The Biology of West African Polynemid Fishes
The biology of two commercially important polynemids has been studied over an eleven-month period on the trawling grounds off Lagos, Nigeria. One species, Pentanemus quinquarius (Bloch), has a normal bisexual reproductive cycle, but in the second species, Galeoides clecadactylus (L.), it was found that only about 25% of the females develop directly from a juvenile stage, the others developing female gonads by passage through a non-functional hermaphroditic stage arising in apparently normal males. Spawning occurs in all months in both species, the intensity being less cyclical in Pentanemus than in Galeoides, which peaks in the dry season and almost ceases spawning during the rainy season. Petersen growth rate studies show that Galeoides up to 3 years old are present in the catches, the values for von Bertalanffy parameters being K = 0·41 and L∞ = 54·5 cm; Pentanemus rarely survive their second year of life. Derivation of lengths at first maturity for Galeoides is complicated by the nature of the reproductive cycle and these range from 15 cm for males to 25·5 cm for secondary females; Pentanemus mature at less than six months old at about 15 cm. The implications of these biological data for the trawl fishery are considered and it is suggested that despite dependence on very few year-classes both species are responding relatively well to the present unregulated exploitation.