An assessment of the demersal fisheries potential of the Cameroonian and Nigerian waters is very difficult on the basis of the data available. On the one hand, in Cameroon catches of the artisanal fisheries are not monitored, and on the other hand, unknown percentages of catches landed in Douala by the industrial demersal and shrimp fleet were taken in Nigerian waters. The Working Group nevertheless applied a global production model to the total demersal catches made by Cameroonian fishing and shrimping vessels combined, using the total standardised effort data expressed in number of fishing days of Njock (in press). The validity of this model is of course limited by the fact that differences in fishing grounds (Nigeria-Cameroon) have occurred as well as differences in fishing strategy (fleet moving inshore, shrimpers being converted into trawlers-shrimpers etc.)
This model illustrated in Figure 5 and Table 23 indicates that the optimum yield is about 19 000 tons of fish and shrimp, the corresponding effort being about 5 400 standardised (400 HP) fishing days. It appears from this crude model that the stock were already fully fished from the early 1970's, as most points observed are to the right of the optimum yield and effort. A drastic decline in overall catches occurred in 1983 when Cameroonian vessels left the Nigerian waters. The position of the 1983 and 1984 points of the graph, would indicate a state of systematic overfishing of the total demersal resource of Cameroon which is corroborated by the very inshore trawling of the industrial fleets which operate even in estuaries and within the 2 mile boundary reserved for artisanal fishermen. Mesh sizes used are very small and of the order of 32–36 (shrimpers) and 36–41 (trawlers) mm. Although no systematic length sampling of the catch was done prior to June 1983, it is said that the average size of the main species in the catches has declined.
Because of the differences of surface areas of the muddy biotopes of Penaeus notialis which in Nigeria are 6.8 times greater than those of Cameroon (Burukowski and Bulanenkov, 1969), the pink shrimp stocks in Nigerian waters are much more important than in Cameroon. In the period 1970–1982 Cameroonian shrimpers have operated in the eastern part of the Nigerian waters where the shrimp is most abundant. In 1980 a change in strategy occurred when a number of old Cameroonian shrimpers were replaced by new trawler-shrimpers (Njock, in press). Standardised effort data (from the CRECAM Company, Njock in press, Table 24) are available for shrimpers since 1970. The Working Group applied general production models for the combined Nigerian-Cameroonian shrimp stocks, under the assumption that the declining percentages of shrimp in the catch of the shrimp trawlers can be fully explained by a decrease in discarded fish. Table 25 illustrates these annual percentages. Supposing that the (standardised) effort of the shrimpers was wholly directed towards the catch of the pink shrimp, which would constitute 'a single stock in Nigeria-Cameroon, total shrimp catches and standardised directed effort data can be used for the period 1970–1979, before the above mentioned change in strategy occurred. Figure 6 and Table 24 illustrate this model. Although the basic data used differ from those available for this area to the Working Group on the assessment of shrimp stocks off Ivory Coast-Congo, which used unstandardised effort data, the average maximum potential catch calculated for the period 1971–1976 and the result of the present analysis are very close : 3 370 t. (FAO, 1979a) against 3 600 t. (Schaefer) and 3 500 t. (Fox) for the present model. Linear correlation coefficients are of the order of 0.69 for both models. Catch levels since 1980 show an increase from 2 000 tons to 3 100 tons in 1984, with a high value of 4 000 tons in 1982. Unstandardised Nigerian effort data (Adetayo, 1982) were also tried in a Schaefer model (Table 24), but, although leading to similar MSY (3 300 tons), showed a very low linear correlation coefficient (r = 0.14).
Coastal lagoons and brackishwater coastal systems
The productivity of the Nigerian estuaries and creeks is still unknown, but, on the basis of comparative productivity of African floodplain systems, the coastal lagoons and the “lagoon-like” estuaries, which are bar-built, should have a productivity range of 50-60 kg/ha or 5-6 t/kg2 (Welcomme, personnal communication). But estuaries with more nutrients derived from the hinterland would have typical productivity values of 100–200 kg/ha or 10–20 t/km2 (Kapetsky, personnal communication). The following potential yield estimates for only finfish and crustaceans for estuaries, lagoons and creeks are given by Ssentongo, Ajayi and Ukpe (1983) :
Lagos and Ogun States lagoons (about 500 km2) - estimated yield potential is between 5 000– 10 000 t, ;
Bendel State, Benin, Escravos and Forcados Rivers (area about 300 km2) - estimated average yield potential is between 3 000–6 000 t. ;
River State, estuaries and lower river sectors between Ramos River and Imo River (about 1 000 km2) - estimated average yield potential is between 10 000–20 000t. ;
Cross River State, Kwa Ibo River and Cross River (about 600 km2) - estimated average yield potential is between 6 000–12 000 t.
Hence the estimated average yield potential for the entire national brackishwater sector is 24 000 to 48 000 t.
Coastal inshore sector (depth 0–18 m)
The very inshore waters adjoining the estuaries and creeks are likely to have productivity values similar to those of lagoons, creeks and estuaries. On the basis of this, yield potential for several coastal sectors were made by Ssentongo, Ajayi and Ukpe (1983) ;
Cross River State coastal area (0–18 m) is about 2 800 km2; the estimated yield potential is of the order of 2 800–5 600 t. ;
River State coastal area (0–18 m) is about 5 200 km2, the estimated yield potential is of the order of 5 200–10 400 t.;
Bendel State coastal area (0-18 m) is about 1 200 km2, the estimated yield potential is of the order of 1 200-2 400 t. ;
Ondo, Ogun and Lagos States coastal area (0-18m) is about 2 520 km2, the estimated yield potential is of the order of 2 500-5 000 t.
The total potential for the sector 0-18 m depth is 11 700- 23 400 t.
Coastal sectors beyond 18 m depth
In the Gulf of Guinea, an intensive survey of the demersal resources was carried out by the Guinean Trawling Survey (Williams, 1968). A more systematic survey was made off Nigeria coast by Longhurst (1965).
Longhurst (1965) used the catch rates and estimated the potential of the trawl catches in deeper waters (more than 18-20 m) to be about 15 000 t.
In 1976 a survey was performed by the FAO/USSR vessel FIOLENT The commercial demersal fish stock for the surveyed area was estimated. at 28 600 tons (Robertson, 1977; see Table 26 and Figure 7).
On basis of comparative figures from similar neighbouring and higly productive coastal systems lead to an estimated total maximum potential yield of 100 000 t. for the inshore artisanal fisheries of Nigeria. Table 27 summarizes the trawling surveys carried out in Nigerian waters (Van der Knaap, 1985).
FAO (1979b) cites the main results of the assessments made by Longhurst (1965) and Williams (1968). These yielded respectively estimates for Cameroon of 4 900 tons of demersal potential and 9 000-12 000 tons. The potential for Equatorial Guinea based on the GTS data was estimated at 1 500-2 000 t. for the continental part and 500 t. for Malabo Island (SCET, 1980b). The DR. FRIDTJOF NANSEN survey made at a depth of 50-200 m, in August-September 1981, estimated the pelagic biomass at 120 000 tons and the demersal biomass at 6 000 tons for the area of Cameroon and Malabo Island (formerly Fernando Poo) combined (Strømme et al, 1983). Table 28 summarizes the trawling surveys carried out in Cameroonian waters (Van der Knaap, 1985).
The FAO/USSR research vessel FIOLENT carried out a comprehensive survey in the Central and Southern Gulf of Guinea (Robertson; 1977). Two cruises were made, one in the cold season (May) and one in the warm season (September). Demersal fish densities are presented in Figures 8 and 9. Yields of several species and species groups are given in Table 29. The commercial demersal fish stock for the surveyed area was estimate at 1 700 tons in May and at 3 600 tons in September (Tables 30 and 31).
In October-November 1979, the Spanish research vessel PINZO PRIMERO surveyed the shelf and slope areas of both insular and continental Equatorial Guinea. 52 stations were fished, of which most were deeper than 100 meters (Ariz, 1979). Trawlable grounds were only found north and northwest of Malabo Island. Off continental Guinea, the area shallower than 50 m is difficult to fish because of the strong currents, and deeper than 50 m grounds are of irregular nature and successful fishing requires a detailed geographical knowledge. Catches of crustaceans for which the optimum fishing season is May-September were low but of good quality. Cephalopod catches were equally low and a fisheries potential was thought to be non-existent. Fish catches were dominated by 8 species of sparids. This preliminary survey indicates that this resource may be of local interest, but could hardly support an intensive industrial fishery.
In August 1970, the Soviet research vessel VYKHMA made a trawling survey in the waters of continental Equatorial Guinea (ex Rio Muni). A total of 38 trawl stations were fished along 7 transacts perpendicular to the isobaths in the depth range of 20-70 m, while several catches were made at 150–250 m depth (PTITSYN and SIGAEV, 1972). This survey distinguished three zones, of which the central one (1°27'-1°44'N) is muddy and dominated by Galeoides, Brachydeuterus, Gerres and Ilisha, whereas the northern (1°44' -2°17'N) and southern (1°07' - 1°27'N) zones are rocky, dominated by Lutjanus, Sparidae, Ephippidae and Carangidae. The central area is trawlable and yielded commercially interesting catches of Penaeus notialis (15–30 kg/hr) off Benito River mouth, whereas the northern and southern areas would be fishable with longlines, young cephalopods were constantly present in the catches on the shelf, but the adults appeared to live over greater depth. Survey data were not used to assess the demersal fisheries potential.
