Results of the 1985 surveys and objectives for the new surveys.
Under the same programme in 1985 four seasonal surveys were made of the continental shelf off Congo and Gabon with the general objective of investigating the composition, distribution and abundance of the coastal fish resources. The methods included acoustic integration technique particularly to study the small pelagic schooling fish and a bottom trawling programme to study the demersal resources using the swept area method for abundance estimates.
The main findings from these surveys can briefly be summarised as follows.
Gabon
For the small pelagic schooling fish Cape Lopez seems to represent a point of division for their distribution. No significant aggregations were located north of the Cape in any of the surveys. South of Cape Lopez surveys in March and June gave biomass estimates of small pelagic fish of 25 - 30 thousand tonnes which consisted of horse mackerel and scads offshore and bumper and ilisha in the inshore waters. Sardinellas were only found incidentally. The September survey showed, however, a marked increased biomass estimate with 160 thousand tonnes. The main part of this consisted of juvenile and immature stages (less than 20 cm total length) of horse mackerel and sardinellas, mostly round sardinella. Some anchovy also formed pan of the biomass. Finally in December the estimated biomass was about 100 thousand tonnes less than in September, but still some aggregations of small sized sardinellas and horse mackerel were found over the outer shelf.
The biological interpretation suggested by these findings is that the Gabon shelf south of Cape Lopez serves as a nursery ground for the sardinellas especially the round sardinella and the horse mackerels. The main spawning grounds from which the juveniles are recruited are located to the south probably off Congo and Cabinda and the current system transports the eggs and larvae northwards along the shelf in the early cold season.
Using the bottom trawl stations and the swept area method the total biomass of demersal resources on the Gabon shelf south of Cape Lopez was estimated at 120 thousand tonnes. About half of this represents commercial types offish of which seabreams, snappers and groupers were the most important. It is thought that the extreme inshore fauna, mainly croakers and catfish were not well represented in the survey sampling.
The objectives for the 1989 surveys for Gabon were discussed with Government authorities in a visit to Libreville on 27 January 1989. The following main points were agreed:
Since the previous surveys demonstrated that the main part (about 80 per cent) of the resources are located south of Cape Lopez the new surveys should be restricted to this southern part of the coast to allow a high survey intensity.
Design and methods should be similar to those of the previous surveys to allow verification of the results, but in the distribution of survey effort emphasis should be given to the inshore shallow areas and the types of resources on which the artisanal fishery is based.
Congo
A brief review of the findings from the four coverages in 1985 is as follows:
Small pelagic fish. A biomass estimate based on the acoustic integration technique gave the following results:
|
Survey 1, March 1985 |
28 000 |
tonnes |
|
Survey 2, June 1985 |
51 000 |
" |
|
Survey 3, September 1985 |
57 000 |
" |
|
Survey 4, December 1985 |
7 000 |
" |
Demersal fish. Only a total of 24 bottom trawl hauls are available for estimating the biomass of the demersal fish by the swept area method. The estimates of densities of all demersal species taken together for all the shelf and by depth strata were as follows (tonnes/nm2).
|
ALL SHELF |
0-30M |
30-50M |
50-100M |
100-300M |
|
11.1 |
14.8 |
14.9 |
8.5 |
8.3 |
For Congo the objectives of the 1989 surveys were discussed with the scientists participating in survey 1/89. The overall objective is a description of the composition, distribution and abundance of all the main components of the shelf resources. As in Gabon it is desirable to give emphasis to the inshore area. Important groups here are croakers and other commercial demersal fish and sardinellas. The shrimp Penaeus notialis for which both day and night testing should be made is also an important inshore species. There is a fishery for deep sea shrimp in the slope and these grounds should also be tested.
Participation
The following participated from the region:
Agnes Boulingui Ilama, Direction de Peches Maritime et Cultures Marines, Libreville, GabonThe scientific staff from IMR was:
Jean Alogho Nang, Direction General de l'Economie Forestiere, Libreville, Gabon
Felix Domba, Secretariat General a la Peche, Brazaville, Congo.
Prosper M'Fina, ORSTOM, Pointe Noire, Congo
G. Saetersdal, O. Alvheim, Diana Zaera, M. Dahl and E. Molvaer.Narrative
After calling on Owendo on 27 January to embark the participants from Gabon the work started that evening with the hydrographic profile over the shelf on the Equator. During 28 January to 3 February the Gabon shelf south of the protection zone off Cap Lopez was covered with course tracks and fishing stations as shown in Figure 1. The survey intensity was highest in the inshore area. Fishing with the bottom trawl was in many areas restricted by rough bottom. Observations on the type of bottom were recorded and will be used to update the chart of bottom conditions prepared earlier. Some trawl hauls were made in the slope at depths between 200 and 600m where bottom conditions were favourable. A hydrographical profile was worked off Pte. Panga. The vessel steamed 1100 nm in Gabonese waters during the survey and 55 trawl stations were made.
The shelf off Congo was covered from 3 to 8 February. Also here the survey intensity was highest in the inshore area, but a number of trawl stations were made also in the slope where bottom conditions are good from 200 to about 700m. A hydrographical profile was worked off Pointe Noire. The distance steamed on the Congo shelf was 640 nm and 41 trawl stations were made. The survey ended in Pointe Noire on 8 February.
