Reproductive biology of four Diplodus species Diplodus vulgaris, D. annularis, D. sargus sargus and D. puntazzo (Sparidae) in the Gulf of Tunis (central Mediterranean)

The reproductive features of four commercially important species of the Diplodus genus were studied in the Gulf of Tunis to contribute to better fisheries management on the local scale. The study was specifically aimed to estimate sex-ratio and size at first maturity, to determine spawning season, to analyse hepatic and muscular energy reserve changes and to highlight possible regional difference over the Diplodus geographical distribution. A total of 570 Diplodus vulgaris (two-banded seabream), 428 Diplodus annularis (annular seabream), 350 Diplodus sargus sargus (white seabream) and 218 Diplodus puntazzo (sharpsnout seabream) were collected from the commercial catches of the artisanal fleet between October 2004 and June 2006. Male:female ratio was initially skewed in favour of females for the four Diplodus species and especially among larger-sized individuals. Length and age at first maturity was 17.4 ± 0.2 cm total length (TL) (4 years) for D. vulgaris, 9.7 cm TL (2 years) for D. annularis, 21.0 ± 0.3 cm TL (4 years) for D. s. sargus, 21.5 ± 0.2 cm TL (3 years) for D. puntazzo and a recommendation is made for the increasing of the captured length. According to the monthly succession of gonad maturity stages and the yearly gonadosomatic index (GSI) fluctuation, the spawning season of Diplodus species present a temporal succession along the year. The two-banded seabream has a winter spawning period, the white seabream is mainly a spring spawner, the annular seabream spawns essentially in summer months and the spawning of sharpsnout seabream takes place in autumn. Generally, hepatosomatic index presented an inverse trend to GSI in relation to the breeding activity. The condition factor showed significant monthly variations affected by the sexual cycle of the Diplodus species.