Population structure and reproduction of the African bonytongue Heterotis niloticus in the Sô River-floodplain system (West Africa): implications for management

Abstract –  Population structure of the African bonytongue, Heterotis niloticus , was examined in southern Benin within Lake Hlan and a region of the Sô River floodplain located approximately 60 km downstream from the lake. Both locations support important fisheries in which Heterotis is the principal target species during the flood period. Ripe adults comprised over 40% of the population in Lake Hlan, whereas only 3.5% of individuals captured from river sites were adults. Monthly averages for the gonadosomatic index and percentages of individuals with mature gonads peaked as water levels increased during the wet season then declined during the peak flood period. Oocyte size frequency distributions from ovaries suggested a potential to produce an additional cohort in the event of nesting disruption. During the peak spawning period (May to August), between 37 and 51 active nests per hectare per month were observed in Lake Hlan. The number of larvae per nest ranged from 3953 to 6125. Lake Hlan bonytongues appear to constitute an important source subpopulation that exports new recruits to river/floodplain areas downstream where intense fisheries harvest mostly juveniles and subadults. Consequently, restriction of harvest of adult bonytongues in Lake Hlan may be essential for sustenance of commercial fishing in downstream reaches of the Sô River.