Results of 20 years of monitoring of the Ichthyofauna in West Africa - a summary report

ONCHOCERCIASIS CONTROL PROGRAMME IN WEST AFRICA PROGRAMME DE LUTTE CONTRE L'ONCHOCERCOSE EN AFRIQUE DE L'OUEST ECOLOGICAL GROUP Eighteenth session Accra. 29-31 Janrary 1997 OCP/VCU/HYBIO/97.8 ORIGINAL: French RESIJLTS OF 20 YEARS OF MONITORING OF THE ICHTHYOFAI.JNA IN WEST AT'RICA . A SI.JMMARY REPORT Y. Fermon, D. Paugy, E.K. Abban, M.E. Diop, C. Kou.assi & Z. Bah The working method adopted for the analysis of 20 years of ichthyological monitoring data consists in following the evolution of the ecological system concerned in order to see whether, after being disturbed, it deviates, from its natural courses of variation. To know the evolution of the structure of the fish populations, factorial correspondence analyses (FCA) were carried out on the CPUEs data (in numbers) and the fish condition (K) of the principal species. For the interpretation of the results, abiotic (discharge, pesticides) and anthropic factors were taken into account. Catch per unit effort (CPIJE) The factorial correspondence analysis encompassing all of the drawings from all the selected stations (16 out of 56 sampled) clearly shows on axis 1, the difference between the stations of C6te d'Ivoire and those of the other countries, The other axes make it possible to separate out the stations of C6te d'Ivoire from those of Mali, Ghana and Guinea which are very close to one another. This approach in the analysis led us therefore to consider the stations separately. The various catches provide data that are often difficult to interpret, namely due to the lack of regularity in the sampling as well as of the reference sites. However, some long series of date make it possible to obtain information that is fairly well supported. Thus, it was possible in a number of stations, to show the close relation between the quality or the quantlty of the catches and the flood regime of the rivers. On one hand it was possible to clearly establish that catches were fairly well affected during high water periods and, on the other hand that he years with a deficit in rainfall had negative consequences on fishing results. As a result, it may be considered that unfavorable factors affect both the diet (see fish condition below) and the success of reproduction of fishes. In one case (site 01), it was also observed that human resettlement in the valleys definitely had an impact on the quality and quantity of fish populations. Some stations, namely the one at Mandiana on the Sankarani (site 63), have shown how managements could also have a significant impact on the composition and structure of catches and therefore necessarily on the fish populations. Every time one of the factors mentioned (water level, human pressure, land development, ...) is sufficiently marked, the impact of the parameter considered is always preponderant and influences the composition and structure of the catches. When this is not the case, however, some of the variations may then be related to the intensity and quality of 2insecticide spraying. This is the case, for instance, in the Comoe (site 02), Bandama (site 03), Pru (site 20), Milo (site 60) and Dion rivers (site 68). In all of these cases, the direct influence of the pesticides cannot be advocated. Nonetheless in the absence of any other explanation, the influence of larvicides cannot, a priori, be ruled out. Even though this influence exists, however, it can be seen that in any case it is of little scope and generally masked by other natural (water level , . . . ) or non natural factors (human pressure, . . . ) . Fish condition K The coefficient of condition reflects the physiological state of the fish, especially the amount of flesh. This factor may express either a direct effect (action of the insecticide on the physiology of the fish) or an indirect effect (action on available food). In this study no separation was made between males and females because most often no significant difference in K was observed between the sexes, This is not necessarily the case between young and adults as most of the species present and increasing allometry. Therefore, only a restricted size interval was taken into consideration in order to be able to work with as homogeneous samples as possible. A comparison of species between station was deliberately given greater importance in order to verify whether some eventually significant variations were related to one or some stations or whether they were more general, in which case they would be rather due to a chance factor independent from the events likely to occur at a particular site (water level, pesticides, land development, ...). Apart from S. mystus (3901) for which some doubt persists, no influence of larviciding on the condition of fishes could be observed in any of the stations for all of the other species. This could have been predicted for omnivorous species such as B. macrolepidotus (1704), but it is reassuring to note that this also applies to fairly exclusively insectivorous species such as P. bovei (1203), C. guntheri (6401) or even Chrysichtys spp or Synodontis schall (5414). On the other hand, it was observed that the water level as well as water managerment along the river network could have a dominant influence. For instance, the seasonal hydrological cycle of tropical rivers often directly influences the rate of reproduction of some species, which, as a result, affects the condition (K). As observed, this is the case for B. nurse (1709) and B. leuciscus (1711). Here therefore, the influence of the flood regime is a direct one. The rate of the discharge during the annual flood period may also have a direct affect on the K. Thus it was observed in S. schall (5414) that a low level flood immediately caused a general drop of the K of individuals regardless of the rivers or the stations. In the Sudanian zote, the low water season is generally considered a period of "food shortage" for fishes. Any deficit in water level is therefore also an additional period of food scarcity for them. This could then explain the drop in the general condition of the insectivorous populations subjected to these unfavourable conditions. Lastly, it was noted that fish condition could be affected by the multiplication of development works such as dams. This is the case in particular for two migrating species of the Bandama such as A. baremoze (1702) and S. mandibularis (3902). It is therefore comporting for the OCP Programme to note that the spraying of more or less selective insecticides for nearly twenty years has not affected the potential sources of food for fishes in any particular way. Even though the populations of certain prey taxa have significantly decreased, the fishes have obviously been able to adjust their diet to other available food resources.