The sensitivity of coastal pelagic species to environmental conditions is now well demonstrated, especially for the clupeids: the reproduction, survival of larvae, the condition, abundance and availability of these species depend on certain conditions, the most important of which are the upwellings which determine the food availability of the ecosystem. The wind, by its action on the stability and the dynamics of the surface layer, river flow and its role on desalination of the coastal region and currents as a factor of transport of eggs or larvae, are the parameters which interact and condition the recruitment and abundance of coastal pelagic fish.
Mr Pezennec summarized the results from the examination of satellite imagery obtained by the METEOSAT satellite, received and processed by UTIS, Dakar-Thiaroye. The cooling (upwelling) which affects coastal waters of the northern zone of the Gulf of Guinea has been observed through the thermal infra-red medium of METEOSAT on a daily and five-day basis covering a period of six years, from July 1985 to July 1990. The main results of this study are:
the quality of the observations in the upwelling period is good but varies depending on the season: in the minor cold season (January-March), 20% of the five-day syntheses are affected by cloud cover while in the major cold season (August-September) this percentage is close to 40. In addition, and especially during the major cold season, the visibility of the surface waters (ignoring cloud cover) appears less positively linked with the intensity of the upwelling.
the cooling observed (Figure 3) along the Ghana-Côte d'Ivoire coastline corresponds well, spatially and temporally, to the conclusions drawn from examination of coastal temperatures. The “onset” and “maturation” phases of the upwelling could be followed, but those corresponding to their “disappearance” are very diffuse.
the principal contribution of this type of studies lies, in the possibility of having access to information concerning: (i) the offshore extension of cooling; (ii) the position of the horizontal thermal gradients (fronts); and (iii) the mesoscale structure of the upwelling.
in addition to the Ghana-Ivorian upwelling, effective coolings were also observed each year from January to March along the coast of Sierra Leone and Liberia; they coincide with and are physically linked with those that affect the western part of Côte d'Ivoire during this period.
The conclusions drawn from these observations are positive and can only urge a further utilization of this type of information in the study of the general dynamics of the pelagic ecosystem of the area.
The results below were drawn from the work of Arfi et al (1991). The upwelling indices for 12 stations on the Ghana-Ivorian coastline were available (Figure 1) on a temporal basis, the minor scale being fortnightly. The available indices were for Ghana since 1970 (except Tema, 1963) and for Côte d'Ivoire since 1978. The main conclusions drawn from the examination of these beach temperatures are:
the existence of one minor cold season, strongly marked in Côte d'Ivoire, especially in the west and less noticeable in Ghana.
a pronounced major cold season on the whole Ghanaian-Ivorian shelf (except in front of the western part of Cape Three Points)
the importance of morphological factors, notably cape effect, which, in addition to the relative position of the coast in relation to the dominant winds, make the two regions situated on the east of Cape Palmas and Cape Three Points the two principal sites for the development of the upwelling.
Figure 4 represents the annual upwelling indices and for each of the two cold seasons at Tema from 1963 to 1989.
A symbolic representation of the intensity of the coastal cooling is shown on a trial basis; it utilizes the global upwelling indices (Annex 4) by station and by year or for each of the cold seasons, represented schematically with the aid of two simple criteria:
one of these representations (Zone/Zone mode or station-by-station) treats each station separately: it puts each point (index) in five-point intervals defined with the aid of the mean and the standard deviation concerned.
the other representation (Max-Zone mode) considers all the stations and only the largest mean of the twelve stations is chosen.
The first representation is favourable for inter-annual analysis by station, whereas the second is preferred for a comparative approach as it evidences the station with the greatest cooling.
The principal results drawn from the observation of the grids (Figures 5a–5c) are in agreement and complete the description in the previous paragraph:
The sites at the level of which the cooling is most intense appear distinctly. Thus, for the minor cold season, west Côte d'Ivoire (Tabou, San Pedro) was clearly indicated while for the major cold season Takoradi appears as the major site.
Examining the station by station representation, the inter-annual evolution of each station and eventual similarities between stations can be traced. Thus for the major cold season, the years 1967 (Tema), 1971–72, 1975–76, 1982 and 1986 are the most intense on the entire coastline of Ghana. In Côte d'Ivoire, 1978, 1980–82 were years of intense upwelling, but irregularly along the coast.
The weak upwelling of 1986–89 in Côte d'Ivoire and 1987–89 in Ghana is also noted. For the minor cold season, the 1985–87 and especially the 1990 sequence can be distinguished because the minor upwelling was intense for the whole coastline. Years 1988 and 1989 appear, on the contrary, uniformly weak.
Several approximations were made to assess the total discharge of freshwater into the sea. The results below represent fair approximations of the amount of freshwater entering the sea from the main rivers of Côte d'Ivoire and Ghana:
| COTE D'IVOIRE | GHANA | ||||
|---|---|---|---|---|---|
| Rivers | Cavally | Sassandra | Bandama | Comoe | Volta |
| Periods | 66–88 | 66–88 | 66–88 | 66–88 | 66–80 |
| Mean annual module (m3/s) | 489 | 432 | 204 | 156 | 740 |
| Mean annual volume (m3) | 15.109 | 13.109 | 6.109 | 5.109 | 23.109 |
| Total volume (m3) | 39.109 | 23.109 | |||
The annual mean volume of freshwater carried to the sea is of the order of 39 x 109 m3 for the four principal rivers in Côte d'Ivoire and 23 x 109 mm3 in Ghana for the Volta River alone (data on other Ghanaian rivers were not available).
Figure 6 (see also Table 10) represents the variation of total annual mean flow of the four principal rivers of Côte d'Ivoire. The maximum flow was observed in 1968 and the minimum in 1983.
Between 1967 and 1972, a considerable fall in river flow affecting the hydrographic network was observed in Côte d'Ivoire due to the construction of the first hydro-electric dam. Since 1972, the flow has varied around a mean of 1 200 m3/s without any apparent trend.
Data concerning coastal currents were too difficult to obtain and till now are not being collected.
The existence of a numerical model for the inter-tropical Atlantic (OPERA model) under study at the “Laboratoire d'Océanographie Dynamique de l'Université Paris 6 (LODYC)” was mentioned. This model simulates from wind data, vectors of the surface and bottom currents for the whole of the tropical Atlantic. One coastal sub-model concerning the western Gulf of Guinea could be put in place by ORSTOM in the years 1992–93.
A possible input of remote sensing into the quantitative analysis of the dynamics of surface currents (drift of upwelled water) should be considered.
Recent work by (Roy, (1990)) shows, among other results, the analysis of wind data collected by merchant ships off Côte d'Ivoire. This work indicated a progressive intensification of the wind force during the past ten years which could be the beginning of the thermocline coming closer to the surface of the ocean thus increasing the productive capacities of the environment.
In Ghana no such data had been collected, and efforts should be made to identify the probable sources of these data for collection later.
