Morphological and meristic characterization of the African bonytongue, Heterotis niloticus (Cuvier, 1829), from Lake Hlan and Sô River, Southern Benin, West Africa: The need for habitat protection

Abstract

The African bonytongue, Heterotis niloticus (Pisces: Osteoglossiformes: Osteoglossidae) is one of the largest omnivorous fish species of high commercial and economic value in African freshwater fisheries and in semi- intensive and extensive aquaculture systems. Heterotis specimens were sampled in the Lake Hlan - Sô River (South-Benin) system in order to investigate the morphological and taxonomic characterization of this threatened species under overfishing and various environmental degradations, and to detect whether the population is morphologically and taxonomically separable. The population exhibited significant (P<0.05) morphological relationships with positive slopes varying between 0.39 and 2.97 and coefficients of correlation “r” between 0.70 and 0.99 for most regressions. The positive slopes “b”<3 (2.97 for Lake Hlan; 2.67 for Sô River) from the Total Length - Body Weight relationships suggest that Heterotis niloticus exhibited an allometric growth in the Lake Hlan - Sô River system. Overall, the results showed insignificant (0.0001 ≤ F 1,38 ≤ 3.275; P≥0.05) sexual variation for most morphometric features, ratios and meristic counts for both sub-populations, Lake Hlan and Sô River. Also, insignificant (0.003 ≤ F 1,38 ≤ 3.978; P≥0.05) variation where found between Lake Hlan and Sô River (males and females indiscriminately) for most morphological and meristic traits. The significant (P<0.05) variations recorded in few morphological (ratios HL/ED and HHe/ED) and meristic (NSpH; NTLoJ) traits between males and females and between Lake Hlan and Sô River were probably due to habitats characteristics, environment degradation and geographical factors. As results, the population of Heterotis in the Lake Hlan-Sô River system of southern Benin is morphologically and taxonomically inseparable indicating that Heterotis may probably be a unique species in the Lake Hlan – Sô River system and, therefore, may not be genetically diversified. Further studies on morphological and meristic characterization of bonytongues in all Benin freshwater systems, coupled with molecular analysis are required to implement ecological sound management program including habitat protection, conservation scheme, aquacultural valorization and sustainable fisheries management.