The spawning and recruitment patterns of the major Cichlid fish species, including Hemichromis fasciatus, Oreochromis niloticus, Sarotherodon galilaeus, Tilapia zillii and other species, Auchenoglanis occidentalis, Brycinus nurse, Clarias gariepinus and Marcusenius senegalensis were studied for 24 months in Bontanga Reservoir, Ghana, using length-based fish stock assessment approaches. The species spawned throughout the year, with two spawning pulses described as major and minor spawning seasons. The major spawning season occurred from March to September for the Cichlids, and from May to September for the other species. The minor spawning season, indicative of extended spawning, occurred from October to March for all the fish stocks. Fish spawning began with the onset of the rains in April ⁄ May, peaking by June ⁄ July, before the rainfall peak in August for all the fish stocks studied. Recruitment was found to occur throughout the year, with major and minor pulses coinciding with the major and minor spawning seasons. Accordingly, the most appropriate time for a possible closed fishing period appears to be from June to August, 1 month after the start of, and before the end of, the rains. The estimated mean standard length (L m ) for first time spawners of A. occidentalis, B. nurse, C. gariepinus and H. faciatus were 11.7, 12.7, 2.7 and 7.5 cm respectively. The estimated maturity-length ratio of 0.4 and 0.2 for O. niloticus, S. galilaeus and T. zillii were lower than the known 0.7 for normal growth, suggesting the tilapias matured faster, and at a smaller size, in the reservoir. Apparent sexual precocity associated with early maturity, yearround spawning and recruitment were some important adaptations found to have sustained the reservoir fisheries, even during high fishing pressures. For conservation and sustainable exploitation of the fisheries, instituting a closed fishing season, mesh size regulations, withdrawal rights and a community-based fishery management system are recommended.
The exploitation rates of eight major component fishery species, Auchenoglanis occidentalis, Brycinus nurse, Clarias gariepinus, Hemichromis fasciatus, Marcusenius senegalensis, Oreochromis niloticus, Sarotherodon galilaeus and Tilapia zillii, of gill net fishery of Bontanga reservoir, were studied from March 2004 to March 2006 based on lengthbased models. The exploitation rates (E) of A. occidentalis and H. fasciatus were 0.7 and 0.6, which exceeded the optimization (E opt) criterion of 0.5 for sustainable exploitation of fisheries, indicating that these species were over exploited. For B. nurse and S. galilaeus, E ≈ 0.5, indicating that these species were at their maximum rates of exploitation. For C. gariepinus, M. senegalensis, O. niloticus and T. zillii, E < 0.5, indicating that these species were not over exploited. Based on the selection factor of the legal minimum gill net mesh size of 5 cm of Ghana Fisheries Act 625, the estimated mesh size for catching the eight species ranged from 9.9-19.3 cm, calling for an upward review of the legal minimum mesh size of gill nets from 5 to 10 cm for sustainable exploitation of reservoir fisheries. The adoption of minimum mesh size of gill nets of 10 cm, restriction of further entry into the fishery, control rights and community based co-management system are some management options suggested to enhance sustainable exploitation and management of the fisheries. The objective of the study is to assess the current exploitation rates of the major component fishery species of the reservoir and suggest management options to enhance management of the reservoir fisheries for sustainable exploitation. Materials and methods Study area The Bontanga reservoir (Fig. 1) was formed by construction of a dam across Bontanga river, a tributary of the White Volta. The constructional works of the reservoir commenced in 1981 and the reservoir was commissioned in 1986. The reservoir was primarily constructed to support irrigation
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