An investigation into the food and feeding ecology of a potential aquaculture candidate, Sarotherodon galilaeus multifasciatus in a meteoritic crater lake in Ghana
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چکیده
An investigation into the food and feeding ecology of the cichlid, Sarotherodon galilaeus multifasciatus in a tropical meteoritic lake in Ghana was undertaken. Stomachs of the fishes, at two length classes, representing adults and juveniles, were examined and the food items identified. The juveniles of Sarotherodon galilaeus multifasciatus feed mainly on insect and insect larvae and relatively small quantities of diatoms but avoiding macrophytes. A striking feature in the composition of juvenile diet was the considerable quantities of ingested sand, which constituted about 16% of stomach contents. Ingested sand may be helpful in maceration of food in the pyloric stomach. A sandy substratum may, therefore, appear ecologically important for juveniles of the species. Adult stomachs contained mainly phytoplankton, chironomid and insect larvae with no ingested sand. No significant differences were found in the stomach contents of the fishes for the dry and wet seasons due, partly, to the lack of any considerable seasonal differences in allochthonous inputs into the lake waters. The fishes exploit a wide range of food resources in the open, sub-littoral and littoral zones of the lake. Further studies to include growth and recruitment are recommended to enhance future production of the species under culture conditions. @ JASEM Lake Bosumtwi is a crater lake located 30 km south east of Kumasi at the northern tip of the Adansi mountains 06o 30 ́ N, 01o 24 ́ W in the forest zone of Ghana. The lake is circular in shape and has a radial drainage system. The maximum diameter and depth of the lake is estimated to be 10.5 km and 80 km respectively (Jones et al., 1985). The lake covers a total surface area of 49 km. Studies and research have confirmed that the lake was formed as a result of a meteorite impact (Whyte, 1975; Jones et. al., 1981; Koeberl and Reimond, 1996). The lake, currently, holds 12 species of fish, all belonging to the taxonomic family of Cichlidae. Sarotherodon galilaeus multifasciatus constitutes the bulk of the cichlids captured and appears to be the most commercially exploited fish by, at least, 22 communities surrounding the lake. Investigations are considered necessary for a better understanding of the food and feeding ecology of both juvenile and adult of the species to enhance production of the species in aquaculture development. MATERIALS AND METHODS Fish capture and examination A combination of sampling methods for lake fishing was employed to maximise the capture of both juvenile and adult Sarotherodon galilaeus multifasciatus. Nets of 5-mm stretched mesh were used for sampling adult and juvenile fishes. Fish capture was done twice within each month between November 2003 and October 2004 to cover the dry season (November 2003 to April, 2004) and the rainy season (May 2004 to October 2004. All fish samples were removed between 06.00 hours and 08.00 hours. In addition, ten 5-mm wire mesh traps were set up in the shallows, and trapped fishes were removed within 24-hour intervals for examination and analysis of stomach contents. Habitat and benthic substrate characteristics were noted. Analysis of stomach contents Fish captured were immediately identified, counted and their total lengths measured to the nearest mm. Fish were stored under ice and rapidly transported to the laboratory for analysis. Fish lengths were categorised into two size groups of 20-49 mm Total Length (T.L) representing juvenile fish, and 50 mm (T.L) and over to represent adult fish. Representative samples of juvenile (329) and adult (334) fishes were dissected, guts removed and fixed in a mixture of 10% formaldehyde, glacial acetic acid and 50% ethanol in a ratio of 5:5:90. Stomach contents and stomach fullness were examined using the Points Method (Windell and Bowen, 1978; Hyslop, 1980). In this method each food item was awarded points in relation to its estimated contribution to stomach volume. Thus points were awarded from 1 to 4 as follows: 1 = 1⁄4 full stomach; 2 = 1⁄2 full stomach; 3 = 3⁄4 full stomach; 4 = full stomach. The percentage contribution of each food item to stomach fullness in each size class was then determined on the basis of the awarded points. Monthly mean stomach fullness was determined for juveniles and for adults to elucidate the impact of spawning and mouth brooding on feeding intensity. RESULTS AND DISCUSSION Sarotherodon galilaeus multifasciatus has a wide feeding range. The percentage contribution of food items to stomach fullness in juvenile and adult fishes of Sarotherodon galilaeus multifasciatus are presented (Fig. 1). There were significant differences in the stomach contents for the two length classes (Levene’s test of Equality of Variances, p< 0.05), over the study period. An investigation into the food and feeding ecology of a potential aquaculture * Corresponding author: Amisah, S 16
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