Longhurst (1965) has demonstrated the uniformity of demersal populations of fish and crustaceans along the coast of tropical West Africa. He has also confirmed the partial observations made, especially, by Postel (1955) in Guinea, Salzen (1957) in Ghana, Longhurst (1958) in Gambia and Sierra Leone, Crosnier (1964) in Cameroon, Crosnier and Berrit (1966) in Dahomey and Togo. In his preliminary summary, Longhurst points out the influence of depth and type of substrata together with the effect of the thermocline, which is very clear cut in the Gulf of Guinea. The depth of the low point of the thermocline varies seasonally and locally between about 50 m and the surface. He pinpoints the following communities:
- a croaker community living over soft sediment from the coast out to the depth of the base of the thermocline. A variation of this community is characteristic of estuaries and other less salty parts,Once the findings of the Gulf of Guinea trawling survey were analyzed (Williams, 1968), Longhurst was able to break down the various community associations more quantitatively and to modify the pattern then known in the following ways;- a community of sparids living over all kinds of bottoms of the continental shelf below the thermocline. A variation of this community is found over hard substrata in the waters of the thermocline,
- a community of the continental slope and
- a group of eurybathic species.
(a) The sparid subcommunity over hard bottoms in the waters of the thermocline is actually made up of a mixed snapper community (lutjanids) corresponding to residual coralline fauna and elements of the intrathermoclinal community.Table 1 lists the main species making up each of these communities and which were retained once this second study had been concluded (from Longhurst, 1969). The environmental conditions under which these groups live are found in Table 2 (from Williams, 1968).(b) The term "continental shelf community" in fact covers two distinct groups:
- a community living over the deepest part and on the edge of the continental shelf and
- a community of the continental slope.
To get some order of the magnitude of the area occupied by the most important communities (croakers, sea breams), the Working Group included in its report (Table 3) the estimations by country, sector and depth ranges of areas of the different bottom types. These are classified schematically into soft (muddy to sandy-mud), hard and sandy, and rocky (not trawlable). Domain presented this section.
