GROWTH, MORTAILY AND OPTIMUM CATCHABLE SIZE OF COILIA MYSTUS IN OUJIANG RIVER ESTUARY

Coilia mystus is one of the main fishing species in Oujiang River Estuary and it has high economic value. Based on the fishery resources monitoring data collected in Oujiang River Estuary in Wenzhou from 2015 and 2018 to 2020, the present study analyzed the biological characteristics (n=1359) of C. mystus, including body length, total length and body weight. The population growth and death parameters were estimated by body length frequency distribution. The results indicated that the average body length of C. mystus in Oujiang River Estuary was (15.1±3.0) cm, with the group of predominant body length 14.0—18.0 cm, accounting for 54.3%; the average body weight was (16.8±10.1) g, with the group of predominant body weight 5.0—25.0 g, accounting for 72.0%. The distribution of body length and body weight differed significantly between males and females (P<0.05). The average body length and body weight of females were greater than those of males, but the body length-weight relationship was not significantly different (P>0.05). The relationship between body length and weight was expressed as: W=0.0035×L^3.0783 (R²=0.9509). The power exponent b was 3.0783, which is consistent with uniform growth characteristics. The growth parameters of von Bertalanffy formula estimated by ELEFAN were L_∞=26.60 cm, k=0.47, and t₀=−0.55 a. The total mortality coefficient (Z) was calculated using a length-converted catch curve as 2.30. The natural mortality coefficient (M) estimated referring to the Pauly empirical formula, was 1.00. Therefore, fishing mortality coefficient (F) was 1.30, and the exploitation rate (E) equaled 0.567, demonstrating that the stock had been over-exploited. The catch curve also determined that the current first capture L₅₀ was 14.50 cm, corresponding to an age of 1.13, which was lower than both the critical age (1.36 a) and the turning point age (1.85 a). This indicated that the main fishing objectives were juveniles and included the recruitment group of C. mystus. Under the current resource status in Oujiang River Estuary, great amount of effort should be made to reduce fishing mortality and protect C. mystus habitat. It was suggested that the control of catchable length should be the main management strategy for the trend of miniaturization and low age of C. mystus resources. According to the Beverton-Holt dynamic model, the minimum capture size for C. mystus should be 15.50 cm (age 1.31 years).