Mitigation of negative ecological and socio-economic impacts of the Diama dam on the Senegal River Delta wetland (Mauritania), using a model based decision support system

International audienceAbstract: The delta of the River Senegal was modified substantially by the construction of the Diama dam in 1986 and the floodplain and estuarine areas on the Mauritanian bank were affected severely by the absence of floods. In 1994, managed flood releases were initiated in the Bell basin (4000 ha) of the Diawling National Park, as part of a rehabilitation effort. The basin was designated as a joint management area between traditional users and the Park authority and a revised management plan was developed through a participatory approach based on a topographical, hydro-climatic, ecological and socio-economic data. Hydraulic modelling was developed as a tool to support stakeholder negotiations on the desired characteristics of the managed flood releases. Initially, a water balance model was developed. The data were then integrated into a one-dimensional hydraulic model, MIKE 11 (DHI, 2000). When associated with a Digital Elevation Model and a Geographic Information System, (Arc View), the model provided a dynamic description of floods. Flood extent, water depth and flood duration data were combined with ecological and socio-economic data. The water requirements of the different stakeholders were converted to flood scenarios and the benefits and constraints analysed. A consensus scenario was reached through a participatory process. The volume of flood release required to restore the delta does not affect hydro-power generation, navigation or intensive irrigation, for which the dams in the basin were constructed. Hydraulic modelling provided useful inputs to stakeholder discussions and allows investigation of untested flood scenarios. Keywords: wetland restoration, water use conflicts, equity, Senegal River delta, Mauritania, Diawling National Par

Life history, growth and production of <i>Neomysis integer</i> in the Westerschelde estuary (SW Netherlands)

The Neomysis integer (Leach, 1814) (Crustacea, Mysidacea) population of the brackish part of the Westerschelde estuary was sampled on a fortnightly basis from November 1990 to December 1991. Density, biomass, population structure and brood size were recorded. The Bhattacharya method was applied to the length-frequency data for the detection and separation of cohorts. Growth is described both by a generalised von Bertalanffy function incorporating seasonal oscillations in growth. Secondary production was estimated for each cohort using 4 approaches. The seasonal pattern in density and biomass showed 3 peaks: a relatively small yet distinct, peak in early March (30 ind. m -2, 60 mg AFDW m-2) and 2 main peaks in late spring (160 ind. m-2, 225 mg AFDW m-2) and in summer (140 ind. m-2, 125 mg AFDW m-2). Throughout winter, N. integer density remained well below 30 ind. m-2. Three periods of increased reproductive activity and subsequent input of juveniles were found. This suggests that 3 cohorts were produced per year. The overwintering generation lived from autumn until the following spring. The spring generation was born in early spring and lived for about three mo, while the summer generation lived from summer until early winter. The 3 cohorts showed marked differences in their biology. The overwintering generation showed seasonal growth oscillations, larger brood size and a larger size at maturity. Individuals belonging to the other two cohorts generally grew faster, produced less young per female, and attained maturity at a smaller size. Within each cohort, both sexes exhibited different growth characteristics: females generally lived longer, grew faster and consequently became larger than males. The size-frequency, growth summation and removal summation methods yielded comparable production estimates. The annual production was 0.3 g AFDW m -2 yr-1 with an annual P/B ratio of 6. The average cohort P/B was 3. The size-frequency method gave similar results only when applied to the 3 cohorts and to both sexes separately. The spring cohort accounted for almost half of the annual production. Despite the longer life span on the overwintering generation, it generated only a quarter of the annual production. An independent estimate of production using the mortality rate of the different cohorts resulted in values comparable to those obtained by the other methods for the overwintering cohort, while the production of the other 2 cohorts was overestimated

Life history, food consumption and resource partitioning in two sympatric gobies <i>Pomatoschistus minutus</i> and <i>P. lozanoi</i> in the Belgian coastal waters

Gobies were obtained monthly from the bycatch of a commercial shrimp trawler operating in the shallow waters (less than 20m depth) of the Westdiep area, Southern Bight of the North Sea, from May through December 1984. Pomatoschistus lozanoi juveniles appear later than P. minutus juveniles, indicating a temporal segregation of reproduction. Yearly average density of P. minutus is about twice that of P. lozanoi. Food consumption by the two species amounts to 1-2 g AFDW/m2/year. Stomach analysis of several hundred gobies shows that P. minutus is a food generalist that obtains most of its food from the benthos and epibenthos. P. lozanoi is a more specialised feeder, strongly dependant on the hyperbenthos, on Mysidacea in particular. The absence of a food niche shift in P. lozanoi when no P. minutus are present suggests that present-day competition is unimportant in shaping the food niche segregation observed

The food of early post-larval plaice, <i>Pleuronectes platessa</i>

Food-limitation in late larval and early post-larval plaice, Pleuronectes platessa, may be an important factor in the determination of year class strength. In this study data on stomach content, hepato-somatic index and condition index of metamorphosing and early post-larval plaice from a sandy beach in the mouth of the Western Schelde are compared to data from the Zwin lagoon. Settlement in the area starts at the end of february. All plaice examined from the sandy beach were fasting. Their condition, as measured by hepato-somatic index and condition index, did not differ significantly from the condition of feeding post-larvae from the lagoon. Thus starvation during metamorphosis is rejected as an important factor in determining year class strength. In the Zwin lagoon the juveniles of less than 20 mm standard length feed mainly on Calanoidea, Polychaetes and Harpacticoidea

Functional guilds of fishes and macrocrustaceans in the Westerschelde and Oosterschelde estuary

Several subhabitats (e.g. intertidal and subtidal soft substratum or saltmarsh creeks) have been defined within the estuarine reaches of the Westerschelde and the Oosterschelde. This complex system can serve as spawning ground, nursery area, feeding ground and as a pathway in diadromous migrations. This study concentrated on the significance and role of these selected habitats in the life cycle of fishes and macrocrustaceans. Therefore, both habitat availability and habitat use were quantified. The importance of estuarine biotopes as habitats for fish was evaluated through a Habitat Utilisation Index. Next to the ecological guilds, also feeding guilds were developed to assess the relationship with the lower consumers. A generalised food web structure within the Westerschelde was defined, which proved the importance of mysids and other crustaceans in the food of juvenile fish and shrimps. The results were compared with other systems to examine if the Westerschelde and the Oosterschelde comply with what is inspected in European estuaries

Land and water grabbing in an east african coastal wetland : the case of the Tana delta

International audienceno abstrac

The mobile epibenthic fauna of soft bottoms in the Dutch Delta (south-west Netherlands): spatial structure

During 1989 monthly beam trawl samples were collected at 48 stations in the Dutch Delta. The annual mean densities of the demersal fishes and mobile epibenthic invertebrates at these stations were calculated. These data were then subjected to multivariate statistical techniques for an analysis of the spatial structure of the communities found and to study the relationship between these communities and their environment. The present study confirms the results of HENDERSON (1989) that, given a fairly limited number of environmental variables, mainly salinity and exposure/substratum type, quite accurate predictions of the type of community expected at a certain site can be made. The explicit inclusion of epibenthic invertebrates, a dominant group in most assemblages, in studies mainly targeted at demersal fishes, is strongly recommended. In the Dutch Delta rich and varied communities exist in the Voordelta. A number of groups, such as salmonids, anadromous species and starfish, expected in the Westerschelde are absent or extremely rare probably as a consequence of pollution stress. The Oosterschelde is relatively poor in density terms but has a highly diverse epibenthic fauna dominated by fishes