Nature and Science, 2012; 10 (2); http://www.sciencepub.net/nature 19 Aspects of reproductive biology of big eye grunt Brachydeuterus auritus (Valenciennes, 1832) Fatimat Adenike Adebiyi* Department of Marine Sciences, Faculty of Science, University of Lagos, Akoka, Yaba, Lagos, Nigeria adebiyi_fatima@yahoo.com Abstract: Aspects of reproductive biology of big eye grunt Brachydeuterus auritus off Lagos coast, Nigeria were investigated and focussed on sex ratio, gonadosomatic index, stages of gonadal development and fecundity. A total of 450 fish with body length of 11.7 – 19.3 cm (total length) and body weight of 12.4 – 110.5g were used for this study. The sex ratio of B. auritus was 1: 1.11 (male to female). The females were more than the males. However, there was no significant difference (p > 0.05) in sex ratio from the expected 1:1 distribution. Gonadosomatic index ranged from 0.18 – 12.73%. The mean gonadosomatic index was 3.96 ± 0.19%. High gonadosomatic indices were observed in July, August and September which indicated the spawning period and falls within the wet season. Quiescent, maturing and mature ovarian and testicular stages of gonadal development were observed in all samples of B. auritus collected for this study. Fecundity ranged from 12,503 – 74, 045 eggs with a mean of 32, 344 ± 1,889 eggs. This suggested that B. auritus is a low fecund fish. Fecundity–length and fecundity-weight relationships showed positive correlations. The Fecundity-length and fecundity–weight relationships were determined by regression analysis with the regression equation Log F = 1.8985 + 2.2706 Log L (r = 0.5083) for fecundity–length relationship and Log F = 4.0292 + 0.2657 Log W (r = 0.3090) for fecundity-weight relationship. Fecundity was more related to length than to weight. This information is relevant for fisheries recruitment and population dynamics as well as breeding programmes in aquaculture of B. auritus. [Fatimat Adenike Adebiyi. Aspects of reproductive biology of big eye grunt Brachydeuterus auritus Valenciennes, 1832) . Nature and Science 2012;10(2):19-24]. (ISSN: 1545-0740). http://www.sciencepub.net . 3 Key words: Brachydeuterus auritus, fecundity-length relationship, fecundity-weight relationship, gonadal development, Lagos coast, reproductive biology, sex ratio. 1. Introduction Sex ratio, gonadosomatic index, stages of gonadal development and fecundity are some important aspects of fish reproductive biology, which give information necessary for successful fisheries management and recruitment in natural water bodies and aquaculture of fish species. Sex ratio provides information on the representation of male and female fish by stating the proportion of male fish to female fish in a population and indicates the dominating sex of fish in a population which constitutes basic information in assessing reproductive potentials and estimating stock size in fish population (Vicentini and Araujo, 2003). In order to determine female spawning biomass, estimates of reproductive potential can be added to sex ratio information to give a better understanding and assessment of stock status relative to a biological indicator, which has been observed for some fish stocks (Morgan, 2008). Gonadosomatic index is an index of the size of gonad relative to size of the fish (Dadzie and Wangila, 1980) and is an indicator of gonadal development in fish. Gonadosomatic index determines the percentage of body weight of fish used for egg production. Fecundity is one of the important aspects of reproductive biology of fish species which provides information on the number of eggs in the ovary prior to the next spawning season (Bagenal, 1978). Fecundity studies are useful for systematics in racial studies that has connection with total population estimation and is also relevant in studies of population dynamics and productivity. The Lagos coast contains many economically important fish species for fishery. B. auritus is one of the fish species known as grunts and is one of the demersal resources of the Lagos coast. B. auritus forms part of the trawl fishery catch in the Lagos coast and belongs to family Haemulidae and inhabits soft muddy sandy bottoms at depth between 15-50m (Mensah and Quaatey, 2002). Information on reproductive biology of some economically important fish species in the Lagos coast have been reported by some authors (Marcus and Kusemiju, 1984; Anyanwu, 1990). There are few references to the study of the biology of grunts off the Lagos coast, Nigeria. It is pertinent to note that there is paucity of information on the reproductive biology of B. auritus off Lagos coast. The reproductive biology of B. auritus has not been widely reported. Hence, the need to carry out this study which aimed at some aspects of reproductive