Rock Assessment of the Five Major Commercial Species in Yeji Area (Stratum Vii) of the Volta

Abstract

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 - e x p ( - 0.60 (t + 0.12))] 5 [ 1 - e x p ( - 0 . 5 5 (t + 0.45))] Of 1 - e x p ( - 0.80 (t + 0.38))] The estimated longevity (t max - 3/K) for the major stocks were between 4 and 6 years indicating that they are short-lived. This was expected because tropical fish are known to be fast growing and short-lived. This increase in growth rate accompanied by a decrease in size appeared to be adaptation for survival by the species in the midst of increasing effort. The estimated ages at first capture of the major species based on the estimated length at first capture (Lo50) show that they are caught before reaching one year old which implies that there is growth over-fishing. The length frequency analysis also showed that the fishery operated on three pseudo-cohorts of the major fish stocks. According to the the relative yield-per-recruit analysis, the present level of fishing (Epresent) was greater than the maximum expected exploitation (Emax). Based on the Eopt = 0.5 optimization criterion, this implied that there was over-exploitation of the major stocks. This grave situation is aggravated by the progressive reduction in water level, brought about by poor rains. In order to forestall this situation, some options for management of the fisheries have been recommended. These include: a) limitation of entry into the fishery; b) enforcement of mesh size regulations; c) control over changing fishery technology; d) establishment of “lake reserves” through closure of fishing in selected areas; and e) adoption of a rights-based fisheries management system.