Ecologie benthique de la Lagune Ebrié. Théorie sur la méthode du DR' pour la mise en évidence de biocoenoses

The theoretical aspect of the DR' method was presented. It suggestes the association of some characteristic transects, the organisms of which will be used to describe the first biocoenoses of the Ebrié lagoon.

Influence de la réouverture du canal de Bassam sur la pêcherie de crabes de Vitré II en Lagune Ebrié (Côte d'Ivoire)

This work is the result of one year of investigation from May 1987 to June 1988 among fishermen of Vitré II village located in the eastern region of the Ebrié lagoon.

Estimations des potentiels de pêche des stocks démersaux ivoiriens par les modèles globaux. Effets de la prolifération du Baliste (Balistes capriscus)

Statistical data (1959-1977) of the trawling fishery off the continental shelf of Côte d'Ivoire has been fitted to the Fox (PRODFIT) global model. Owing to the proliferation of baliste (B. capriscus) since the years 1971-1972, data were divided into two groups. Maximum Sustainable Yield MSY: PMMC in the text), for the whole of commercial species in the continental shelf, decreased from 8800t to 5900t between the two periods; the difference represents balistes potentialities at bottom level. The model has also been fitted to data which concern Sciaenidae coastal community and Sparidae community which are parts by the 50 m isobathe. Deep layer (50-120m) MSY is at 2350t during the whole period of study. Until 1977, this potentiality was never reached, because of the low productivity of the Sparidae community.

Zooplancton et micronecton dans la région du Dôme d'Angola en février-mars 1971 et rélation avec les thons

Diurnal variations and geographic distribution of zooplankton and micronekton are studied in the Angola Dome. The small zooplankton species (Copepodids, Copepods, Amphipods, Ostracods, Chaetognaths, etc.) undertake vertical migrations within a 100 m water layer. Most of the micronekton species are below this layer during the day and move towards the surface during the night. The whole region prospected is rich in zooplankton and micronekton species that are present in the upper 100 m layer during the day. Only the South-West region is poor. On the contrary, this latter region is abundant in species that migrate below this layer during the day. The authors think there are 3 main difficulties in establishing good relationships between micronekton and tunas distribution: 1 - inability of micronekton nets to catch the tunas preys; 2 - the great diversity of tunas food; and 3 - the too large delay between micronekton studies and those of stomach contents of tunas.

Premiers résultats sur l'excrétion et la production du zooplancton de la Lagune Ebrié (Côte d'Ivoire)

Biomass and metabolic rates (total nitrogen and phosphorus excretion and respiration) were measured at 4 stations, representative of the lagoon environment, during high-water (Oct-Nov), dry (Dec-Jan) and rainy (July) seasons. In low-salinity waters (4o/oo) Acartia clausi is almost the only species, whereas a marine and diversified fauna is brought in from the ocean during the dry season. O/NT and O/PT atomic ratios between respiration (O) and total nitrogen (NT) and phosphorus (PT) excretions are high (15.1 and 111, respectively) and show a marked hydrocarbon feeding of zooplankton. Production was assessed from excretion via the net growth efficiency coefficient, K2 , calculated from N/P ratios for particles (a1), zooplankton excretion (a2) and constitution (a3). Daily productivity indices (i.e. daily production/biomass ratio) are high and equivalent to 1.2-3.8 day turn-over times. These high values may be ascribed to high temperatures (26.5-30 C) and phytoplankton richness (surface chlorophyll 'a' concentrations are always greater than 4 mg/m-3). Finally, the paper deals with trophic relationships between phyto- and zooplankton (ingestion /primary production ratio and transfer coefficient) and the question of relationships between zooplankton and predators.

Variabilité, circulation et chlorophylle dans la région du Dôme d'Angola en février-mars 1971

In February-March 1971 the hydrological conditions off Angola did not display the thermal dome mapped by Mazeika's averages (1967). Cold water cells observed are connected at the surface to a sinuous boundary between low-salinity coastal waters and high-salinity tropical oceanic waters. That boundary coincides rather regularly with an area where trades and SW winds alternate; photosynthesis growths rapidly in a thermoclinal layer that rises until 10 m of the surface but never outcrops. Below a poor and permanent homogeneous surface layer, chlorophyll concentrations show a distribution which is typical of divergence areas. Geostrophical and measured currents show off a transient process in horizontal and vertical movements, however the general curvature of the circulation is propitious to upwelling. Oxygen oversaturations of about 110%, suggest a moderate potential primary production which confirms slowness and alternation of movements. Also, the regular range of the various chemical and biological levels and moderate chlorophyll concentrations suggest an ecosystem where nutrients supply rapidly equilibrate phytoplankton consumption and not at all a 'phytoplankton bloom' area as that which exists in coastal upwelling. Values of Richardson's number show that instability becomes visible at the bottom of the euphotic layer. An evaluation of the vertical motion is inferred by the peculiar distribution and diurnal alternance of the winds shows that 'doming' structures may be sustained by local meteorological events.