Comparative Studies of the Dynamics and Management Of Fish Populations in an Open and Closed Lagoon In Ghana

Abstract

Comparative studies have been done on the dynamics and management of the fish populations in Abrubi lagoon (closed lagoon) and Densu delta (open lagoon) in Ghana from May 1994 to April 1996.Some environmental parameters of the lagoons, which affected fish population were also monitored. There was a noticeable increase in temperature during the dry season (November to March) in both study areas. Temperatures ranged from 26°C to 33°C. On the whole, changes in pH were gradual and steady with no definite pattern in both study areas. The major rains were from May to July with minor rains from September to November; December to March were generally dry months. The depth of the water increased with the onset of the rains. Fluctuations in salinity were closely linked to amount of rainfall. The three stations in each study area showed a pattern of salinity gradation with lowest salinity occurring at Station 1 (riverine portion) and highest salinity recorded at Station 3 the area nearest to the sea. Dissolved oxygen levels were high in the wet season but low in the dry season. Differences in oxygen levels were not significant between months and between stations in both study areas. Analysis of variance tests showed that levels of phosphate and nitrate were not significantly different between months and between stations in both study areas. Levels of total dissolved solids and conductivity followed each other closely. Maximum Catch per Unit Effort (CPUE) values were recorded when the depth of the lagoon was low. Eighty-three thousand, eight hundred and forty-three kilogrammes of fish were obtained from the Densu delta by experimental cast net fishing during the study period and sixty-six thousand nine hundred and ninety-nine kilogrammes of fish from Abrubi lagoon. The five dominant species in Abrubi lagoon were Sa.rotherod.on melanotheron, Hemichromis fasciatus, Tilapia zillii, Liza falcipinnis, and Ethmalosa fimbriata. In the Densu delta they were Sarotherodon melanotheron, Gobiodes ansorgii, Tilapia zillii, Mugil cephalus and Ethmalosa fimbriata. Sarotherodon melanotheron was the most dominant species, accounting for 68% of the catch in the Densu delta and 75% of the catch in Abrubi lagoon during the study period. The overall species diversity was not markedly different between Abrubi lagoon and Densu delta. At 5% significance level (Mann Whitney U test), the species diversity in the dry season was different from that of the wet season in Stations 1 and 3 but not in Station 2 in both study areas. The Eveness component varied in a manner similar to diversity. Breaching of the lagoon affected species diversity in Abrubi lagoon. The peak of diversity occurred just before the peak of rainfall in Abrubi lagoon. Marine incursions and the opening and closing of the sluice gates of the Weija dam affected diversity in the Densu delta. The Forage / Carnivore ratio of 2.86 for Abrubi lagoon and 5.15 for Densu delta indicates that both study areas had balanced fish populations. The Multi-Dimensional-Scaling (MDS) obtained for the six sites in the two study areas indicate that temperature and salinity were strong environmental factors determining the presence of fish species There was monthly variation in male: female ratio with females predominating most of the time. From the Gonadosomatic Index (GSI) obtained S. melanotheron spawns from February to March. The overall length -frequency distribution showed a modal length of 8.5-9.0 cm SL in Abrubi lagoon and 5.0-5.5 cm SL in Densu delta. The length at first maturity was 7.2 cm SL in Abrubi lagoon and 4.8 cm SL in Densu delta. The von Bertalanffy growth parameter (Loo = 12.0 cm SL , K = 0.67 for Densu delta and Loo =15 cm SL, K= 0.79 for Abrubi lagoon implies fast growth which might compensate for the rather high fishing and natural mortality rates. Direct and indirect management practices can be used to manage the fisheries. Direct management involves gear control, catch control, fishermen control and season control. Indirect management involves mangrove afforestation, environmental protection and introduction of alternative trades to the fishermen.