The survey was planned as a 30-days-survey covering the shelf from Togo to Congo with the shelf off Nigeria and Cameroon covered only between 50-200 m depth limit. Due to a breakdown of a generator the survey had to be shortened by over a week, making the sampling intensity less than planned.
Generally speaking the findings reveal an area with a very scattered fish distribution, both on small pelagic and demersal species. Dense plankton recordings occurred on most part of the shelf, making an accurate determination of the scant fish resources difficult. The estimates given reflect thus more an order of magnitude than absolute estimates. Both the northern and southern region were estimated to hold an average fish density of 39 tonnes/nm2 .In absolute figures the Togo - Cameroon 50-200 m depth region is estimated to hold 310,000 tonnes of fish while the Equatorial Guinea - Congo shelf region comprise 580,000 tonnes. Of this about 40% is allocated to demersal fish in the northern part and 25% in the southern. Only the sardinellas (Sardinella aurita and S. maderensis) off Gabon and Congo seem to form any stocks in terms of resources. The pelagic resources off Gabon and Congo were estimated to 350,000 tonnes of fish in the surveyed area, and about 50% of the biomass should belong to the sardinellas if one should split according to the catches. But this is a weakly based assumption as the hauls were few, the presence of the sardinellas were very concentrated and the trawl-avoidance is likely to be species dependent. Furthermore the sardinellas off Gabon - Congo belongs to stocks extending into Angola (TRODEC and GARCIA 1979) and the local abundance is thus heavily influenced by fluctuations according to the migratory pattern.
The biomass in the region have earlier been estimated on basis of results from two surveys, the Guinea Trawling Survey (GTS) in 1963-64 and the "Fiolent 1976 Eastern Central Atlantic Coastal Fishery Resource Survey". Comparing the fish densities obtained from these surveys (WILLIAMS 1979, ROBERTSON 1977) with the results from "Fridtjof Nansen", the last one stand out among the others with its distinctively higher figures (Table 5), At first glance it seems as if higher reliance should be attached the GTS and Fiolent figures, as these are fairly in accordance, but it should be borne in mind that both surveys have uncertainties in calibrating the equipment. The calculations from the GTS survey have as its main assumption that all and only all fish within the track of the trawl are assumed to be caught, and no fish above the headline of the trawl are taken into the calculations. Like-vise during the Fiolent survey the acoustic system was calibrated by means of simultaneous trawling and integration of the 6 meter bottom stratum. And again all fish within the track of the trawl are assumed to be caught. On the other hand the density figures from Fiolent include pelagic fish detected acoustically off the bottom stratum. In lack of more reliable calibration data none of the estimates points out to be more accurate than the others. It is seen from Table 5 that the ratio between the pelagic estimates in Gabon and in Congo is inverted from the GTS to the "Fridtjof Nansen" survey. This could be ascribed the seasonal migration of the sardinellas in the region.
Table 5. Estimates on fish densities in Gulf of Guinea. Tonnes per run on shelf area in four countries from three surveys.
|
|
GTS |
FIOLENT |
FIOLENT |
DR. FRIDTJOF NANSEN |
|
|
bottom trawl |
acoustic/b. trawl |
bottom trawl |
acoustic |
|
Nigeria |
9 |
9 |
6 |
|
|
Cameroon |
9 |
10 |
5 |
|
|
Gabon |
12 |
- |
9 |
45 |
|
Congo |
24 |
53 |
23 |
23 |
