3.1 Nature of bottoms in the area between 19°N and 4°30'N
3.2 Relationship between the type of bottom and distribution of demersal fish species
For purpose of distribution of demersal fish stocks, the area from Cape Timiris to Cape Palmas has been divided into four sectors, namely:
(i) Southern Mauritania areaIn each of these sub-areas, three groups of demersal fish stocks have been identified based on the work of Longhurst (1965) and the Guinean Trawling Survey (GTS).
(ii) Northern Guinea Bissau and the Senegal/Gambia (Senegambia) area
(iii) Guinea and Southern Guinea Bissau area
(iv) Sierra Leone - Liberia area
(i) The purely coastal species inhabiting muddy or soft bottoms, e.g., rays, polynemidae, pomadasydae, sciaenidae, carangidae and cynoglossidae.(ii) The coastal species inhabiting hard bottoms, e.g., serranidae, lutjanidae, some pomadasydae, sparidae, mullidae, drepanidae, balistidae, dactylopteridae, psettodidae, soleidae and cephalopods.
(iii) The deep water communities inhabiting the deeper part of the shelf and the edge of the slope of the continental shelf, e.g., some serranidae, sparidae (e.g., Dentex macropthalmus and Dentex angolensis), Brotula barbata and triglidae.
3.1.1. Muddy bottoms.
3.1.2. Hard sandy bottoms
3.1.3. Rocky banks
3.1.4. Sandy-mud bottom at the edge of the continental shelf
In order to understand the geographical distribution of demersal fish stocks, it is useful to briefly describe the nature of bottoms found in the area.
Generally four principal types of bottom are found on the continental shelf of West Africa (Figures 1 and 2).
In the area between Mauritania and Liberia, four zones of muddy sea beds are identified:
(i) Cape Timiris: immediately to the south of Cape Timiris is muddy bottom between 20 and 50 m depth.In areas of heavy outflow of water, especially around river estuaries, the bottom generally consists of fine sand which becomes muddier from the coast. This forms a transition between sand and mud.(ii) St. Louis area: there is extensive muddy bottom off the estuary of the Senegal River extending from 15°10'N to 16°20'N and between the depth of 20 and 60 m.
(iii) Cape Roxo area: there is also an extensive muddy bottom off Guinea Bissau from 11°30'N to 12°20'N between 15 and 100 m depth.
(iv) Cape St. Ann area: this muddy bottom is situated in the same latitude as the Sherbro Islands and it is not extensive. Prom Cape St. Ann to the Ivory Coast there exists series of narrow thick layers of mud. Not much information is however available for this area.
This type of bottom is found alternating with the muddy areas. The size of the sand particles varies greatly. Sparidae are normally found in these areas. A number of outcrops of scattered rocks corresponding to the old shore line are found on this type of bottom. These areas are generally not trawlable. Fish species in these areas are exploited normally by the artisanal fishermen notably in the Senegal-Mauritania sector.
The rocky areas are not trawlable and fish fauna in these areas are exploited exclusively by artisanal fishermen.
This is a relatively homogenous band consisting of coarse sand and mud. The coarse sand is of madreporic origin. Between depths of 120 and 200 m are discontinuous formations of madrepores which make trawling in this area difficult.
The distribution of demersal fish species is related to the type of bottom found in the area. Generally soft or muddy bottoms are associated with the fish species or groups of species given in Appendix 3. Polynemidae, pomadasydae, sciaenidae, cynoglossidae, ariidae and rajidae are families commonly associated with this type of bottom.
Fish families such as Sparidae, Domadasydae, Drepanidae, Latjanidae and Serranidae are usually associated with hard bottoms (Appendix 4). Pseudotolithus typus, Pseudotolithus brachygnathus, Pseudotolithus (Fonticulus) elongatua and fishes in the families mugilidae, elopidae are found in estuarine areas with heavy outflow of water. This type of fauna is particularly important in areas such as Guinea, Guinea Bissau and Sierra Leone.
