The study of the numerous areas of biology of fish produces ample and relevant information that aids scientist to explore, know and maintain the survival, growth, continuity and sustainability of life of various fish species. A total of 94 specimens of Gymnarchus niloticus were collected from River Niger at Agenebode during the sample period that ranged from 14 – 94 cm total length (54.88± 17.92); weighed 424 - 2281g (1569.61 ± 422.85). The length weight relationship (LWR) of G. niloticus in this study showed a negative (b<3) allometric growth pattern. Out of the 94 specimens analysed 54 males and 40 females were examined with a ratio of 1:1.4. Insects and decapods were the major food of the juveniles of Gymnarchus niloticus while the major food of preference of the sub adults and adults were Fish and insects with 27.12%, and 27.97% by FO. Gymnarchus niloticus possesses a single ovary in the female and single testis in the male which is often referred to as unpaired gonad. Fecundity ranged between 620 to 1378 eggs with a mean of 957.69 ± 112.31 eggs in females. The relationship between fecundity (F) and weight (W) and fecundity and standard length (SL) showed that fecundity is more related to weight than length of fish. Log F = 820.66 + 0.11 W and Log F = 151.23 + 5.31 SL. Fecundity had a positive significant correlation (P<0.05) with both somatic weight (r = 0.714) and standard length (r = 0.657) of fish. Ova diameter showed a ranged between 3.5 and 7.9 mm with a mean of 4.72 ± SD1.69 mm and it had significant relationship (P<0.05) with total weight, gonad weight and maturity stage. Gymnarchus niloticus in the Lower River Niger at Agenebode showed that G. niloticus has an unpaired gonad in both sexes, a gonochoristic reproductive pattern. Fecundity is low and the ova are large. They are small brood spawners with asynchronous breeding strategy. The attributes of G. niloticus obtained from this study reveals that the fish species is a good candidate for fish culture.

Background. The African bonytongue, Heterotis niloticus, is readily accommodated in the fresh water of the Niger River. It is available all year round with a large population of juveniles and adults due to its fast growth and versatile feeding habits. This commercial fish is a highly preferred source of food because of its high protein content and hardy flesh, thus forming a very important component in the diet of many Nigerians. It is highly valued because of its socioeconomic importance and benefits. Hence, this research is designed with the aim of studying the biology of the African bonytongue, Heterotis niloticus, and providing viable information about its importance in fish culture in order to make an available added variety of culturable and affordable fish species in Nigeria. Results. The oesophagus is a muscular organ that is short and distensible. The oesophagus leads to the gizzard-like stomach, a reddish bilobed organ that is muscular and tough. H. niloticus is an omnivorous macrophage detritor, consuming a wide variety of bottom-dwelling food items. The histology of the gastrointestinal tract reveals four conspicuous layers from the inside to the outside: mucosa, submucosa, inner circular layer and outer longitudinal layer of muscularis, and serosa. The presence of numerous mucus glands and longitudinal folds with a prominent columnar epithelium provides durable length or an extension of the gut mucus to aid lubrication and easy passage of food materials, protecting the mucosal epithelium from mechanical or chemical injuries arising from interactions with digestive tract contents or enzymes. In addition, the presence of absorptive cells helps in the absorption of valuable nutritive substances. Conclusions. The anatomy of the mouth and gut and the aforementioned histology are modified to accommodate the feeding habits. H. niloticus has a single ovary that rests on the right side of the fish; it is reproductively active as the flood plains rise and peaks at the peak of the rains, being a moderately fecund fish. The high fecundity is complemented with peaks of GSI observed during the months of September, October, and November, which reveals spawning periods. Hence, the aforementioned attributes of H. niloticus make the fish a viable fish species for culture.

This study was conducted to record the anatomy and histological features of the gastrointestinal tract of the trunk fish Gymnarchus niloticus captured at Agenebode in the Lower River Niger, Edo State. The gastrointestinal tract was segmented into the mouth, oesophagus, stomach, pyloric caeca, and intestine. The slender (taeniform) body that is fuller at the head tappers at its end. There are no spines on this fish, and the fins used for locomotion are the dorsal fins and not the typical pectoral fins as in other fish. The oesophagus, stomach, pyloric caeca, intestine, and cloaca (anterior, intermediate, posterior, and rectum) were analysed for histological examination. The stomach had significantly higher villi and thicker inner circular muscles compared to the intestine and oesophagus. The lamina propria was thickest in the stomach which is significantly higher when compared with the oesophagus, but not with the intestine. However, the intestine showed significantly thicker outer longitudinal muscle, while gastric glands were observed only in the stomach. The histological features were closely associated with the functions of the different segments of the gastrointestinal tract. In conclusion, the histological features of the gastrointestinal tract of G. niloticus are consistent with the feeding habit of a carnivorous fish. In conclusion, the GIT is a complex structure composed of organs that reveals that it is a higher vertebrate created for a hardy survival, consistent with the feeding habit of a carnivorous fish. The histology reveals a GIT formed by four conspicuous layers from the inside to the outside of the mucosa, submucosa, and an inner longitudinal layer of the muscularis and the outer circular of the muscularis typical of higher vertebrates.