7. EVALUATION OF THE PELAGIC STOCK BIOMASS

7.1 Senegal-Mauritanian Zone
7.2 Sherbro Division

7.1 Senegal-Mauritanian Zone

7.1.1 Soviet Work
7.1.2 CRODT Work

7.1.1 Soviet Work

Several surveys were made in 1976 and in 1977 on the Mauritanian continental shelf between 16°N and 21°N. The following results are deemed to be trustworthy for bottom fishes, but some reservations must be made concerning the pelagic stocks (Mylnikov and Overko, pers. comm.):

Period (month and year)	SPECIES
	T. trecae		T. trachurus
	No. individuals (000 000)	Biomass ('000 t)	No. individuals (000 000)	Biomass ('000 t)
I.76	889.0	28.7	9.6	1 870
XII.76 and I.77	1 891.5	52.8	16.0	2 720
II - III.77	759	26.2	57.8	406
VII - VIII.77	3 965	73.0	92.6	1 804

7.1.2 CRODT Work

Several echo-surveys have been made in the open sea off the coast of Senegal and Mauritania, The method offers trustworthy results but has well-defined limitations which depend on both the equipment used and the behaviour of each species, particularly the sardinella, A fairly constant biomass is found from Dakar to Cape Roxo in the cold season estimated at about 800 000 t (Gerlotto et al., 1978):

January	1974 - 890 000 t
April	1975 - 850 000 t
April	1976 - 750 000 t
April	1977 - 725 000 t

It is definitely much smaller in the hot season.

The breakdown of this biomass by species, or even by groups of species, is a delicate undertaking. It was attempted by a survey in October and November 1974 between Cape Roxo (12°N) and Cape Barbas (22°20 N) on the basis of data from samples taken by oceanographic vessels and from fishing fleets working in the area of exploration (Marchal and Boëly, 1977). The results are summed up in the following table, expressed in thousands of tons:

	BIOMASS (103 t)
	Total	Sardinella		Horse	Mackerel
		S. aurita	S. maderensis	mackerel
11°N/12°N	132	Breakdown not feasible
12°N/14°45N	543	81	23	140	63
14°45N/17°N	717	104	63	152	11
17°N/19°10N	542	26	77	369	5
19°10N/22°20N	1 189	402	-	618	-
Total %	2 991	613 20	163 5	1 279 43	76 3
22°20N/26°10N	1 788	Breakdown not feasible
Grand total	4 911

These results, related to the total of the continental shelf give a total biomass of 12 million t. Toward the south, in Guinea-Bissau, the fish biomass has been estimated at 870 000 t over the continental shelf. In addition a recent evaluation of the pelagic resources of Guinea Bissau with acoustic methods gives a biomass of 210 000 t (Boëly and Marchal, 1977).

7.2 Sherbro Division

The evaluation of the pelagic biomass of Sierra Leone continental shelf between 7°N and 9°N was made by Soviet scientists from the end of December 1976 to the beginning of January 1977 by means of echo-surveys and trawling. Studies focused mainly on the three most productive zones, i.e., Sherbro Island, the Orontes Bank and the northern region (Fig. 2). The total biomass of pelagic stocks for these three zones is estimated at 158 900 t. This evaluation does not take into account coastal species caught by the artisanal fisheries (Fisheries Division, Freetown, Annex 16).

The biomasses of the most abundant species were the following (Ivanov et al., 1977):

Species	Average weight (kg)	Estimate of total number of individuals	Biomass (t)
Caranx chrysos	0.35	182.4 × 106	64 000
Sardinella spp.	0.08	167.23 × 106	14 200
Chloroscombrus chrysurus	0.08	25 × 106	2 000
Balistes capriscus	0.01	767.8 × 106	76 200