Surveillance of the aquatic environment in Gunea ichtyofauna: summary

ONCHOCERCIASIS CONTROL PROGMMME IN WEST AFRICA PROGRAMME DE LUTTE CONTRE L'ONCHOCERCOSE EN AFRIQUE DE L'OUEST GROUPE ECOLOGIQUE Twenty one session Bamako, 23-25 February 2000 OCP/VCU/HYBIO/2OOO.3 ORIGINAL : French SURVEILLANCE OF THE AQUATIC ENVIRONMENT IN GUINEA ICHTYOFAUNA Moussa Elimane Diop Summary The processing of field data collected in 1999 and the resulting analyses confirmed the dynamics of the principal parameters of the surveillance of the aquatic environment, which had already been observed in the previous seasons. The quality of the water was regularly examined on the basis of the following principal factors: temperature, salinity, conductivity, turbidity, nitrate and phosphate content, etc..(tables l, !l and !!!). We noticed through the various larviciding sequences (table lV) that the larviciding strategy was strictly adhered to, i.e, rotating the various larvicides in order to minimize the risk of vector resistance; adapting insecticide doses to the hydrological conditions of the treated rivers; reducing the number of annual cycles of "hard" larvicides (Permethrin), and of the consecutive cycles of Pyraclofos and Phoxim. After standardizing the data base (tables V, Vl and Vlll), it appears that the total CPUEs, by number and by weight, are following the same trends. The number of CPUEs obtained in 1999 was slightly lower than those of the two preceding years at Baranama, and those of 1998 at Mandiana and Bousoule (fig. 5, 6 and 7). The modes appear in November 1997 and 1998 respectively. The reduction in catches could be due to the late occurrence of the spate in 1998/99. The interannual overall upward trend of CPUEs is however maintained. On the whole, seasonal variations kept to the classic evolutionary pattern of the previous years, namely: total high CPUEs during low water Ievels and low CPUEs in the spate (weight). lt is to be noted that for the CPUEs expressed in terms of fish numbers, abundance is generally recorded in November, during the period of low water levels. CPUEs by net category show a higher rate of efficiency of the three net types in 1998 at Mandiana and Boussoule, and in 1997 at Baranama (figures 8,9 and 10). Basically, the small-sized nets (12.5, 15 and 17.5) caught the highest number of fish, as compared to the medium and large-sized nets. The medium-sized nets (20, 22.5 and 25) were, however, more efficient in terms of biomass. The large-sized nets (30 and 40) proved to be less efficient, due probably to the fishermen who catch the same sizes of fish. The cumulative results from 1990 to 1999 are comparable (figures 11,12 and 13). The general trend of CPUEs of the three net sizes reflects that of the total CPUES. 2The specific richness increases and is kept at a relatively high level at all the stations. Despite a relative decrease that was observed at Mandiana in 1998 and 1999, the specific richness shows an overall upward trend (figures 14, 15 and 16). The cumulative richness, from 1987 to 1999, is somewhat stable; this shows the limits of the efficiency of mesh nets in the surveillance stations (fig. 17). No total and lasting disappearance of species was recorded. The mean condition coefficients of the principal species show some kind of constancy at both the seasonal and interannual levels, hence the deduction that their physiological condition is good and is being maintained (figures 19,20,21 and22). Therefore, it could be said in conclusion that the results of the aquatic surveillance in 1999 are on the whole good compared to those of the previous years. As far as we are able to determine, they do not show any drastic effects of the larvicides on the fish fauna. Among the three cases of fish mortality, two of which were reported by aerial operations and the other by the authorities, none, according to our inquiries, was attributed to the activities of the Programme (see background document).