The Brown Meagre from Tunisian coasts, Sciaena umbra (Sciaenidae): sexual cycle, age and growth

The aim of that work was to study reproduction and growth of Sciaena umbra, for which the biology still remains largely unknown on its whole distribution area. Brown Meagres, caught by artisanal fishing gears and trawlers, are bought in the central market of Tunis and come from Tunis bay. Fish were sampled several times by month from September 1995 to August 1996. The whole sample comprises 227 juveniles, 343 females and 238 males, with standard lengths (SL) ranging respectively from 9 to 19.6 cm, 10.4 to 41.2 cm and 10.7 to 36.6 cm. For the reproduction study, we have established a scale of gonad maturity using macroscopic observation; then, the percentage of mature specimens by sexes was calculated for each size class and the sizes related to 50% and 100% taken as the length at first maturity. The periods of sexual cycle were determined by analysing monthly the evolution of the gonadosomatic index (GSI). Moreover, to obtain some information on the species physiology, the hepatosomatic index (HIS), the condition factor (K) the nutrition index (NI) and the length-weight relationships were calculated for each period of the sexual cycle and statistically compared. Concerning the growth study, we have used, for the first time to our knowledge, scales to estimate the age for this species. The annulus was described in detail and photographed. The monthly evolution of marginal increment was analysed. Retrocalculated lengths were estimated by the non-linear proportional method of back-calculation attributed to Monastyrsky (in Francis, 1990) considering the obody proportional hypothesiso or BPH. Retrocalculated lengths were modelised taking into account the biological age (birth date on July and annulus formation on April). Sizes (SL) related to 50% and 100% of matured individuals were, respectively, for males 20 and 22 cm and for females 21 and 23 cm. Sciaena umbra reproduction happens from March to August. Gonad maturation takes place from March to June; spawning occurs in July and August while sexual resting extends from September to March. During the sexual resting period, Brown Meagres accumulate hepatic lipidic reserves. Feeding is significantly reduced throughout gonads maturation. Spawning leads to a decrease of condition and this is also confirmed by the fact that length-weight relationships point out a negative allometric growth only during spawning and an isometric one during the rest of the year. Only one annulus was registered on the scale in relation