Some biological aspects and fisheries assessment of Diplodus vulgaris (Geoffrey Saint-Hilaire, 1817) (Teleostei: Sparidae) caught by gillnets (Egyptian Mediterranean waters, Alexandria)

Abstract

This study was conducted to estimate some biological aspects and fisheries assessment of Diplodus vulgaris caught by gillnets during the period 2018–2020 from the Egyptian Mediterranean waters of Alexandria. From an overall of 1004 individuals of D. vulgaris (10.0 – 24.0 cm TL) examined, lengths from 13.0 to 17.0 cm were representing 82.67% of the total fish number. The length-weight relationships for total and gutted weights of the D. vulgaris were: Log W = −1.7144 + 2.9063 Log L (R² = 0.9982) and Log W = −1.8508 + 2.9903 Log L (R² = 0.9967), respectively. The condition factor (k) was found to be fluctuating between 1.268 and 1.534 with an average value of 1.395 ± 0.0597. The overall sex ratio (M: F) was 1.00: 0.63 and the chi-square test revealed highly significant difference (χ² = 24.80; df = 1; P < 0.01). The spawning season of D. vulgaris was observed from December to February for both sexes. The gonadosomatic index (GSI) reached its highest value in December for both sexes. The age composition of the D. vulgaris catch was estimated to be four years and the first age dominated the population. The present von Bertalanffy growth parameters were calculated as follows: the coefficient of growth (K) = 0.128 yr.^-1, the asymptotic length (L_∞) = 37.06 cm, and the hypothetical age at zero size (t_o) = −2.773 yr.^-1. The growth performance index (Ф') was: Ф'_L = 2.24, and Ф'_W = 1.00. The reproductive load (L_m/L_∞) was low for males (0.39) and females (0.37). The ratio L_max^D/ L_m^D was 1.36 and 1.39 for males and females, respectively and it was found to be compatible with the Pauly's Gill-Oxygen Limitation Theory (GOLT). The total mortality coefficient (Z) was 1.79, the natural mortality (M) was 0.39 and fishing mortality (F) was 1.40. The M/K was 3.0615 and Lc/L_∞ was 0.371. At the level of the current exploitation rate (E_cur = 0.78), the stock is considered highly exploited by gillnets fishery and only 9.34% of the stock biomass is protected. Fishery management for this wild stock is highly required.