SynopsisThe African bonytongue, Heterotis niloticus (Osteoglossidae), is an important fisheries and aquaculture species in West Africa. This species has frequently been characterized either as an omnivore, insectivore or detritivore, the latter, in part, because of its benthic feeding habitats and possession of a gizzard (thickwalled pyloric stomach). We examined diets of two populations of H. niloticus in the SoˆRiver in southern Benin. A population from the river channel and seasonally flooded marginal plains was dominated by juvenile and subadult size classes. Adults size classes were common in a second population from Lake Hlan, a natural lake in the river floodplain located upstream from the channel study region. Heterotis of all sizes consumed a variety of food resources, ranging from aquatic invertebrates to small seeds. Aquatic invertebrates composed a large proportion of the diets of juveniles, and adults consumed a mixture of aquatic invertebrates, seeds, and detritus. Seasonal dietary variation was observed in both populations, and diet breadth was not significantly different between populations. Aquatic invertebrates remained significant in diets of larger size classes; diets of fish between 100 and 200 mm began to include seeds and detritus, with a marked increase in the volumetric proportion of detritus in diets of fish between 300 and 400 mm in Lake Hlan and between 500-600 mm in the river. Relative gut length was inversely related to body size, which supports the notion that Heterotis is an omnivore and not a specialized detritivore. The thick-walled gizzard of Heterotis, which generally contained sand, probably aids digestion of seed coats. Because Heterotis consume mostly invertebrates and grass seeds in shallow waters of seasonal aquatic habitats and lakes in the river floodplain, foraging success and fishery production should be strongly dependent on the annual flood pulse.

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Two consecutive experiments were conducted to study the e¡ects of stocking density on growth, food utilization, production and farming pro¢tability of Nile tilapia (Oreochromis niloticus) ¢ngerlings (initial mean weight:16.2 AE 0.2 g) fed Azolla, as a main component in diet. In experiment 1, ¢sh were hand-fed twice daily with three isonitrogenous (28.5% crude protein) and isocaloric (14.5 kJ g À1 ) diets A30, A35 and A40 containing 30%, 35% and 40% Azolla, respectively, for 90 days. Diets were formulated by mixing Azolla with locally available by-products. No signi¢cant di¡erences were found in growth parameters and production (P40.05). Total investment cost was signi¢cantly higher with A30 (Po0.05), but same pro¢tability values were obtained with all diets (P40.05). In experiment 2, three stocking densities, 1, 3 and 5 m À 2 , were assigned to three treatments T 1 , T 2 and T 3 respectively. Fish were hand-fed twice daily with diet A40. The ¢nal mean weight (89.53^115.12 g), the mean weight gain (0.811.10 g day À1 ), the speci¢c growth rate (1.90^2.20% day À1 ) and the apparent food conversion ratio (1.29^1.58) were a¡ected by stocking density, with signi¢cant di¡erence (Po0.05) at 5 m À 2 , compared with the other densities. Stocking density did not affect survival rate (P40.05).Yield and annual production increased with increasing stocking density, ranging from 7.10 AE 0.90 to 25.01 AE 1.84 kg are À1 and 28.79 AE 3.66 to 101.42 AE 7.48 kg are À1 year À1 , respectively, with signi¢cant di¡erences between all densities (Po0.05). Higher stocking density resulted in higher gross return and lower cost of ¢sh production, with signi¢cant variations (Po0.05). The net return increased with increasing stocking density (Po0.05). However, both densities of 3 and 5 m À 2 produced the same pro¢tability values. On the basis of growth values and economic return, it was concluded that Nile tilapia could be raised at a density of 3 ¢sh m À 2 with A40 to improve production and generate pro¢t for nutritional security and poverty alleviation in rural areas.

Population structure and reproduction of the African bonytongue Heterotis niloticus in the Sô River-floodplain system (West Africa): implications for management Un resumen en espan˜ol se incluye detra´s del texto principal de este artı´culo.