Ontogenetic, seasonal, and spatial variation in the diet of Heterotis niloticus (Osteoglossiformes; Osteoglossidae) in the Sô River and Lake Hlan, Benin, West Africa

Alphonse Adite (Univ. Abomey-Calavi, Benin), Kirk O. Winemiller and Emile D. Fiogbe (Univ. Abomey-Calavi, Benin)

The African bonytongue, Heterotis niloticus, is an important species in fisheries and aquaculture in West Africa.  This osteoglossid has frequently been characterized as a detritivore, in part because of its benthic feeding habitats and possession of a gizzard (thick-walled pyloric stomach).  Diets of two populations in the Sô River in Southern Benin were examined over 18 months (1,461 specimens).  A population from the river channel and seasonally flooded marginal plains was dominated by juvenile and subadult size classes.  Adults, including reproductively active individuals, were common in second population from Lake Hlan, a natural lake in the river floodplain located 35 km 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.  Individuals <100 mm SL fed primarily on aquatic invertebrates. Aquatic invertebrates remained significant in the diet of larger size classes.  Diets of larger fish began to include seeds and detritus, with a marked increase in the volumetric proportion of detritus in the diet between 300 to 400 mm in Lake Hlan and between 500 and 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.

Population structure and reproduction of Heterotis niloticus (Osteoglossiformes: Osteoglossidae) in the Sô River-floodplain system (Benin, West Africa)

Alphonse Adite (Univ. Abomey-Calavi, Benin), Kirk O. Winemiller, and Emile D. Fiogbe (Univ. Abomey-Calavi, Benin)

Reproductive ecology of the African bonytongue, Heterotis niloticus, was examined in the lower Sô River, its flooded plains, and Lake Hlan, a flooplain lake located approximately 60 km upstream from the river study sites in southern Benin.  Both locations support important fisheries in which Heterotis is the principal target species during the flood period.  Ripe and nesting adults were common in Lake Hlan, whereas only 3.5% of individuals captured from river sites were adults.  Monthly averages for the gonadosomatic index and the percentage of individuals with mature gonads peaked as water levels increased during the wet season (May-August) then gradually declined during the flood period (September-November).  Frequency distributions of oocyte size classes within ripe ovaries indicated production of multiple cohorts per spawning season.  Median maturation size was estimated at 675 mm TL for both genders.  Fecundity (number of mature oocytes in the ovary) increased with body size.  Lowest and highest fecundities recorded for individual fish were 2,697 (500 mm SL) and 27,508 oocytes (735 mm SL), respectively.  Averages ranging from 37-51active nests per hectare per month were observed in Lake Hlan during the peak spawning period (May-August).  The number of larvae per nest ranged from 3,953 to 6,125.  At 5-7 days posthatch, larvae averaged 13.10 ± 0.51 mm SL and 15.79 ± 1.09 mg.  Yolk sacs were resorbed at 6-7 days posthatch.  At 5-6 days posthatch, larvae formed swarming schools during daytime, and at 25-30 days posthatch fry foraged independently.  Lake Hlan bonytongues constitute an important source subpopulation that exports individuals to river/floodplain areas downstream where intense fisheries harvest almost entirely juveniles and subadults.

Updated Monday February 27, 2006