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Les mangroves sont bien plus que des palétuviers. Forêts dans la mer, ou encore marais maritimes convertis en rizières, elles constituent l'un des paysages les plus originaux des littoraux intertropicaux. Dès leur découverte par les navigateurs portugais, elles suscitent des discours contradictoires et sont l'objet d'enjeux multiples et souvent conflictuels entre « conservationnistes » et « productivistes ». Cette synthèse régionale dresse un état des connaissances sur la mangrove, met en évidence les interactions variées entre les sociétés et leur environnement littoral, explicite la spécificité des « Rivières du Sud », littoraux à mangrove compris entre le Sénégal et la Sierra Leone. Elle réunit un ensemble d'informations originales (volume 1) et un recueil bibliographique de plus de 3 000 références (volume 2), et constitue un outil indispensable pour les acteurs, scientifiques comme décideurs, concernés par la gestion des littoraux ouest-africains. Par un dialogue entre sciences de la nature et de la société, cet ouvrage ouvre des perspectives conceptuelles et méthodologiques et contribue ainsi à la réflexion plus générale sur les rapports entre environnement et développement, sciences et société. Mangroves are much more than just mangles. Forests in the sea or sea marshes converted into rice fields, they form one of the most original coastal landscape types in the tropics. Since their discovery by Portuguese navigators, they have generated contradictory statements and have been the centre of many issues often leading to contention between 'conservationists' and those in favour of 'productivity'. This regional synthesis reviews the state of knowledge on mangrove, shows the varied interactions between societies and their coastal environment and explains the specific features of the 'Rivières du Sud' (southern rivers), the coastal mangrove areas from Senegal to Sierra Leone. It consists of a set of original information (Volume 1) and a bibliography of more than 3,000 references (Volume 2), forming an essential tool for all those - both scientists and decision-makers - involved in the management of West African coastal zones. By establishing a dialogue between natural and social sciences, the work opens up conceptual and methodological prospects and contributes to more general reflection on the relations between the environment and development and between science and society.

International audience; A comparative study, combining three different assessment methods (fish gillnet sampling, artisanal fisheries surveys and hydroacoustics) was conducted in Mali where two man-made reservoirs (Sélingué and Manantali) are particularly suited for investigating the impact of fishing effort on the fish assemblage. These two ecosystems have relatively similar areas, edaphic and environmental properties but are subjected to different levels of fishing exploitation (low at Manantali, high at Sélingué). The comparison is based on two indicator parameters: the abundance indices and the size spectra distributions, obtained by the three methods at two contrasting hydrological seasons (April and October). The results were compared first between the two seasons, and then between the two lakes. The present work is based on two main hypotheses: (1) that there is a higher fish abundance in October associated with smaller overall sizes, after spawning; (2) a lower abundance and smaller sizes in the Sélingué reservoir than in Manantali, because of the much higher fishing pressure in Sélingué. The relevance of each method to the selected indicators is discussed. On the one hand, the three methodologies on the whole gave similar conclusions and they also complement each other. On the other hand, some results do not match the hypotheses because of biases due to difficulties and technical limitations of each method in such ecosystems (shallow water with vegetation and stumps of former forests)

International audience; In most estuarine ecosystems, the fish biomass is difficult to estimate, since most of the standard sampling methods are difficult to implement and may be limited to certain biotopes. Moreover, Protected Areas are no longer limited to marine ecosystems and several protected areas have been created in estuaries, lagoons or mangrove environments. In these areas, turbid waters do not allow direct diving observations and fish sampling, often performed using destructive – because efficient and non selective - gears such as trawling or seine nets, is always an ethic problem. As a consequence there is an increased need for liable, non-destructive methods to study fish assemblages in these environments. In order to describe the fish biomass distribution and evolution in the Gambia estuary, five research surveys have been conducted at different hydrological seasons. The fish assemblages were sampled by vertical echo-sounding at moored stations and at the same time and same location with a purse seine. The purpose of this work, still in process, is to compare the two methods, to try to inter-calibrate them and to seek for their complementarity. Though significant, the global direct correlation between estimations of biomass performed with the two sampling methods were too low to be considered satisfactory. Moreover the level of correlation between the two methods varies extensively following the survey, the season or the zone of the estuary taken into account. The role of the main environmental variables: depth, salinity, transparency and current strength on the quality of the correlation of the estimation made by the two methods was emphasized.

La gestion des p��ches artisanales n��cessite souvent l���usage de relations biom��triques pour transformer des donn��es collect��es sur le terrain en indicateurs utilisables pour cette gestion. L���une des relations les plus communes est celle qui relie la longueur d���un poisson �� un poids estim��. �� partir d���informations collect��es lors de programmes d�����tude des peuplements ichtyens des estuaires et lagunes ouest-africaines, le document propos�� fournit des clefs longueur-poids pour 52 esp��ces de poissons vivant dans ces ��cosyst��mes estuariens et lagunaires.

Although tilapia culture in the Senegal river basin was promising at its beginnings, it now faces major difficulties (production of less than 50 tonnes). In spite of the many positive elements such as a vast water potential, abundance of by-products from agricultural and animal farming, and a deficit of about 21,000 t in fish supply from the river watershed, tilapia culture has not been successful. Its development has been affected by many constraints : (1) environmental - irregularity in the natural water system, poor mineral content in the rivers, high turbidity during the rainy season and low water temperatures from November to March ; (2) technical - poor choice of sites, insufficient depth of the fishponds and inadequate dike construction, poor water quality control and poor fry quality ; (3) administrative - poor definition of responsibilities, insufficient management support services and constant confusion in the experimental and extension objectivs; and (4) socioeconomic - farmers' perception of fish farming as a secondary activity, high costs of construction and management, and competition with the fisheries sector. However, the construction of the Diama and Manantali dams seems to offer better prospects. (Résumé d'auteur)

In the Ebrié lagoon (Côte d'Ivoire), the violent conflicts opposing "individuals" and "collective" artisanal fishermen in 1982 originated in the collapse of Bonga catches and led to the total ban of purse seines for reasons of overfishing. The results of bio-ecological research, mainly in the fields of reproductive biology and distribution of the species in the lagoon, did not agree with the hypothesis of overfishing, particularly because of the great abundance of Etmalosa in several unexploited sectors of the lagoon. Finally, sociologist emphasized the long-lasting conflict between "individual" autochtonous fishermen and "collective" fishermen (most of them being "foreigners"). The coming together of several scientific disciplines provided a better understanding of the 1982 conflict, the origin of which seems to be more of a sociological nature rather than a biological one. A global and multidisciplinary approach seems to be the only way to comprehend the complexity of artisanal fisheries systems. (Résumé d'auteur)

A positive linear relationship between maximum observed length (MOL) and growth rate is shown for 69 marine, fresh- or brackishwater African fish species in the natural environment and for six cultured species. When accurate data on growth are missing, the MOL appears to be one of the most useful criteria for the preselection of candidate fish species for aquaculture. (Résumé d'auteur)

Les Gerreidae sont représentés dans les lagunes de Côte d'Ivoire par deux espèces : #Gerres nigri$ et #Eucinostomus melanopterus$. Bien que très proches, par leur morphologie et leur comportement général, elles se distinguent par des cycles bio-écologiques différents. Avec plus de 10 000 individus capturés dans les pêches expérimentales, #C. nigri$ est, en lagune Ebrié, la plus abondante des deux espèces; il est présent dans tout le domaine lagunaire, y compris dans les zones extrêmes, douces ou oligohalines, mais avec une abondance maximale marquée dans le secteur maritime... Moins abondant (4000 individus capturés), #Eucinostomus melanopterus$ a également une large répartition lagunaire mais n'a pas été observé dans le secteur le plus continental. Son abondance semble maximale en zone maritime et en fin de saison sèche mais la variabilité est importante. Comme l'espèce précédence, #E. melanopterus$ est très euryhalin et d'une manière générale peu exigeant en ce qui concerne les conditions environnementales... #C. nigri$ dont le cycle biologique se déroule totalement en milieu saumâtre est une espèce "lagunaire" véritable alors que #E. melanopterus$ est une forme amphibiotique

La réouverture artificielle de la passe de Grand-Bassam, pratiquée en septembre 1987 dans le but de limiter la prolifération des végétaux aquatiques, a modifié l'environnement et la structure des peuplements ichtyologiques de la partie orientale de la lagune Ebrié (Côte d'Ivoire). Les résultats de deux séries d'observations réalisées en mai 1988 montrent une élévation notable de la salinité et une augmentation de la composante marine au sein des peuplements de poissons par rapport à la situation antérieure (années de référence : 1990 et 1981). (Résumé d'auteur)