No abstract

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.

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.

SynopsisA preliminary study of the ecology and commercial fish catches was carried out in the Toho-Todougba, Ahouangan and Dati lakes (1500 ha) in southern Benin (West Africa) over a period of 18 consecutive months .Water quality, species richness, and the reproductive biology of the dominant species, Sarotherodon galilaeus, were examined . Unlike Lagoon Toho-Todougba, lakes Ahouangan and Dati are less favourable for fish production because of their high total iron content and their relative low pH . The study revealed the existence of 19 species belonging to eleven (11) families : 16 species in Lagoon Toho-Todougba, 12 in Lake Ahouangan, and 7 in Lake Dati . Lakes Ahouangan and Dati had a relatively high faunal similarity whereas Lagoon TohoTodougba exhibited a relatively low faunal similarity with the former systems . In Lagoon Toho-Todougba, six cichlid species were the most important component of the fishery : Sarotherodon galilaeus, Tilapia guineensis, T zillii, T mariae, Chromidotilapia guntheri, Hemichromis fasciatus . Three catfish species Chrysichthys auratus, Clarias agboyiensis, and Clarias lazera were harvested in low abundances . Some other fish, such as Polypterus senegalus senegalus, Protopterus annectens and Heterotis niloticus were captured sporadically, with H. niloticus appearing more consistently in catches than the others . Although not abundant, Gymnarchus niloticus and the African pike, Hepsetus odoe, were almost always present in the catches . Ctenopoma kingsleyae appeared only in Ahouangan . Some predatory species, such as Notopterus afer and Xenomystus nigri, are encountered only in lakes Ahouangan and Dati . In Lagoon Toho-Todougba, Sarotherodon galilaeus had a sex ratio (6 : ?) of 0 .49 :1, and size at maturation was estimated as 12.3 cm TL . The spawning period extended from May to September with the peak occurring in July . Fecundity was correlated to log-body length (r = 0 .74) and log-body weight (r = 0 .76) . Fish catches were estimated at 389 kg ha 1 year 1 ; about 73 % of which was Sarotherodon galilaeus . The overexploitation of juvenile fish and the use of this lagoon for the irrigation of palm tree plantations have reduced fish recruitment and mean size .

Examination of eight microsatellite DNA loci revealed high levels of genetic differentiation among populations of the African bonytongue Heterotis niloticus from three river basins that constitute important fishing areas in Benin. Low levels of population genetic differentiation were detected within the Ouemé-Sô River floodplain system. These results have important implications for conservation and management of stocks supporting important inland fisheries in West Africa.