Rock Assessment Of The Five Major Commercial Sh Species In Yeji Area (Stratum Vii) Of The Volta Vke

SUMMARY An assessment of the fishery of five major commercial species which together contribute more than 70% of the catches in stratum VII (near Yeji) of the Lake Volta has been made based on studies undertaken between March, 1995 and December, 1996. The species studied are: Hemisynodontis membranaceus (Geoffroy St. Hilaire, 1809), Chrysichthys nigrodigitatus (Lacepede, 1809), Chrysichthys auratus (Geoffroy St. Hilaire, 1809), Oreochromis niloticus (Linne, 1757), and Schilbe intermedius (Ruppell, 1832). The study aimed at contributing information to provide a basis for comprehensive developmental strategies, management and sustainable exploitation or conservation of the fisheries particularly in stratum VII of the lake. Sixty-six fish species representing 39 genera and 19 families were encountered throughout the studies. Gill-net is the most important gear and the catches were dominated by the bagrids Chrysichthys auratus, C. nigrodigitatus and the cyprinid Labeo spp. Data on gonadosomatic index and and gonad maturity stages of the major species indicate that Chrysichthys nigrodigitatus and C. auratus-, and the cichlid Oreochromis niloticus spawn throughout the year while the mochokid Hemisynodontis membranaceus shows two clear spawning seasons in February and July The mean annual catch-per-unit of effort (CPUE) of canoes utilizing gill-nets during the study period were estimated respectively as 10-18 kg canoe' 1 d' 1 and 7-15 kg canoe 1 d 1 for 1995 and 1996 which indicated declining catches over those years. The total annual canoe catch estimate in stratum VII alone during 1996 was about 25,000 metric tonnes. Among the physico-chemical variables measured during the study, the mean monthly CPUE was found to be positively correlated with the ammonia concentration in the water; total dissolved solids (TDS), and conductivity. The relation with ammonia concentration (AMN) was best described by the equation: Log m CPUE = 0.6076 Log I0AMN + 1.3522 (r 2 = 0.6992) Growth parameters were estimated using length-based stock-assessment methods and the von Bertalanffy growth model for the major species are described as follows: H. membranaceus: Lt = 44. C. nigrodigitatus: Lt = 44. C. auratus: Lt = 31. O. niloticus : Lt = 33. S. intermedius: L, = 30. 0 [ 1 - exp (- 0.55 (t + 0.26))] 5 [ 1 - exp (- 0.65 (t + 0.22))] Of 1-exp (- 0.60 (t + 0.12))] 5[1-exp (-0.55 (t + 0.45))] Of 1-exp (- 0.80 (t + 0.38))]