Food and feeding habits of grey Mullets (Pisces: Mugilidae) in two estuaries in Ghana

Food and feeding habits of grey mullets (Mugilidae) in the River Volta and River Pra estuaries in Ghana were studied between February 1997 and July 1998 as part of efforts to encourage their culture. Stomach contents of fish samples, obtained with a cast net and a drag net, were analysed using the ‘points’ and frequency of occurrence method. Diatoms, detrital material and sand particles were the major items in the stomachs of all the species from the two estuaries. Their diet did not show any substantial seasonality, neither did it change with size. The various species ingested sand particles of selected range with Liza dumerilii ingesting the widest range in both estuaries, 41.2-1195.8 ìm in the Volta estuary, and 33.0-1649.0 ìm in the Pra estuary. Species that ingested the same modal size of sand particles showed preferences for different food items. The shortest mean relative gut length (gut length to body length ratio) of 1.82 and 1.72 were calculated for L. dumerilii in the Volta and Pra estuaries, respectively, while the longest mean relative gut length of 4.56 was calculated for Mugil cephalus in the Volta estuary and 4.33 for Liza grandisquamis in the Pra estuary. All the species showed a diurnal feeding habit, with the main feeding period occurring between 08.00 and 12.00 h. The peak feeding time, however, differed among the species

Acute toxic effects of deltamethrin on tilapia, Oreochromis niloticus (Linnaeus, 1758)

The effect of toxic compounds are variable among living organisms in different habitats. Toxicity tests are conducted to measure the effects of different pollutants on one or more species of organisms and in the form of LC50s, assist in the development and application of water quality criteria for the protection of the aquatic environment. Active tilapia, Oreochromis niloticus, samples with size ranging between 4 and 6 cm standard length were exposed to different concentrations (between 0 and 30 ìg/l) of Deltamethrin for 96 h in 50 l glass aquaria. This was to assess the dose-response of O. niloticus to different concentrations of the pesticide, and to estimate the LC50 using Probit analysis. The very low LC50 value of 15.47 ìg/l obtained indicates that Deltamethrin is highly toxic to O. niloticus. This LC50 value should serve as a guide for the application of Deltamethrin in agriculture. Further study into the processes and mechanism by which Deltamethrin adversely affects the physiology of fishes have been recommended

Review of catch trends and changes in fish species composition of the Volta lake during its 45 years of existence

Existing catch data for the whole of the Volta lake from 1969 to 2004 and those for certain segments of the lake (Strata II, III, and IV) covering various periods were analysed in relation to lake levels, to find out changes in catch trends and species composition. The study was undertaken as a baseline activity aimed at enhancing fisheries productivity and  management of the lake. Higher catches were made at the inception of the lake, with about 65,000 t being recorded in 1969. This dropped and  fluctuated between 36,000 anct48,000 t from 1971 to 1994. Catches kept increasing rapidly from 1995 with about 80,000 t being recorded in 1999, the highest in the history of the lake. In relation to annual fake water level fluctuations, high catches were made during periods of low water level compared to periods of high water level. On the long term, decreasing lake water level corresponded with higher fish catches. There was a change frominsectivorous fish species (e.g., Chrysichthys, Schilhe and Synodontis) at the initial stages of the formation of the lake to those with vegetarian food habits dominated by the tilapias. The dominance oftilapias in catches, which persisted till the early parts of the 1990s, has given way to Chrysichthys spp., suggesting that the composition of fish species in the lake is still undergoing changes 45 years after its formation

Catch composition and efficiency of major fishing gears used in stratum II of the Volta lake– implications for managing the fisheries

Catch composition and efficiency of major fishing gears viz; gillnets, basket traps, net traps and ‘atidza’ (brush park), deployed by fishers on the Volta lake at Dzemeni (Stratum II), was assessed over a 16-month period to determine how their deployment can impact on the fish stocks. Fifty thousand seven hundred and ninety four individual specimens were recorded during the study period. This represented 13 taxonomic families, 21 genera and 32 fish species. Only five genera constituted 95 per cent and 84 per cent by number and weight, respectively, of the total catch. Chrysichthys spp. was the most abundant genus (59.1%) followed by the tilapias (15.7%), Hydrocynus spp. (9.7%), Synodontis spp. (8.8%) and Bagrus spp. (2.6%). Catches by ‘atidza’ and basket traps were predominantly Tilapias and Chrysichthys spp., respectively, while that by gillnets and net traps were more heterogeneous. The highest catch of 20 t during the period was made by basket traps, while that from the other gears ranged from 0.33 to 6 t indicating that basket traps were very efficient. Knowledge about the catch composition by the various gears and their efficiency will help to regulate their use when it comes to formulating measures to manage the fisheries of the lake

Impact of mining operations on the ecology of river Offin in Ghana

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Potential for culture of grey mullets (Pisces:Mugilidae) in Ghana

The potential for culture of grey mullets in Ghana was investigated in fresh and brackish water ponds. Growth performance of fish that were given supplementary feed was better than those cultured without supplementary feeding. However, the difference in growth was not significant. The difference between the growth of fish raised in fresh water and that of those raised in brackish water ponds was also not significant. Feed conversion ratio of 6.0 was recorded in fresh and brackish water ponds while survival rates ranged from 43.4 to 68.8 per cent in the various ponds. Within a 270- day culture period, fish production of 336.5 and 287.0 kg ha-l were recorded in ponds with and without feed, respectively. The mean final weight of Mugil curema (Valenciennes) was the highest among the species cultured. The results for this study compared favourably with other published works and indicated a good potential for the culture of grey mullets in Ghana. Ghana Journal of Science Vol. 44, 2004: 19-2