Pour les vingt ans du C.R.O.

Founded in 1959, the Centre de Recherches Océanographiques is now twenty years old. During its existence, its administrative structure and its internal organization have considerably changed. It is now a research center placed under the supervision of the State Secretary for Scientific Research but, for management, it has been committed to the care of Orstom, a French organization specialized into scientific and technological research in the developing countries. Orstom also partially finances the research work, whose aims are decided by special commissions, which in their choices take into account the fundamental options of the plans of development of the government of Côte d'Ivoire.

Étude du crabe rouge profond Geryon quinquedens en Côte d'Ivoire. 2 - Éléments de biologie et d'écologie avec références aux résultats obtenus au Congo

Geryon quinquedens is present along the West African continental slope at depths from 300 to 1000 m, on silt-clay sediments. Geryon is a cold and rather poorly oxygenated water loving species. It is easily caught by traps as it is a scavenger and predatory crustacea. In a given area its distribution does not appear to be homogeneous: for example, densities of red crabs are higher in the eastern and western region of Côte d'Ivoire than in the central zone. Similar observations can be made off Congo, Angola and United States. It can be assumed that there is a relation between the abundance of Geryon and the productivity level of the area. Geographical variations of sex ratio are suspected to be correlated with the density distribution. Males and females have not the same bathymetric distribution: females are only common in the shallower waters (300-500 m) whereas males are present in the whole biotope. Seasonal migrations occur down and up the slope in both the sexes and are certainly related to the reproductive biology. Knowledge of the reproductive biology is also necessary to understand fishing-trap catch rate: egg maturation extends over several months and ovigerous females are exceptionally caught by traps; males also are less available during the same period (March to August) when migrations are less important; in this period, mean size increases and probably this happens at the end of a moult. From September to February the catch-rates increase. Growth is slow compared with other littoral Guinean Crustacea (Peneides). Females become sexually mature at a size of 80 mm (carapace width): modification in the allometric relations of abdomen and carapace are then conspicuous.

Résultats du groupe de travail FAO/COPACE sur l'évaluation des stocks démerseaux côtiers du secteur Côte d'Ivoire - Congo

The exploitation rate of demersal stocks in the Côte d'Ivoire-Congo area is in most cases below the level permitting maximum sustainable yield. Any increase in total catch would be achieved through an increase in catch per effort which implies bigger mesh sizes than those in use now (40-45 mm). A first step would be to fix the minimum legal mesh size to 60 mm. New, probably limited resources (crab, squids, benthic sharks) are to be sought along the continental slope.

Contribution à l'écologie de quelques taxons du zooplancton de Côte d'Ivoire. 2 - Dolioles, Salpes, Appendiculaires

The seasonal variations of vertical distribution and abundance over the shelf are investigated for Doliolids, Salps and Larvacea. The 3 groups present many similar ecological features. Two maxima of abundance occur during the little and main cool seasons. The second maximum is usually the most important, except for Salps. The 3 taxons inhabit more superficial layers in cool than in warm seasons. This allows them to follow the phytoplankton maximum which drifts near the thermocline during the warm season. Pelagic Tunicates come back to the phytoplankton enrichment areas by the deeper Ivorian under-current. A relationship between the vertical distribution pattern and the life cycle (sexual or asexual generation) is suggested.

Contribution à l'écologie de quelques taxons du zooplancton de Côte d'Ivoire. 3 - Larves de décapodes et lucifer

The warm season is the abundance period of the planktonic larval stages of Decapod Crustacea and of Lucifer faxonii in Ivoirian waters. Two or three maxima occur each year during the enrichments interrupting the warm and oligotropic season: February (small upwellings), June - some years - (first rains) and September - November (flood of rivers, end of cold season). Vertical distribution follows seasonal variations and varies little among the taxons. In a general way, Decapod larvae and Lucifer inhabit superficial layers in cold season and sink down during the warm season. It allows them to follow the maximum of primary production. Lucifer faxonii is breeding almost the year long. Breeds succede at rate of 3,7 weeks approximately.

Contribution à l'écologie de quelques taxons du zooplancton de Côte d'Ivoire. 4 - Euphausiaces

Euphausiids vertical distribution on the Côte d'Ivoire continental shelf during different seasons, average zonal distrubution, mean distance to the shore, and seasonal variations in abundance computed for five years, suggest that the appearance, and the shifting of euphausiids across and along the shelf are related with the variations of the distance from the shore of the ivorian under-current. The author reports that Nyctiphanes capensis has not been found in the region.

Étude du crabe rouge profond Geryon quinquedens en Côte d'Ivoire. 1 - Prospection le long du talus continental: résultats des pêches

From Aug 1975 to Jan 1976 a fishing survey for deep-sea red crab Geryon quinquedens was executed along the Ivorian coast between 3{degree} and 7{degree}30'W. Three regions could be distinguished: a central one with poor catches (1.6 kg/trap/20h); an east region with the best catches (5 kg/trap/20h); and a west region, also with good catches (4.5 kg/trap/20h). For the whole survey, catches only took place between 300 and 700m, the best at 400 m. A distinct sexual segregation was observed according to bathymetry with the percentage of males increasing with depth. Nearly the same abundance was observed in Côte d'Ivoire, in Congo and North Angola, while in South Angola catches are somewhat higher.

Excrétion organique du phytoplancton et activité bacterienne hétérotrophe dans le Dôme de Guinée (Océan Atlantique Tropical Est)

Primary production rates in the Gulf the Guinea (east tropical Atlantic) were measured with in situ incubations. The extracellular organic carbon passed through 0.8 u pore size membrane filters averaged 29.5% of the carbon fixed by photosynthesis. These is a good linear correlation (r=0.86) between assimilation and organic excretion. The significance and limits of the method for the measurements of low rates of photosynthesis is discussed. The influence of ecological factors (nutrients, light and stability) cannot be demonstrated. The importance of bacterial regeneration processes by reutilization of algal extracellular products is assessed.

Notes sur l'écologie de quelques taxons du zooplancton de Côte d'Ivoire. 1 - Ostracodes, Cladocères et Cirripèdes

Seasonal variations of abundance and vertical distribution over the shelf are investigated for Ostracoda, Cladocera and Cirripede larvae. The main characteristics of the environment are the periodical enrichments mainly caused by upwellings, secondly by the river floods. Ostracoda abundance variations approximately follow phytoplankton outburst. Breeding occurs all over the year. Their vertical distribution is correlated with a discontinuity layer. Diurnal migration, when it occurs in warm season consists in an upward movement during the night towards surface layers. The Ostracoda inhabit bottom layers during the day and migrate at night in intermediate and surface layers. For the main two species of Cladocera, Penilia avirostris and Evadne tergestina, abundance periods follow upwellings, especially during the main cool season. However, Cladocera can grow in low salinity but rich waters. On average Penilia inhabits more superficial waters in cold than in warm seasons. Cirripede nauplii and cypris are more abundant off rocky coasts. Their maxima are in the upwelling periods.

La circulation superficielle dans la partie occidentale du Golfe de Guinée

Direct current measurements carried out for 10 years in the Gulf of Guinea clearly show the importance of transverse circulation. The general pattern of currents is characterized by an anticyclonic gyre. The seasonal variations of this circulation are calculated through the study of the 3-monthly averaged geostrophic currents. The biological consequences of such a circulation are outlined.

Note sur les biomasses pélagiques évaluées par écho-intégration dans la zone équatoriale du Golfe de Guinée: premiers résultats

An Echo-Integration survey on the equator made during he GARP Atlantic Tropical Experiment indicated an important biomass in the layer 0/100m. The changes of that biomass have been connected to hydrological conditions. Measurements of target strength have been made, and the biggest schools have been measured.

Méthodes des mesures des biovolumes, poids secs, poids secs sans cendres et des éléments C, N, P du mésozooplancton utilisées au C.R.O. d'Abidjan

Methods for the estimation of zooplankton biomasses, used in the Oceanographic Research Center of Abidjan are presented. They deal with settled and displacement volumes, dry weight and ash-free dry weight, elementary carbon, nitrogen and phosphorus composition. The dry weight method is detailed: elimination of salt by a fresh water stream, preservation of dry samples at -20 degrees Celsius, rehydration during weighing. A few comments on the 'CHN' analysed values are made: at 1,100 degrees Celsius, most of the carbon is organic, only 10% of the mineral fraction being analysed.

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.

Équation de production: adaptation du modèle de Ricker à un stock de poissons exploité par différents engins

The authors have developed the method used by Pianet and Le Hir (Doc.Sci.Cent. ORSTOM Pointe-Noire, 17, 1971) for the study of albacore (Thunnus albacares) in the Pointe-Noire region. The method is based on the fact that the ratio between unit of effort and number of fish for two fishing gears is equal to the ratio of their catchability coefficients.

Rélations hydrométéorologiques sur les côtes du Golfe de Guinée: influence de l'orientation de la côte

The Gulf of Guinea is remarkable for its abundant precipitations and negative anomalies of the surface water temperature. This originality comes from the southern monsoon which transfers the cold season to the middle of northern summer up to latitude 10 degrees North. Yearly precipitations, which can vary along the coast, are well in correlation with coast crossing air flow (r=0.71) and with the sea-air temperature difference (r=0.72). Precipitations provide a better correlation with surface temperatures (0.72) than with salinities (-0.63). The wind influence upon negative anomaly of the surface temperature is more clear on N-S coast (r=0.98) than on W-E coast (r=0.73) of the Gulf. Temporal correlations calculated on 16 years of observations in Pointe-Noire are in connection with previous spatial correlations. Coastal hydroclimates are thus likely to be deduced from meteorology.