Surveillance of the aquatic environment in Guinea in relation to antiblackfly larvicides: summary

ONCHOCERCIASIS CONTROL PROGRAMME IN WEST AFRICA PROGRAMME DE LUTTE CONTRE L'ONCHOCERCOSE EN AFRIQUE DE L'OUEST ECOLOGICAL GROUP Twenty one session Bamako, 23-25 February 2000 OCP/VCU/HYBIO/2OOO.1 ORIGINAL: French SURVEILLANCE OF THE AQUATIC ENVIRONMENT IN GUINEA IN RELATION TO ANTIBLACKFLY LARVICIDES Keletigui Nabe Summary Two distinct periods have marked the 15 years of surveillance of the aquatic environment. The second period spans from 1987 to 1999, and is characterized by the relative use of 7 larvicides, from the less selective ones (Permethrin, Vectron) to the most selective (Abate, Pyraclofos, Bf and Teknar). ln the course of the 1999 season, three watercourses: the Milo, Niandan and Dion were treated, predominantly with Bf and Teknar on the Dion, Niandan and Tere. Abate was also used on a large scale in larviciding on the Tere. There were also several suspension periods, as compared to 1997. I. STUDY OF THE FAUNA AT BOUSSOULE The total densities of the fauna recorded in 1999 were slightly lower than those of 1998. The densities of the Trichoptera had trends similar to those of the total fauna, whereas the densities of the Ephemera and the Diptera were relatively stable, as compared to those recorded in 1998. ln 1999, 18 taxa were collected in the organisms of the rock slabs as against 34 in the pre-larviciding period; this gives a relative presence of 52.94o/o. ln 1999, non- target Diptera were predominant (48.79%), followed by the Trichoptera (43.18%) and the Ephem era (7 .29o/o). ln the night drift, 26 taxa were recorded in 1999 as compared to 36 during the pre-larviciding period; this gives a presence rate of 72.22% of drifted fauna. This fauna was largely made up of non-target Diptera, with the Ephemera alone constituting more than 50% of the total drift. II. STUDY OF THE FAUNA AT SANSAMBAYA I The densities of the total fauna in 1999 were not too different from those obtained during the previous season. The density of the Ephemera was the same as that of the previous year; that of the non target Diptera was slightly higher, while the density of the Trichoptera has slightly decreased (fi9. 11). The relative composition of this saxicolous fauna shows a heavy population of non-target Diptera (50.56%), followed by the Trichoptera (37%) and the Ephemera (8.63%) respectively, with the other taxa making up the remaining 3.72o/o. 2The Ceratopogonidae, Dytiscydae and Hydracarina were absent from the samples. Besides, 21 taxa, as against 33, were recorded in the pre-larviciding period, which gives a relative presence of 63.63% in the Surber samples of this season, as compared to the pre-larviciding period. The taxonomic richness of the night drift during this season was richer and more diversified than that of the saxicolous fauna, i.e. 26 taxa for the drift, compared to 21 taxa for the saxicolous fauna. The Ephemera (Baetidae and Caenidae) and the non-target Diptera (Orthocladiinae, Tanypodinae and Chironomini) formed the essential base of this drifted fauna. III. STUDY OF THE FAUNA AT TERE For lack of pre-larviciding data, the analysis of the saxicolous fauna is based on the 1990-1999 period. lt was observed that the densities of the saxicolous fauna showed an upward trend between 1990 and 1999. The densities recorded in 1999 were relatively lower that those of 1998. The non target Diptera made up the majority of organisms of the saxicolous fauna, followed by the Trichoptera (18.18%) and the Ep hemera (2.30%) respectively. IV. INVESTIGATIONS UPSTREAM FROM BOUSSOULE The Milo and Niandan are neighbouring rivers on which the same larviciding cycles were applied. The decrease in faunal density observed on the Milo seems to be the result of larviciding combined with comtamination from mining activity. We could not ascertain the type of products which were dumped into the river in connection with this mining activity. The Niandan is not only poor in tributaries compared to the Milo, but also it presents no source of pollution likely to disturb the populations of aquatic insects. V. AQUATIC HABITAT QUALIW ASSESSMENT The preliminary results recorded in the context of this study show two categories of parameters 1. Parameters variable in the short term : changes in the water stretch - Discharge rate ; 2. Parameters variable in the longer term : epifaunal substrate - Characteristics of the substrate of the water stretch - Sediments deposit - Changes in the river bed - Bends in the water course - Bank stability - Bank protection by plant cover - Width of riverside vegetation. From the review of these parameters in relation to the benthic fauna, it appears that very few organisms are collected when the water level drops down to the base of the two banks or fills the river bed up to75 %.