The Biology and Potential for Culture of Grey Mullets (Pisces: Mugilidae) in Ghana

Abstract

Aspects of the biology of grey mullets were studied between February 1997 and July 1998 in the Volta and Pra River estuaries in Ghana and their potential for culture in brackish and freshwater ponds was evaluated. Six species of grey mullets were identified during the study. These were: sickle fin mullet, Liza falcipinnis (Valenciennes, 1836), large-scaled mullet, Liza grandisquamis (Valenciennes, 1836), grooved mullet, Lizu dumerilii (Steindachner, 1870) white mullet, Mugil curema Valenciennes (1836), banana mullet, Mugil bananensis (Pellegrin, 1928), and striped mullet, Mugil cephalus Linnaeus (1758). With the exception of L. grandisquamis, which was found only in the Pra estuary, all the other species were common to both estuaries. M. curema and L. falcipinnis were the most abundant species in the Volta and Pra estuaries respectively, while M cephalus was the least abundant species in both places. Specimens of all the species from the Volta estuary were relatively larger than their counterparts in the Pra estuary. With the exception of L. grandisquamis, whose growth was isometric, the other species from both estuaries exhibited allometric growth. Monthly changes in gonadosomatic index, occurrence of mature and spent fish in samples as well as oocyte size distribution, suggest that the populations in both estuaries were multiple spawners; those from the Volta estuary spawned between February and August while those from the Pra estuary spawned between March and November. Males of all the species matured at a smaller size than the females in both estuaries but species in the Pra estuary matured at much smaller size than their counterparts in the Volta estuary. Stomach contents of grey mullets in the two estuaries were quite similar consisting mainly of diatoms, detrital material and sand particles. Their diet did not show any substantial seasonality neither did it change with the size of fish. All the species showed a strong diurnal feeding habit, with the main feeding period occurring between 08.00 h and 12.00 h. Growth performance of fish that were given supplementary feed was better than those cultured under natural pond conditions; that of fish fed on wheat bran was also better than those fed on rice bran. The differences in growth of fish in both treatments were, however, not statistically significant. Food conversion ratios between 3.6 and 7.7 and survival rates ranging from 43.4 to 68.8 % were recorded during the culture trial. Within a 270-day culture period, fish production of 336.5 kg ha-1 and 287.0 kg ha-1 were achieved in ponds with and without feed respectively. The mean final weight of M curema was the highest among the species during the 270-day culture period. Results obtained in this study compare favourably with other published works and indicate a good potential for the culture of grey mullets in Ghana.