Contribution to knowledge of the biology of Alestes macrophthalmus gunther (Pisces: Characidae)

Summary Contributions to the biology of Alestes macrophthalmus in Central Africa are reviewed and methods of data collection for the purposes of the present work are given. The structure and sexual composition of the L. Bangweulu A. macrophthalmus population, based on the data collected, are discussed. It was found that fish had four stages in the life cycle: a juvenile inshore stage, up to 50 to 70 mm; a juvenile offshore stage, 70–110 mm; an adult, predominantly male (70%), inshore population with the mode at 140 mm; and an adult, predominantly female (70–80%) offshore stage with the population mode at 200 mm. The population died out by 340 mm. The occurrence method was used in the analyses of stomach contents. Thirty two different food constituents were found, but seven of these constituted 94.6% of all food taken. Feeding behaviour was found to change as fish grew, with minnows dominating other constituents after a length of 160 mm had been reached. The question of size of fish prey in relation to shape is discussed. Seasonal changes in diet appeared to be dependent on availability of food. The Bangweulu Alestes were found not to be anadromous, breeding taking place primarily along the sandy wave washed western shoreline. Eggs were demersal. Breeding occurred throughout the year, but was concentrated in September and December. Male fish were more precocious than females and breeding appeared to be dependent on female activity. The linear regression of eggs on fork lengths for lacustine fish plotted as egg production per unit length show that as fish increase in length the proportionate number of eggs produced also increases, but at an ever decreasing rate. Lake fish appear to be more fecund than swamp fish. Mean egg production of 39 fish subjected to egg counts was approximately 10,000. Individual fish bred at least twice each year. A growth curve is postulated on evidence which is not entirely satisfactory. It is suggested that fish take a year to reach 80 mm, reach 120 mm by the end of their second year, 190 mm by the end of their third, 250 mm by their fifth, during which year they die. The possible reasons for the migrations of age groups, and the ecological effects of the species in Lake Bangweulu, are discussed. It is concluded that, on the evidence offered by this work, biological utilisation appears to be adequate and that the low productivity of Lake Bangweulu must be related to edaphic factors.Field work was carried out while with The Joint Fisheries Research Organisation, P. O. Box 48, Samfya, Northern Rhodesia. (New Zambia).