Geographical variation of the structure and population parameters of the freshwater prawn Macrobrachium macrobrachion (Herklots, 1851) in West African coastal hydrosystems: Insights from Benin

The freshwater prawn Macrobrachium macrobrachion (Herklots, 1851) is widely exploited in the Oueme Delta and the low Mono in Benin. The structure and population parameters of M. macrobrachion were evaluated to provide accurate scientific information for the better exploitation and rational management of freshwater prawn species in these areas. In Oueme Delta (1928 specimens) and low Mono (1045 specimens), 2973 specimens were sampled by artisanal and experimental fishing and analysed. The prawns were caught between January and December 2016. Total length and total weight were taken from each specimen. Global and monthly size–frequency distributions showed that recruitment in both populations takes place from August to October with a peak in September. The population parameters obtained by analysing frequency data with FiSAT II software (FAO-ICLARM) showed an L∞ = 15.50 cm, K = 0.35 year⁻¹, t₀ = −0.56 year in Oueme Delta and L∞ = 15.30 cm, K = 0.45 year⁻¹, t₀ = −0.43 year in low Mono. The growth performance index (Φ′) was 1.93 and 2.02 for the Oueme Delta and low Mono populations, respectively. The exploitation rates (E_max and E_0.5) suggest that more than half of the M. macrobrachion population was exploited in the two areas. Natural and fishing mortality rates showed that both populations of M. macrobrachion were vulnerable to natural and fishing death. The estimated first capture size (L_c) revealed that the majority of the two populations of M. macrobrachion were captured before they reached the size at first sexual maturity (L₅₀), reflecting fishing pressure on small individuals and growth overfishing in both areas. These results showed that the used fishing gears and the fishing frequencies in both areas did not allow the majority of the prawn to grow and reproduce at least once before being caught.