Stock Assessment and Management of Dilplodus: Speciee in Abu Qir Bay, Alexadria, Egypt

The present study deals with the fisheries of two Sparid fish species in Abu Qir Bay. Family Sparidae represents about 17 % of the landed catch from the Bay. Among the family; genus Diplodus is generally caught by trammel nets, beach and purse seines but mostly by longlines. Two species of this genus were considered here for study. They are namely D. sargus and D. vulgaris where they represent by about 4.2% of the landed catch during the year 2008 in the Bay (GAFRD, 2008). The length weight relationship of both species under study showed a value of exponent “b” of very slight negative allometry. Analysis of the catch length structure of the two species revealed that the catch of D. sargus is composed of 21 length groups, the least length was 7.5 cm and the biggest was 27.5 cm with mean length 16.16 cm, while the catch of D. vulgaris is composed of 19 length groups starting from 8.5 cm to 26.5 cm with mean length 16.23 cm. The most abundant length group was 17.5 cm for the two species and both have nearly the same mean length. Study of the sex ratio showed that the females dominate the population in both species. The study of condition factor “K” showed that the value of “K” decreases as the fish increases in length and also varies with season. Length scale relationship was found to be linear. Back calculations of the total lengths by the end of each year of life, showed that the rate of growth of D. sargus was higher than D. vulgaris in both length and weight. Thus D. sargus was 11.42 cm total length at age I and reached 25.98 cm total length at the sixth year of age. For D. vulgaris it was 10.34 cm total length ate age I and reached 25.52 cm total length at the sixth year of age.Growth in weight studies show that for D. sargus, the fish was 26.77 gm total weight at the end of the first year of life, while it was 300.47 gm at the sixth year age of life. For D. vulgaris, it reached 18.30 gm total weight at one year old, while it was 267.24 gm by the end of the sixth year age of life. No difference in rate of growth between males and females (Linear and Ponderal). This is true for both species. Besides, the values of Von Bertalanffy parameters were estimated and compared for the two species. D. sargus comprises seven age groups from age group 0 to age group VI as well as D. vulgaris. The most abundant age groups are I and II for D. sargus, while in D. vulgaris it was age group II. The values of total, natural and fishing mortalities for both species were estimated. They were slightly higher in D. sargus than in D. vulgaris. The same was true for the values of growth performance in length and weight (ФL and Фw). The Survival rate was slightly lower in D. sargus (0.34), than in D. vulgaris (0.35). The values of exploitation rates were also higher in D. sargus (0.445) than in D. vulgaris (0.428). The length at first capture was nearly the same for both species (12.51 cm and 12.52 cm “Total length”) for D. sargus and D. vulgaris respectively. The corresponding ages were 1.208 year for D. sargus and 1.42 year for D. vulgaris. Fishes belonging to D. sargus reach 10.43 cm TL when they recruit to the population, while D. vulgaris reach 10.62 cm TL at recruitment. These correspond to 0.81 years in D. sargus and 1.05 years in D. vulgaris. The yield per recruit obtained by the analytical model of Beverton and Holt, 1957 was found to be 27.76 gm for D. sargus and 25.99 gm for D.vulgaris. The biomass per recruit for D. sargus is 57.16 gm, while it is 57.90 gm for D. vulgaris. The estimated yield per recruit and biomass per recruit at F0.1 (Target reference point) were 27.7617 gm and 57.16 gm for D. sargus and were 26.5552 gm and 54.87 gm for D. vulgaris respectively. These values were corresponding to fishing mortality values of 0.486 year-1for D. sargus and 0.484 year-1for D. vulgaris. The maximum yield per recruit (Y/Rmax) for D. sargus was 31.1233 gm which correspond to a value of Fmax= 1.239, while for D. vulgaris it was 29.7867 gm corresponding to Fmax =1.245. At Fmax the percentage of biomass per recruit value to virgin biomass was 16.26 % for D. sargus and 16.09 % for D. vulgaris. The effects of fishing mortality together with age at first capture on the values of yield per recruit were estimated for the two species under study. It showed an increase in age at first capture (tc) has little effect on Y/R in both species under constant value of fishing mortality. Cohort analysis (VPA, age based) represent the estimated values of the population numbers, Survivors, Natural and fishing mortalities for each year of life of D. sargus and D. vulgaris in Abu Qir Bay. It is noticed that, the populations of the two species under study decreased gradually with age, this is due to the exposure to the sequence of natural mortality (which decrease with age too) and fishing mortality (which appeared in different trends in the two species). It showed that, the catch is based mostly on fish of age groups I, II and III in both species under study. On the other hand, the effect of F on Y/R at different values of natural mortality (M) showed that the Y/Rmax value corresponding to higher values of fishing mortality at higher values of natural mortality, for both species. From another point of view, it was found that the value of Y/R decreases as the value of natural mortality “M” increases. This is true in both species. This shows the effect of water pollution on fish production. This study concluded that, the fisheries status of D. sargus and D. vulgaris in Abu Qir Bay reached the target reference point (F0.1) but did not reach the overexploited phase, because they did not reach the limit reference point (Fmax) for the two species under study.

Unpublished