Ontogenetic Variation in the Diet of Chrysichthys nigrodigitatus (Lacepede, 1803) in a Tropical Estuarine Ecosystem in Nigeria

摘要

The onto genetic variation in the diet of Chrysichthys nigrodigitatus (Lacepede, 1803) was investigated. The study was carried out for a period of 15 months (Feb.-Sept. 1996 and March- Sept. 1997) based on 660 specimens of the fish. It was aimed at a detailed analysis of the diet composition of C. nigrodigitatus and the variation in diet with size class. The diet composition was analysed using the occurrence, numerical and gravimetric methods. These were combined into a composite Index of Relative Importance (IRI) which formed the basis of determining the importance of each food category. C. nigrodigitatus is a benthic omnivorous feeder subsisting on a variety of benthic food items, with significant ontogenetic shifts in diet composition. The size classes 0-30.00 and 31-60.00 cm adult had preference for plankton, 61-90.00 cm preferred fish and crabs while 91-120.00 cm had preferential diet of shrimps. Index of Relative Importance (IRI) was as follows: fish-31.8%, Crustaceans-23.6%, bivalves-10.61%, as major food items. The minor food items included plankton, polychaetes, palm fruits, plant materials and detritus. Variation of Condition Factor (CF) with fish length was studied with highest CF value of 1.86 for size group 10-19 cm and lowest CF value of 0.71 for size group 40-49 cm. This study has not only identified C. nigrodigitatus as a benthic predator in the estuary, but has also provided data that can make significant inputs into the trophic modeling and multispecies management of the estuary.