Following the declining stocks of Sardinella aurita within the coastal waters of Ghana, this study aimed at examining some population parameters of Sardinella aurita as a guide for managing this important stock sustainably. Length-frequency data of 717 samples were obtained from June, 2014 to January 2015 and measured for total length with the resultant data analyzed using FiSAT II. The asymptotic length (L∞) and growth rate (K) were 21.53 cm SL and 0.25yr-1 respectively. The theoretical age at birth (t0), longevity (tmax) and growth performance index (ϕ) were -0.74yr-1, 12 years and 1.849 respectively. Total mortality rate (Z), natural mortality rate (M) and fishing mortality rate (F) were 3.17, 0.76 and 2.41yr-1 respectively. The ages at first recruitment and first capture signaled future collapse of the stock, in the absence of proper management interventions. VPA outcome showed that mid- lengths of 11 cm and 12 cm SL experienced the highest harvesting rate with MSY estimated at 7733 tons. The recruitment pattern was continuous with two major recruitment pulses. Exploitation rate (Ecurr=0.76) was higher than the maximum exploitation rate (Emax=0.56), indicating unsustainable exploitation. Further, the fishing regime fell within the overfished stage based on the Quadrant Rule. For sustainable exploitation of this commercial fish species, implementation of relevant biological reference points through reduction in fishing efforts, creation of marine protected areas and mesh size regulation are urgently advocated.Res. Agric. Livest. Fish.4(3): 237-248, December 2017
Population parameters for Red Pandora, Pagellus bellottii (Steindachner, 1882) from the eastern coastline of Ghana, using length-frequency data from June 2014 to January 2015 were assessed. A total of 440 samples of Pagellus bellottii were measured for standard length and analyzed with FiSAT II. The asymptotic length (L∞) and growth rate (K) were 19.43 cm and 0.42 per year. Theoretical age at birth (t0 ) and growth performance index (ϕ) were -0.44 per year and 2.20. The age at first maturity (tm50) and longevity (tmax) were 2.2 years and 7 years. The Bhattacharya’s method revealed three modal lengths groups: 9.3 cm, 12.1 cm, and 14.6 cm. Total mortality rate (Z), natural mortality rate (M) and fishing mortality rate (F) were 2.58 per year, 1.10 per year, and 1.48 per year. Exploitation rate (Ecurrent) and maximum exploitation rate (Emax) were estimated as 0.57 and 0.93 respectively. The recruitment pattern was continuous with two major recruitment pulses. VPA outcome showed that midlengths of 11.5 cm and 12.5 cm experienced the highest harvesting rate. The fishing regime of Pagellus bellottii fell in the developed stage based on the quadrant rule. Maximum Sustainable Yield (MSY) was 7864.18 tons. Results from the study indicated the existence of growth overfishing with the present state of exploitation as a forewarning to the collapse of the fishery in future. With resilience of Red pandora within the medium category, incidence of overfishing will produce economic consequences. The developed stage of fishing suggested that maximization of yield per recruits will lead to low stock biomass of Red pandora.
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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