosynthetic Symbiodinium, enabling us to focus on the cnidarian response. We compared the thermotolerance of individuals from 20 m and 40 m depths from the same site and with replicates from the same colony. On the basis of an innovative statistical analysis of necrosis kinetics and risk, we demonstrated the occurrence of a very different response between depths at this local scale, with lower thermotolerance of deep individuals. Strongly thermotolerant individuals were observed at 20 m with necrosis appearing at higher temperatures than observed in situ. On the basis of nine microsatellite loci, we showed that these marked thermotolerance differences occur within a single population. This suggests the importance of acclimatization processes in adaptation to these different depths. In addition, differences between replicates demonstrated the occurrence of a variability of response between fragments from the same colony with the possibility of an interaction with a tank effect. Our results provide a basis for studying adaptation and acclimatization in Mediterranean octocorals in a heterogeneous environment.

The genetic basis and evolutionary implications of local adaptation in high gene flow marine organisms are still poorly understood. In several Mediterranean fish species, alternative migration patterns exist between individuals entering coastal lagoons that offer favourable conditions for growth and those staying in the sea where environmental conditions are less subject to rapid and stressful change. Whether these coexisting strategies are phenotypically plastic or include a role for local adaptation through differential survival needs to be determined. Here, we explore the genetic basis of alternate habitat use in western Mediterranean populations of the gilthead sea bream (Sparus aurata). Samples from lagoonal and open-sea habitats were typed for three candidate gene microsatellite loci, seven anonymous microsatellites and 44 amplified fragment length polymorphism markers to test for genotype-environment associations. While anonymous markers globally indicated high levels of gene flow across geographic locations and habitats, non-neutral differentiation patterns correlated with habitat type were found at two candidate microsatellite loci located in the promoter region of the growth hormone and prolactin genes. Further analysis of these two genes revealed that a mechanism based on habitat choice alone could not explain the distribution of genotype frequencies at a regional scale, thus implying a role for differential survival between habitats. We also found an association between allele size and habitat type, which, in the light of previous studies, suggests that polymorphisms in the proximal promoter region could influence gene expression by modulating transcription factor binding, thus providing a potential explanatory link between genotype and growth phenotype in nature.