Reproductive biology of wild goldlined seabream, Rhabdosargus sarba, captive breeding and larval development in the Sultanate of Oman

Abstract

This study investigated the, age, growth, mortality and reproductive biology of wild caught goldlined seabream Rhabdosargus sarba in the Sultanate of Oman, captive spawning of broodstock and larval development and feeding. Age was estimated by counting the opaque rings that formed in the sagittal otoliths. It was evident from the marginal zone analysis that one opaque and one hyaline were deposited annually. Length-age and weight-age data were fitted to the Von Bertalanffy growth function (VBGF), which suggested a difference in growth rate between sexes. Males mature at age zero due to the protandrous nature of this species and length at first maturity was L50%= 19.04cm T.L. Females mature at 1+ years of age and length at first maturity was L50%= 23.41cm T.L. Length at first capture was estimated to be (LC50= 13.3cm) which was below the age at first maturity for male. Length converted catch curves gave an estimated instantaneous mortality rate for the sampling site (Z=0.42) and (Z= 0.39) for the fish sampled at the fish market. This total mortality rate gave an exploitation rate of about E=0.15 at both sampling locations which suggested that R. sarba was fished below optimal levels of exploitation. Ovarian development of wild stocks was evaluated histologically. Oocyte development was classified into seven stages including atresia that was subdivided into four stages. These included previtellogenic, vitellogenic, atretic and postovulation phases of ovarian development. Testis was classified into four stages based on type of cells and their relative abundance, which included immature, developing, active and post-spawning. Testes were found to mature one month in advance of ovaries. Based on histological evidence, the natural spawning season occurred in late December to early January and continued until late February. Gonadosomatic index for male and female peaked in January and decreased sharply following spawning activity. Sterology had proven a useful tool to study on the ovarian dynamics of R. sarba. The level of plasma calcium (Ca2+) was elevated during maturity stage four (early vitellogenic phase) suggesting higher vitellogenin activity during this stage. Its level was 20% higher than of stage two maturity. The dynamics of lipid level and composition in female R. sarba during the reproductive season were also studied. Females experienced lipid depletion in liver, muscles, carcass and visceral fat as a result of ovarian development during the reproductive season. The level of total lipid correlated well with the ovarian stage of maturity (p<0.02). Fatty acid composition and profile of the ovaries at various stages of development also varied during the reproductive season. Breeding of wild stock of R. sarba in captivity was investigated. Spawning occurred after a short captive acclimatisation (two months) period without any artificial induction. Histological and ultrastructural changes in R. sarba larva in early life history were also investigated. Morphological and functional differentiation of the digestive tract and the eye of the larvae took place before the completion of yolk and oil globule absorption. Food particles were observed on the third day after hatching in the rectal area of intestine of R. sarba larvae at the time when the larval vision system appeared to be functional. Rotifer enrichment with lipid emulsions improved larval growth rate and survival as a result of improvement in the level of HUFA (DHA and EPA). The highest R. sarba larval growth (P<0.05) was obtained while feeding the larvae with rotifers enriched with oil. No significant difference (P>0.05) in larval survival was observed between feeding the larvae with rotifers enriched in sunflower oil and rotifers fed algae. However, highest R. sarba larval survival (P < 0.05) was obtained while feeding the larvae with rotifers enriched with cod liver oil. Finally it has to be said that R. sarba proven to spawn in captivity and that larval rearing was possible and that it is a candidate species for aquaculture in the Sultanate of Oman. It has to be emphasised that more research could be carried in the future on broodstock management and egg and larval quality in order to reveal more information about the species requirement.