Age and growth of Diplodus vulgaris (Sparidae) in the Gulf of Tunis
Age and growth of the two-banded seabream (Diplodus vulgaris) from the Gulf of Tunis were investigated using scales and otoliths. Total length (TL) and weight (TW) ranged respectively from 10 to 32 cm and from 14 to 525 g (n = 570). The length-weight relationship was slightly but significantly allometric (b = 3.05 ± 0.02 s.e.) when data from both sexes were pooled. Marginal increment analysis proved that annuli were formed once a year, in June. Age readings were precise for the two hard structures, with low values of the index of average percent error. The power function well described the relationship between TL and scale or otolith radius (r > 0.92). The von Bertalanffy growth equation was fitted on mean backcalculated length-at-age data resulting in the following parameter values for scales (L∞ = 39.9 cm, k = 0.11 year-1, t0 = -0.73 year) and for otoliths (L∞ = 39.0 cm, k = 0.10 year-1, t0 = -0.96 year). Although scale readings provided a better fit, otolith readings could also be used with fairly good results. Among Sparids, D. vulgaris is a relatively slow growing and a moderate-lived species, with a maximum observed age of 12 years. Total length at maturity (TL 50) was 17.6 cm for males and 17.1 cm for females, which corresponds to an age of about 4 years. Large discrepancies in growth parameters between geographic areas are the result of different growth patterns.