Dynamique hydrologique et évolution morpho sédimentaire du lac Ahémé au Sud-Ouest du Bénin

Le lac Ahémé dans le complexe estuarien sud-ouest béninois connait de nos jours, un véritable problème de dégradation dû à l’évolution naturelle et aux activités anthropiques ayant contribué à accélérer la vulnérabilité de l’hydrosystème. La présente étude, basée sur l’analyse de l’évolution bathymétrique et sédimentologique des fonds lagunaires vise à caractériser le rythme de comblement du lac Ahémé. La bathymétrie du lac Ahémé indique un plan d’eau encaissé, à fond plat, avec des profondeurs moyennes comprises entre 1,6 et 2,4 mètres. Les sédiments sont vaseux dans la partie centrale du Nord au centre du lac, sablo-vaseux au Sud et à l’Est puis sableux à l’Est et aux abords du lac en contact avec les plateaux de Terre de Barre. L’analyse comparative des cartes sédimentologiques et bathymétriques des années successives, a montré que le lac se comble au fil des ans, particulièrement dans sa partie sud. L’évolution des faciès sédimentaires du lac de 1999 à 2018 montre une plus large extension des vases et vases sableuses dans la partie sud du lac alors qu’elles se cantonnaient autrefois dans la moitié sud du lac. La répartition des vases n’est donc plus uniforme, comme c’était le cas il y a une vingtaine d’années. Une telle évolution bathymétrique est le signe tangible d’un comblement et cet état de chose est préjudiciable aux organismes vivants inféodés dans le milieu.   Lake Ahémé in the south-western Beninese estuary complex is nowadays experiencing a real problem of degradation due to natural evolution and anthropic activities that have contributed to accelerate the vulnerability of the hydro system. The present study, based on the analysis of the bathymetric and sedimentological evolution of the lagoon bottoms, aims at characterizing the filling rate of Lake Ahémé. The bathymetry of Lake Ahémé indicates an entrenched, flat-bottomed body of water with average depths between 1.6 and 2.4 meters. The sediments are muddy in the central northern part of the lake, sandy-muddy in the south and east, then sandy in the east, and on the edges of the lake in contact with the Terre de Barre plateaus. Comparative analysis of the sedimentological and bathymetric maps of successive years has shown that the lake fills up over the years, particularly in its southern part. The evolution of the sedimentary facies of the lake from 1999 to 2018 shows a wider extension of the mud and sandy mud in the southern part of the lake whereas they were formerly confined to the southern half of the lake. The distribution of mud is therefore no longer uniform, as was the case some twenty years ago. Such a bathymetric evolution is the tangible sign of a filling and this state of affairs is detrimental to the living organisms that are dependent on the environment

Exploitation De L’huître Des Mangroves Crassostrea Gasar (Adanson, 1757) Dans Le Lac Nokoué Au Bénin

This manuscript deals with the exploitation of oysters, Crassostrea gasar, specie highly presented in Lake Nokoué, south of Benin Republic. This study aims to characterize this exploitation through the sociodemographic parameters of oyster farmers, the characteristics of different collection sites, the importance and economic characteristics of the exploitation. Socio-demographic and economic information was collected on the basis of a survey of the operators. The physico-chemical data and on the type of substrate used to characterize the sites were collected on the stations Agbato and Atchakpanoukpa. A total of 65 farmers were surveyed. In general, women dominate the people who exploit the oysters (67.7%). Harvesting and trading are two components of the exploitation of oysters. Harvesting is an essentially masculine activity while trading is reserved to women. The stations Agbato, Donoukpa, Jesukô, Togbigba and Atchakpanoukpa near to Cotonou channel are the harvest sites characterized by high salinity (24.057.70 g / L). The oysters’ exploitation in Lake Nokoué is realized at march to september of each year by 60% of populations surveyed. The oysters’ exploitation weekly generates incomes of F CFA 30 150.77 to the collectors, F CFA 29 703.57 to fishmongers and F CFA 28 763.33 to whom lead the two activities. Oysters’ exploitation is intense at Lake Nokoué and constitutes important source of incomes to the operators

Biologie et perspectives d’élevage de la crevette géante d’eau douce Macrobrachium vollenhovenii (Herklots, 1857)

L’objectif principal de cette étude est de faire le point des connaissances sur quelques aspects de la morphologie, de la biologie de reproduction et des besoins alimentaires en élevage de M. vollenhovenii afin d’entreprendre son élevage en captivité. L’approche méthodologique utilisée est basée sur la recherche documentaire et a consisté à l’analyse des publications scientifiques, des thèses et livres portant sur la crevette d’eau douce Macrobrachium en général et de Macrobrachium vollenhovenii en particulier. Les larves vivent exclusivement en eau saumâtre. La nutrition larvaire est basée sur le plancton tandis que les adultes sont omnivores à dominance carnivore. La croissance se fait par des mues donnant une population à structure hétérogène. Une femelle mesurant 11,40 cm de taille peut pondre jusqu’à 56481 oeufs. La gamétogenèse comporte 5 phases : phase dominante des cellules immatures, phase d’évolution des cellules immatures, phase dominante des cellules mûres, phase des cellules mûres et phase d’émission des gamètes. Les oeufs, au bout de deux à trois semaines, donnent à l’éclosion de petites larves appelées nauplius qui évolue en zoé et mysis. Après 15 stades larvaires, la métamorphose intervient entre le 17è et 28è jour pour donner des post-larves, petites répliques en miniature des adultes.© 2016 International Formulae Group. All rights reserved.Mots clés: Macrobrachium vollenhovenii, biologie, aquaculture, BéninEnglish Title:  Biology and breeding prospect of giant fresh water shrimp Macrobrachium vollenhovenii (Herklots, 1857)English AbstractThe main target of this study is to give the point of knowledge on some aspects of morphology, on reproduction biology and food needs of M. vollenhovenii in breeding in order to undertake his breeding in captivity. The methodological approach used is based on desk research and consisted in the analysis of  scientific publications, theses and books on the species of freshwater shrimp Macrobrachium genre in general and in particular Macrobrachium vollenhovenii. The larvae live exclusively out of brackish water. The larval nutrition is based on the plankton while adults are omnivorous with carnivorous predominance. The growth is done by moults giving a population to heterogeneous structure. A female measuring 11.40 cm of size can lay up to 56,481 eggs. The gametogenesis is made up of 5 stages: immature cells dominant stage, immature cells evolution stage, ripe cells dominant stage, ripe cells and gametes issuance stage. Eggs, after two to three weeks, give the hatch small larvae called nauplius that evolves zoe and mysis. After 15 larval stages, metamorphosis occurs between the 17th and 28th day to provide post-larvae witch are small replicas in miniature adults.© 2016 International Formulae Group. All rights reserved.Keywords: Macrobrachium vollenhovenii, biology, aquaculture, Beni

Co-culture Clarias gariepinus-Oreochromis niloticus : quels avantages pour l’amélioration des performances zootechniques et économiques des poissons élevés dans les « whedos » du delta de l’Ouémé au Bénin ?

L’alimentation des poissons est la contrainte majeure pour l’émergence de la pisciculture tropicale. Dans le but d’évaluer l’effet de la co-culture Clarias gariepinus - Oreochromis niloticus sur la croissance de ces deux espèces élevées dans les whedos, une expérience a été conduite durant 56 jours dans le village Aïzè à Ouinhi (Bénin). Des alevins de C. gariepinus (6,74 ± 0,27 g) et de O. niloticus (8,11 ± 0,14 g) ont été placés respectivement dans des happas de 1 m2 (30 alevins / m2) et de 6,25 m2 (30 alevins / happa), les happas contenant C. gariepinus étant insérés dans les happas contenant O. niloticus. Trois types d’aliments ont été testés: aliment importé Sckretting (45% de protéines), aliment local (37% de protéines), et aliment mixte (50% aliment local et 50% aliment importé). Seuls les alevins de C. gariepinus ont été nourris, les alevins de O. niloticus se nourrissant de la production primaire produite par les déjections de C. gariepinus. Les meilleures performances zootechniques et d’utilisation alimentaire ont été obtenues avec l’aliment mixte suivi de l’aliment importé chez les deux espèces. Chez C. gariepinus, le poids moyen final a varié de 20,95 ± 1,12 g à 42,56 ± 2,26 g tandis que chez O. niloticus il a varié de 17,00 ± 1,84 g à 19,63 ± 0,83 g. La présente étude a clairement montré que la co-culture Clarias gariepinus – Oreochromis niloticus avec nourrissage seulement de C. gariepinus est une forme d’élevage de plusieurs espèces de poissons à la fois à coût réduit.© 2015 International Formulae Group. All rights reserved.Mots clés: Co-culture, whedos, happas, delta de l’Ouémé, SckrettingEnglish AbstractThe fish feeding is the major constraint to the emergence of tropical fish farm. In order to assess the effect of co-culture Clarias gariepinus - Oreochromis niloticuson on the growth of these two species bred in whedos, an experiment was conducted during 56 days in the village Aїzè in Ouinhi (Benin). Fingerlings of C. gariepinus (6.74 ± 0.27 g) and O. niloticus (8.11 ± 0.14 g) were placed respectively in happas 1 m2 (30 fingerlings / m2) and 6 25 m2 (30 fingerlings / happa), the happas containing C. gariepinus being inserted into the happas containing O. niloticus. Three kinds of foods were tested: imported food Sckretting (45% protein), local food (37% protein), and mixed food (50% imported and 50% local food). Only C. gariepinus fingerlings were fed, O. niloticus fingerlings feeding on primary production produced by the manure of C. gariepinus. The best growth performance and feed utilization were obtained with the mixed food followed by imported food in both species. In C. gariepinus, the final average weight ranged from 20.95± 1.12 g to 42.56 ± 2.26 g, while in O. niloticus it ranged from 17.00 ± 1.84 to 19 g, 63 ± 0.83 g. This study clearly demonstrated that co-culture Clarias gariepinus - Oreochromis niloticus with only C. gariepinus feeding is a form of rearing several species of fish both at reduced cost.© 2015 International Formulae Group. All rights reserved.Keywords: Co-culture, whedos, happas, Ouémé Delta, Sckretting

Facteurs déterminants de la volonté des pêcheurs artisanaux d'adopter la cogestion des droits de propriété dans le complexe lac Nokoué et lagune de Porto-Novo au Bénin : Approche par une expérience de choix discret

The estuarian and lagoon areas of southern Benin are an atypical lake territory where private property rights are inherited from endogenous law. In this area, people live in stilt dwellings and exclusively for free-to-access fishing. Consequently, an increasing number of fishers with low respect for the State’ general rules for fishing sustainability contribute to the tragedy of commons. Co-management of small-scale fisheries has been advocated to offer various benefits, including improved socio-ecological integration, shared sustainable livelihoods and adherence to biodiversity objectives. This study aims to assess the factors that influence the willingness of small-scale fishers to adopt property rights co-management options in southern Benin. We interviewed a random sample of 277 small-scale fishers. The data collected using the Discrete Choice Experiment were analysed using mixed and latent class logit models. The results show that only 44% of fishers are willing to adopt property rights co-management options. This willingness to adopt is determined by their involvement in the co-management committee, access to a subsidy and livelihood diversification options. However, they would prefer to keep the area of water granted as small as possible, even if they are motivated to contribute to more open space on the water bodies. These fishers are primarily owners of Acadja, members of fishers’ associations, and are the oldest in the sample. Institutional agreements for co-management establishment, subsidies to support small-scale fishers’ livelihood diversification and capacity-building must be set up to reduce their fishing time and ensure fishery viability.6. Clean water and sanitatio

Evaluation histologique de la maturation des gonades chez Labeo parvis (Teleostei, Cyprinidae) au Bénin

A histological analysis was undertaken on the gonadal development of Labeo parvus Boulanger 1902, a medium-sized cyprinid from the Ouémé River, Benin. Samples were collected monthly between April 2006 and March 2007 and were examined using routine macroscopic and histoloogical techniques. Five macroscopic stages of gonad maturation were identified in the ovaries and testes and described histologically.Oocyte development was subdivided into five stages, based on the presence of chromatin nucleolus oocytes, perinucleolus oocytes, yolk vesicle oocytes, vitellogenic oocytes and mature oocytes. Four stages of spermatogenesis (spermatogonia, spermatocytes, spermatids and spermatozoa) were observed. Comparisons of macroscopic and histological observations revealed that the temporal patterns of maturation activity obtained from visual examination closely reflected the seasonal histological trend in gonad development. Histological characteristics of spent ovaries and testes ware similar to those of immature fish, indicating that L. parvus were synchronous spawners

Variations du régime alimentaire de

L'étude du régime alimentaire de C. nigrodigitatus Lacépède (N = 646$) et C. auratus Geoffroy Saint-Hilaire (N = 253) a été réalisée d'après l'analyse des contenus stomacaux de poissons du lac Nokoué et de la lagune de Porto-Novo (Sud-Bénin) en 1990-1991. L'analyse factorielle de correspondance sur l'occurrence de 16 catégories de proies dans 21 catégories de poissons (suivant l'espèce, la taille des poissons, la saison et l'habitat) met en évidence la répartition des ressources alimentaires entre C. auratus et différentes catégories de tailles de C. nigrodigitatus capturés hors acadjas (enclos en branchages). Les indices d'abondance (Iab) et de volume (Iv) des proies et leur régularité (Rab et Rv) dans le régime alimentaire mettent en évidence la tendance benthophage des deux espèces : C. auratus (6-20 cm Ls) est beaucoup plus généraliste (Rv = 0,842) se nourrissant essentiellement de mollusques et de petits crustacés (branchiopodes, copépodes, ostracodes) par fouille du substrat. C. nigrodigitatus, en revanche, se spécialise avec l'âge et la taille (Rv = 0,826 à 0,597 de 6-10 cm à 6-20 cm Ls) vis-à-vis des décapodes (Iv de 13,4 à 49,4 %) et des poissons (œufs et alevins, Iv de 7,8 à 66,9 %). Les différences saisonnières concernent principalement la période des crues, au cours de laquelle les poissons se nourrissent de larves ou de nymphes d'insectes sur des végétaux de la plaine inondée. Dans les acadjas, les deux espèces partagent les mêmes ressources alimentaires. Cette étude de l'écologie trophique des Chrysichthys du complexe lagunaire du Sud-Bénin, mise en parallèle avec leurs potentialités de croissance, suggère que le régime alimentaire généraliste de C. auratus se situe dans un contexte de ségrégation sélective vis-à-vis de C. nigrodigitatus. La valeur adaptative des deux espèces dans un écosystème modifié et exploité par l'homme est brièvement discutée

Diet and food resource partitioning between Chrysichthys nigrodigitatus and C. auratus in South-Bernin lagoons

The diet of Claroteid (formerly Bagrid) fish species, Chrysicthys nigrodigitatus Lacépède (N=646) and C. auratus Geoffroy Saint-Hilaire (N=253), was studied through the analysis of the stomach contents from fish captured in the lagoons of South Benin in 1990-1991. Correspondance analysis on the occurrence of 16 categories of preys in 21 categories of fish (depending on species, fish size, season and habitat) showed resource partitioning between C. auratus and size-related categories of C. nigrodigitatus captured in open environments. From prey volume and abundance indices (Iv and Iab) and equitability (Rv and Rab), it is evident that both species can be ranked as benthophagous. C. auratus (6-20 cm SL) is a generalist (Rv= 0,842), mainly feeding on small molluscs and small crustaceans (branchiopods, copepods, ostracods) living in the substratum. C. nigrodigitatus becomes more specialized with age and size (Rv=0,826 to O,597 from 6-20+ cm SL) towards decapods (Iv from 13,4 to 49,4 %) and fish (eggs and fry, Iv from 7,8 to 66,9 %).Seasonal differences mainly refer to the season of spates, when fish forage on larvae or nymphs of insects on macrophytes in the inundated plain. In "acadjas" biotopes (man-assembled boughs), the two species share the same resources. These traits of the feeding ecology of the two Chrysichthys species inhabiting the lagoons of South Benin are analyzed within the context of life-history strategies and growth potentialities. The generalist diet of C. auratus is probably a consequence of niche segregation with C. nigrodigitatus. The ecological role of the two species in a modified and overfished ecosystem are discussed

Annual cycle of the gonads and the condition in two species of Chrysichthys (Siluriformes, Bagridae) in Nokoué lake and Porto-Novo lagoon in Benin.

The annual cycle of the gonads and the condition have been studied in n=1826 Chrysichthys nigrodigitatus and n=689 C. auratus caught in 1990-1991 by artisanal fishing in Nokoue lake and Porto-Novo lagoon in Benin. The condition factor increases progressively from September to February (maximum in February 1.15 in C. nigrodigitatus and in November 1,1 in C. auratus) then decreases from March to August (absolute minimum in August 0,95 in C. nigrodigitatus and in April 0,88 in C. auratus). C. nigrodigitatus grows faster than C. auratus. Gonads increase slowly in C. nigrodigitatus from February (male I.G.S. = 0,20 %; female I.G.S. = 0,13 %) to April (male I.G.S. = 0,22 %; female I.G.S. = 0,31 %), then rapidly in July (male I.G.S. = 0,46 ; female I.G.S. = 2,8 %). In C. auratus, the I.G.S. cycle is similar with the difference that in August the gonad weight remains high (male IG.S. =3,22 %; female I.G.S. = 3,97 %) and the drop in gonad weight noted in September (male I.G.S. = 0,41 %; female I.G.S. =0,87 %)