Seasonal Variation in Food Preference and Feeding Ecology of Two Juvenile Marine Fishes, Pseudotolithus senegalensis (Sciaenid

Aspects of the feeding habits and diet of juveniles of the cassava croaker Pseudotolithus senegalensis (Valenciennes, 1833) (Sciaenidae) and bigeye grunt Brachydeuterus auritus (Valenciennes, 1831) (Haemulidae) in the inshore waters of Cape Coast, Ghana are reported in the study. Both species had a similar diet consisting of larvae of fish, shrimps and cuttlefish. The croaker ingested prey up to 77% of its total length while the bigeye grunt consumed prey up to 70% of its total length. Analysis of the stomach contents, based on the frequency of occurrence, and numerical and gravimetric compositions of these food items indicated that shrimps were the main food of P. senegalensis while B. auritus fed mainly on fish. While shrimps constituted the predominant food of the croakers all-year-round, the bigeye grunts appeared to shift their preference for fish to shrimps from June to September. The results of the study are compared with those on croakers from other parts of the Gulf of Guinea. The study, however, appears to be the first to give a detailed account of the food preference and feeding ecology of the bigeye grunt. Introduction Juveniles of many marine fish species spend some time in shallow coastal waters where they feed and grow to sub-adults before migrating into deeper waters. Successful recruitment of juveniles to the fishable stock, therefore, depends largely on their survival in the shallow coastal waters (Pitcher & Hart, 1982). In Ghana, the inshore beach seine fishery exploits juveniles of many species, and this could have a negative impact on the successful recruitment of sub-adults into the fishery. An understanding of the ecology of these early developmental stages would be of crucial importance for the effective management of the fishery. Information on the food habits of the different age groups in a fish stock is essential for assessment of the stock (Wallace & Fletcher, 1996) as it defines the trophic requirements of the various developmental stages. It is also an important requirement for the ECOPATH model (Christensen & Pauly,1992) used in elucidating aquatic ecosystem functioning and generating critical biological information for fisheries management (Mendoza, 1993; Silva et al., 1993). In Ghana, the food and feeding habits of only a few marine species of economic importance have been studied (e.g. Kwei, 1978; Blay & Eyeson, 1982; Yankson & Azumah, 1993; Blay, 1995ab). The cassava croaker (Pseudotolithus senegalensis) (Valenciennes, 1833) and the bigeye grunt (Brachydeuterus auritus) (Valenciennes, 1831) are important in the Ghanaian demersal fishery (Koranteng, 2002), and their juveniles constitute a significant proportion of the catches in the nearshore beach seine fishery. Whilst the food and feeding habits of P. senegalensis have been reported in the eastern part of the Gulf of Guinea (Anyanwu & Kusemiju, 1990; Tientcheu & Djama, 1994), there is no known record on the food and feeding habits of B. auritus in this ecosystem. Aspects of the dynamics of B. auritus in Ghana have, however, been reported (Asabere-Ameyaw et al., 2000). This paper reports on the diet and feeding habits of the cassava croaker and bigeye grunt as part of the process of gathering baseline information for a detailed investigation of the trophic structure of fish communities in the Gulf of Guinea ecosystem. Materials and methods Monthly samples of B. auritus and P. senegalensis were obtained from beach seine landings between Cape Coast and Elmina from March 1996 to March 1997. The specimens were immediately preserved in 10% formaldehyde solution in the laboratory and the total length (TL, mm) of individuals was measured and the body weight (W, g) determined. West Africa Journal of Applied Ecology (WAJAE) –ISSN: 0855-4307 Volume 9 (Jan – Jun 2006) www.wajae.org Volume 9 (Jan – Jun 2006) Page 2 of 6 The stomach contents of the fish specimens were identified and individual food items counted and weighed. The length of the largest prey item (Lmax) ingested by each fish was measured to establish the relationship between predator and prey. Further analysis of the stomach contents was performed to determine the percentage frequency of occurrence (%F), numerical percentage composition (Cn) and the gravimetric percentage composition (Cw) of the different food items. The index of relative importance (IRI) of each food item was calculated using the following equation: IRI = (Cn + Cw) %F (Pinkas et al., 1971) Results and discussion A total of 631 stomachs of juvenile B. auritus (43–140 mm TL) were examined and of these 25.4% were empty, while 12.6% of the 435 P. senegalensis stomachs examined (76–225 mm TL) were empty. Both species fed entirely on larvae and fingerlings of fish, shrimps (Peneidae) and cuttlefish (Sepia spp.), indicating stenophagy in the species because of the narrow spectrum of food items consumed. Analysis of the stomach content data (Fig. 1) shows that the main food items of B. auritus were larvae and fingerlings of fish including those of its own kind, which might suggest this species practices cannibalism. Fish larvae and fingerlings occurred in 45.5% of the stomachs, and accounted for 85.8% of the total number of food items while comprising 62.5% of the weight of food consumed. In P. senegalensis, peneid shrimps were the main prey items, occurring in 54.5% of the stomachs, with numerical and gravimetric compositions of 79.4% and 74.8%, respectively. Cuttlefish were, however, less significant in the diet of both species.