REPORT ON THE R/V DR. FRIDTJOF NANSEN FISH RESOURCE SURVEYS OFF WEST AFRICA: MOROCCO TO GHANA, AND CAPE VERDE May 1981-March 1982
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REPORT ON THE R/V DR. FRIDTJOF NANSEN FISH RESOURCE SURVEYS OFF WEST AFRICA: MOROCCO TO GHANA, AND CAPE VERDE May 1981-March 1982 |
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FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
UNITED NATIONS DEVELOPMENT PROGRAMME
Rome, 1984
© FAO
A joint NORAD FAO/UNDP project
Reports on Surveys with the R/V Dr Fridtjof Nansen
2 A SURVEY OF THE FISH RESOURCES IN THE
COASTAL WATERS OF THE REPUBLIC OF CAPE VERDE
November 1981
Institute of Marine Research, Bergen
NOTE
The fishery research vessel Dr. Fridtjof Nansen belongs to the Norwegian Agency for International Development (NORAD) and was designed and built for scientific and exploratory investigations of fishery resources of developing countries. Its projects are conducted under a joint plan with the Fisheries Department of FAO based on a funding operation shared by UNDP/FAO and Norway.
The Institute of Marine Research, Bergen, is responsible for the operation of the vessel and the scientific programme under a sub-contract with NORAD and FAO.
The Norwegian research vessel Dr. Fridtjof Nansen has, under a joint NORAD/UNDP/FAO programme (GLO/79/011), carried out acoustic surveys off West Africa in the period May 1981 to May 1982. The work included in this report comprises three surveys of the shelf from Cape Blanc to Cape Verga with two of these extended down to Sherbro Island, two coverages of the shelf off the western Sahara and single coverages in Morocco north to Agadir and in the region Liberia-Ghana. A single survey in the Republic of Cape Verde is also included in the report as a separate part.
The work included hydrographical sections and trawling for biomass identification purposes, except off western Sahara where only acoustical transects were carried out.
The two coverages from Cape Blanc to Sherbro Island gave estimates of total biomass of 3 (May-June 1981) and 3.4 (Feb.- Mar. 1982) million tonnes, raised to 4.5 and 5.5 million tonnes to roughly adjust the figures for the un-surveyed shallow parts of the continental shelf. The two coverages off western Sahara gave estimates of 0.4 (December 1981) and 1.4 (March 1982) million tonnes, raised to 0.6 and 2.1 million tonnes for the whole shelf. A total abundance of 7.5 million tonnes for the whole shelf from Cape Juby to Sherbro Island is suggested. The survey in Morocco from Cape Juby to Agadir gave 750 000 tonnes as a rough total estimate, and the single coverage Liberia-Ghana gave 800 000 tonnes.
In the report the total biomass estimates are given according to national EEZs and to pelagic and demersal components. By combining trawl data with the acoustic estimates, tentative assessments of the abundance of the main pelagic species within the surveyed area are given. Assessments of the density of the individual stocks were hampered by a considerable migration on and off the continental shelf of western Sahara where no species identification by trawling was made. One exception is the triggerfish off Guinea/Guinea-Bissau where the whole stock seemed to be encompassed by the survey track. This stock was estimated to be 1.1 (May-June 1981) and 1.4 million tonnes (February 1982).
From the survey in the waters of the Republic of Cape Verde a total biomass of 100 000 tonnes was estimated, of which 65 000 tonnes were classified as small pelagic fish.
The main report also contains all catch data and length measurements arranged by species.
The reports presented here form part of a large amount of data and documentation produced each year on the fish resources off West Africa. The results of the surveys with the Dr. Fridtjof Nansen are considered valid for the areas covered and with the limitations imposed by species determination through test fishing and problems of coverage in time and space. The results should be preferably used in combination with similar data obtained from other vessels (e.g., R/Vs Capricorne, Cornide de Saavedra, Ibn Sina), from fishery statistics, catch and effort and biological sampling data. This large-scale acoustic survey allowed a determination of major fish distribution patterns in quantitative terms. For detailed assessments other data will also be required in most cases.
The methodology used in acoustic surveys cannot be fully explained in the context of this report. Those interested can find full explanations in the new manual produced by FAO: Johannesson, K. and R.B. Mitson (1983) A practical manual for aquatic biomass estimation. FAO Fish.Tech.Pap. (240), 249 p.
This document consists of two parts: firstly a summary final report on several resource surveys made off West Africa between May 1981 and March 1982 by the R/V Dr. Fridtjof Nansen, and secondly a single cruise report covering a survey made around the Cape Verde Islands in November 1981.
A preliminary version of Part 1 was presented to the CECAF Working Party on Resource Evaluation, 2–6 February 1982: this version has now become obsolete. A limited number of advance copies of the definitive version were made available to the CECAF Sub-Committee on Management, meeting at Tenerife in September 1983.
Part 2 is a full reprint (with the omission of the original Annex V which is already covered in Part 1) of the Cape Verde cruise report, first issued in September 1982 by the Institute of Marine Research, Bergen.
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