Ms A. Ramos (Spain) introduced the report of the GUINEA-90 Survey carried out by the M/V LAGOAPESCA during April 1990. The areas surveyed were the continental shelves and slope of Sierra Leone, Liberia, Côte d'Ivoire and Ghana between 09°10'N 15°20W and 6°8'N 1°12'E. The scientific team comprised of scientists from Sierra Leone, Liberia, Côte d'Ivoire, Ghana and Spain.
During a preparatory meeting of the cruise, held in Abidjan in March 1990, the main objectives agreed upon relevant to stock assessment, fish biology and distribution of the main demersal stocks were as follows:
Estimation of the catch rates yields of main demersal commercial fish species and associated fauna.
Geographical and bathymetric distribution of the main species.
Size distributions of the fish and crustaceans populations of commercially important species.
Contribution to the knowledge on the biology of the main commercial fish species: sex ratio, sexual maturity, reproduction, growth, etc.
Collection of biological samples of commercial fish species (otoliths, stomach contents, etc.).
The stratification of the area was also discussed during the preparatory meeting taking into account the area to be surveyed and time availability of the vessel.
Two types (1 + 2) of trawl nets were used during the cruise: type 2 was utilized off Côte d'Ivoire, while type 1 was the net used off Sierra Leone, Liberia and Ghana. Type 1 had bobbins. Vertical and horizontal openings were 4.79 and 17.16 m respectively for type 1, and 2.8 m by 18.5 m for type 2 (Ramos, et al. 1990).
This is summarized in the following table:
| SECTOR | ZONE | SURFACE AREA (km2) | NUMBER OF TRANSECTS | DISTANCES APART (n.m) |
|---|---|---|---|---|
| Sector 1 | S. Leone West | 14 405 | 4 | 22 |
| S. Leone East | 5 777 | 3 | 33 | |
| Liberia | 22 897 | 9 | 33 | |
| Sector 2 | Côte d'Ivoire | 10 524 | 5 | 55 |
| Ghana | 18 726 | 6 | 50 |
Depth intervals in the Sierra Leone-Liberia sector were: 20–50, 50–100, 100–200, 200–400, 400–700 m. Off Côte d'Ivoire and Ghana: 20–50, 50–100 and 100 m or more (i.e., >100 m). The duration of each haul was 30 minutes.
Those trawl stations with durations of less than 20 minutes or with observed anomalies were not taken into consideration. Target species were defined during the preparatory meeting. They included 12 fish species or groups of species and the shrimp Penaeus notialis. A second group of target species included other crustaceans, cephalopods, hakes and 15 more fish species that were added to the original list. In the case of by catches, samples were taken, to determine its composition, by weight and number of all species included benthomic invertebrates, size composition (some seabreams, cephalopods and crustaceans) and biological parameters (seabreams, crustaceans plus four fish species). Length/weight relationships were determined for 5 seabreams, deep-sea shrimps and cephalopods. Yields of all species were calculated for each stratum and area.
In order to determine possible differences between the two gears, standardization exercises were carried out in the same area and station. Results of these exercises showed that gear type 2 fished on the average 3.12 times more than gear type 1. This however changed with depth; being 1.36 between 20 to 50 m and 4.33 at depths of more than 100 m. In grouping the results, the communities of Lutjanidae, Sciaenidae, Sparidae, deeper shelf and slope were considered, in addition Chondrictii, Cephalopods, Crustacean, Pelagics and other species.
The total number of hauls was 173. The high proportion of incomplete hauls off the western part of Sierra Leone (36%) should be noted. This was due to the nature of the sea bottom especially in the deeper part of the continental shelf and the slope.
Species diversity observed was high: 294 species were recorded of which 237 were finfishes, 45 crustaceans and 12 cephalopods. Highest diversity was shown by the Sparidae, Scorpaenidae, Carangidae, Macrouridae, Serranidae, Sciaenidae and Soleidae families.
Total catch of the cruise was nearly 21 t, 50% of which was taken off Côte d'Ivoire. Average catches by sector were higher in the eastern zone (Côte d'Ivoire and Ghana, 139 kg/haul) followed by those of Liberia (105 kg/haul). The poorest regions found were the two zones of Sierra Leone (59 and 68 kg/haul). On a stratum basis, highest catch rates were obtained in Côte d'Ivoire and Ghana below 100 m deep (201 kg/haul), and off Liberia between 50 and 100 m (162 kg/haul).
Fish communities highly represented were the seabreams at depths of between 50–200 m and the community of the slope in deeper waters. Seabreams represented 35–80% of the total catch of this strata in the area. The seabreams were mainly Pagellus bellottii bellottii and Dentex angolensis between 20–50 m and 50–100 m respectively off Côte d'Ivoire and Dentex congoensis off Sierra Leone. Brachydeuterus auritus of the sciaenid community yielded 78 kg/30 min. trawling in the first strata of Côte d'Ivoire. This community was mostly caught in shallow waters. Lutjanids were found mainly between 20 and 50 m in the eastern zone of Sierra Leone (23.6%) and in Ghana (16.9%).
Cephalopods were mainly caught off Côte d'Ivoire and Ghana. Cuttlefishes occurred especially between 20 and 100 m deep where the yields were 18.8 and 13.9 kg/30 mn. trawling respectively.
Sizes of Dentex angolensis ranged from 4 to 29 cm. Its demographic structure was very similar for the different countries showing an increment of average sizes with depth. This was especially clear off Liberia where average sizes increased from about 7 cm in waters 20–50 m deep, to 25 cm in the 200–400 m strata. Average sizes of females were smaller at all depths, and their abundance in weight was up to 8 times higher than that of males in some strata. Pagellus bellottii bellottii did not show such a clear pattern as D. angolensis. Modal values of size either increased or decreased with depth in the different zones, without any clear trend. Specimens caught and sampled were between 7 and 25 cm. Size distributions of males and females were similar.
Brachydeuterus auritus had a larger average size off Côte d'Ivoire (modal length 13.2 cm) than in the other countries where these sizes were 9.6 and 11.8 cm.
Size distributions of Sepia officinalis were very similar in Ghana and Côte d'Ivoire. This might suggest the existence of a single stock in the EEZs of the two countries. Sizes ranged between 9 and 26 cm mantle length, showing a clear mode at 19 cm. Females were more abundant than males normally.
During the discussions that followed the presentation, several questions regarding methodology and results were raised. Participants questioned why sampling was done only during the day. It was explained that in view of time constraints, survey operations were carried out during the day to enable the vessel to move between transects at night. This was also considered appropriate to avoid the effect of daily vertical migrations of some fish species on the yields.
Participants also questioned the use of different gears in Côte d'Ivoire, on one hand, and Sierra Leone, Liberia and Ghana on the other. Some recalled the view expressed by scientists at the last Working Group (FAO, 1990) that the same gear must be used for such trawling surveys so that productivity figures from the different EEZs will be comparable. Unfortunately time constraints and the nature of the bottom necessitated the change of gears and consequently the desired objective could not be achieved. The Working Group therefore recommended that future international trawling surveys in the area should use the same gear throughout the whole area.
Regarding the failure to collect environmental data, the Working Group was informed that space, manpower and time limitations as well as vessel facilities did not make it possible to record this information. Some participants underscored the importance of environmental factors in fisheries work. It was suggested that the CECAF Secretariat could obtain some environmental data from satellites for the period during which the cruise took place. It was pointed out that this would provide information about the surface only. In conclusion, the Working Group recommended that this type of information should be recorded in future cruises.
Finally, concern was expressed about non-participation of scientists of the different research centres from the region in the preparation of the final report. The CECAF Secretariat informed the Working Group that the procedure had been followed and that it was one of the main tasks of the Working Group to analyze the preliminary results. The Secretariat stressed that the cruise data are available to all scientists of the region for further analyses.
