7. EVALUATIONS

7.1 Ivory Coast
7.2 Ghana - Togo - Benin
7.3 Nigeria - Cameroon - Equatorial Guinea
7.4 Gabon - Congo - Angola (Cabinda) - Zaire

7.1 Ivory Coast

The data available on trawler fishing made possible the use of two global models for the Ivorien continental shelf as a whole. One is for the period 1959-70, the second is for 1972-77. They show striking modifications of the ecosystem in line with the proliferation of Balistes (Figure 6a).

For the first period we get a shelf-wide (depth range 10-120 m) average maximum potential yield of nearly 10 000 t.

For the second period, we have catch and effort statistics for depth ranges 10-50 m and 50-120 m (Figures 6b and 6c) and we get:

MMCP 10-50 m	4 400 t
MMCP 50-120 m	2 200 t
Total (10-120 m)	6 600 t

This last figure reveals an apparent drop of about one-third of the overall productivity of the continental shelf. If we take into account that a recent demersal biomass survey estimates a figure of 8 000 t for Balistes, the missing portion is of the order of magnitude of that fraction of the triggerfish biomass which could be fished on a steady basis but which is now discarded or avoided. This would signify that the productivity of the shelf has varied little in quantitative terms since the Balistes explosion.

The average maximum potential yield figure for the 50-120 m depth strata is very likely underestimated in terms of the biological potential of this layer where many species are not fished or landed due to their low market value and where fishing operations are only carried out during the cool season when sparids are most available. The fishing effort thus falls short of the biological optimum with respect to the species belonging to this ecosystem.

In order to follow trends in the catch patterns of certain species or groups of species, a year-by-year comparison (Figure 7) was made of percentages in weight of a group of target species (sole, threadfin, croaker, carp, shark and ray), after first checking that the percentage of the total of these target species with respect to total species landed had remained stable during the period under study. This was done for the 0-50 m depth range.

From 1968-77 cpue for bigeye grunter (Brachydeuterus auritus) and shad (Ilisha africana) dropped sharply. The drop in bigeye yields is a direct consequence of the triggerfish explosion as they both have the same bathymetric distribution. The sparid cpue (pandora and sea breams or dorados) is greater. This increase may be due to a resurgence of sparids related to area-wide cooling, Or it might be due to the activities of trawlers working farther offshore in order to avoid the 30-35 m grounds where Balistes predominates. The tonguefish decline may be due to the fact that the most abundant species, C. canariensis, has the same bathymetric distribution as Balistes. The decline would thus be an indirect result of trawlers trying to avoid the latter species. Lastly, the increased percentage of West African croakers (Pseudotolithus spp.) after a phase of decline can most likely be imputed to a lessening of the effort exerted these last few years by the Ivorien fleet as a whole.

Yield per unit of area for 1971-77 is about 4 400 t/4 700 km² for the 10-50 m depth range. This makes 0.94 t/km². The figure for the 50-120 m range was 2 200 t/7 600 km or 0.29 t/km². A recent estimate of the biomass without Balistes is 9 300 t at 10-50 m: 11 500 t at 50-120 m.

The catch/biomass ratios are, respectively:

4 400/9 300 = 0.5 for the 10-50 m depth range and 2 200/11 500 = 0.2 for depths beyond 50 m.

The 10-50 m range can thus be considered fully exploited, whereas the catch in the over 50 m sector could be doubled, biologically speaking, which would make a potential of over 4 400 t. The figure is a theoretical one, however, in that present economic yields are not high enough (lower density, larger number of species of lesser market value) to allow the full exploitation of this ecosystem. Now, if we count in Balistes, their non-exploited biomass of 8 000 t represents an additional supplementary catch of roughly 8 000 x 0.25 = 2 000t, with 900 between 10 and 50 m and 1 100 below.

The following table summarizes the present demersal potential of the Ivorien continental shelf:

	Depth range (m)	Potential (t)	Density (t/km2)
Without Balistes	0-50	4 400	0.94
	50-120	4 400	0.38
		8 800
With Balistes	0-50	5 300	1.13
	50-120	5 500	0.72
		10 800

7.2 Ghana - Togo - Benin

Available data are not such as to permit the use of global models.

Using biomass trawler surveys carried out in 1969 and 1970, Rijavec (1971) in Ghana estimated the demersal biomass between 10-40 fathoms (18-70 m) for an area of 15 000 km² at 47 000 t (density 31 kg/ha). Heaviest densities were observed at 50-70 m.

Since the exploitation rate was high at the time these surveys took place, one can reasonably expect a yearly mean sustainable yield of about 0.4 times the biomass, or 20 000 t, or 1.3 t/km². This last figure is a bit higher than the two for Ivory Coast. This agrees with Caverivière’s findings based on comparing the yields of Ivorien trawlers. His conclusion is that the mean density of Ghanaian fishing grounds for 1968-74 at ranges of 0.50 m is about 10-15 percent higher than for Ivorien fishing grounds of the same type.

Extrapolating the figure 1.3 t/km² to the Ghana/Togo/Benin sector as a whole, we get an average potential maximum yield of 32 500 t, assuming the overall productivity of the area has not changed since the Balistes explosion, which seems to be so in Ivory Coast. Though data are still too scanty to prove it, Balistes may possibly represent half of this potential catch.

7.3 Nigeria - Cameroon - Equatorial Guinea

Longhurst (1964) was the first to publish estimations of the possible demersal potential of the Nigerian continental shelf. According to this estimate, at the beginning of the sixties a Lagos-based fleet of trawlers removed an average of 3 500 t from inshore grounds at depths of under 20 fath between Cotonou (Benin) and Lekke (Nigeria). This sector represents an area of approximately 500 n mi. The corresponding annual production per unit of area is therefore about 7 t/n mi² or 2 t/km². He makes the point that little is discarded, particularly for such small species as Brachydeuterus. The figure of 2 t/km² does not represent maximum yield, because several factors of considerable potential impact have not been taken into consideration.

- while the author does indicate that trawlers were active at the time, we do not know whether the maximum exploitation rate has been reached or exceeded. If it had (or if it had not), the result would be an error of omission;

- peripheral areas were not fished, at least not trawled, and so an immigration toward the central area depleted by trawling would entail an error of overestimation;

- lastly, artisanal production, which is important in an overall sense but is undoubtedly concentrated on pelagic species, is left out (error by default).

trawl fishery	- inshore area (0-50 m)	11 000 t	(0.4 t/km2)1/
	- offshore area (50-200 m)	3 500 t	(0.25 t/km2)
artisanal fishing (mostly coastal area)		25 000 t
Total (approx.)		40 000 t

^1/ Not including artisanal fishing

- they provide a rough notion of the possible potential. The 40 000 t figure corresponds to a per unit of area productivity of about 1 t/km² (plateau area = 41 000 km² - Table 3);

- they show the poor potential of the offshore zone, estimated by the author at only one third of the (trawl) potential of the inshore zone. This deep area was not being fished at the time;

- he observes that the density of the immediate biomass drops off sharply as the Cameroonian shelf is approached.

1.98 x 37 800	=	74 844 t	(0-50 m)	without Brachydeuterus
1.67 x 22 300	=	37 241 t	(+ 50 m)
		112 085 t

Considering the probable level of exploitation during this period, conceivably 30-40 percent (more likely 30 percent) of the biomass of 0-50 m grounds could be caught annually. This would be 22 000-30 000 t, with 15 000-20 000 t for Nigeria, 5 000-7 000 t for Cameroon and 1 000-2 000 t for Equatorial Guinea. At depths of over 50 m, where there was no fishing, we can expect an annual catch of 20 percent of the biomass or about 7 500 t. Out of this figure, theoretically, under 5 000 t for Nigeria, 2 000 t for Cameroon and perhaps 500 t for Equatorial Guinea would be a fair estimate. The potentials calculated from these data correspond to a theoretical maximum which could be reached if all species were fished and marketed and the fishing effort were exerted at the right times and places. The offshore zone, for instance, is still largely unfished throughout the area.

As Longhurst (1965a) had proposed, the GTS data confirm that the absolute value of the potential of the offshore sector would be one-fourth to one-third of the inshore sector. This assumes that the catch would be economically feasible, and applies to the whole of the Nigeria-Cameroon-Equatorial Guinea sector.

Comment

The estimates for Cameroon (7 000-9 000 t in all) do not agree with the catches reported by this country, which have for several years exceeded 15 000 t. It seems that a large part of the catch comes from the more inshore area and from the many and vast river mouth areas, which have so far not been studied at all. It is thought that these resources are mainly fished by the artisanal fleet, and, to a certain extent, have not been properly figured into the assessments, which are based on the trawl fishery. Longhurst's figures (1965a) for Nigeria (15 000 t for the trawl potential and 25 000 t for the artisanal potential) confirm this observation. Independently of the fact that the Douala-based trawler fleet may well fish a wider area than the Cameroonian shelf, it has been suggested that the presence of nutritive salts borne into the area from the numerous watercourses, and of mangrove areas, and of extensive delta zones may well be enriching factors partially compensating for the low oceanic productivity of this portion of the Gulf of Guinea.

7.4 Gabon - Congo - Angola (Cabinda) - Zaire

The 1963-64 GTS survey led to an estimate of the available biomass off the continental shelf by depth ranges and by sectors (0-50 and 50-200 m in the Douala-Cape Lopez, Cape Lopez-southern Gabon and southern Gabon-Congo mouth sectors). Trawling was active at the time along the coastal strip of this latter sector where most of the more valuable species were fished at close to peak levels (Le Guen, 1971; Troadec, 1971). The grounds immediately south of Cape Lopez were also fished, but to a lesser extent, mostly because much of the shelf off Gabon (60 percent according to Rossignol et al., 1962) is untrawlable. The fauna-poor area north of Cape Lopez was not visited by trawlers. Grounds with depths of over 50 m were only fished a few months out of the year, during the cool season, off far southern Gabon, Congo and Cabinda. Using these data as a base and taking an average figure of 0.4 as a coefficient of natural mortality, it was accepted that the potential of the most fished sector (by trawlers from Ponte Negro) was equal to 40 percent of the available biomass at the time of the survey. The virgin sector (offshore zone of southern Gabon) potential was assumed to be only 20 percent of this biomass. For the other sectors, intermediary ratios were used, selected on the basis of probable rate of exploitation. We thus get the following estimations of potential:

Sector	Depth range (m)	"Limited"1/ biomass (t)	Ymax biomass	Theoretical potential (t)
Northern Gabon (N. Cape Lopez)	0-200	6 5002/	0.20	1 300
Southern Gabon (S. Cape Lopez)	0-50	20 000	0.30	6 000
	50-200	37 000	0.20	7 400
Congo-Cabinda-Zaïre	0-50	41 000	0.40	16 400
	50-200	57 000	0.25	14 250

^1/ i.e., excluding pelagic species and Brachydeuterus auritus

^2/ Obtained from extrapolation of the biomass in the southern Gabon sector. The factor of extrapolation was taken as equal to the ratio of respective areas multiplied by that of the densities; Rossignol et al. (1962) having shown that during the same season the biomass was 2.7 times less dense than south of the Cape (Cape Lopez)

(a) Since 60 percent of the Gabonese continental shelf is untrawlable, full realization of the 15 000 t potential corresponding to the Gabon shelf area would be dependent on the possibility to fully exploit the hard-bottom area. This means successful, full implementation of gear other than trawls (lines, nets, traps and the like);

(b) Estimations for the Ivorien shelf have shown that for the deep strata it was not now possible for economic reasons to fish more than a bare two-fifths of the biologically feasible potential. Therefore it would now be impossible to get more than 9 000 t from this layer over the entire area considered here. The same consideration applies to the inshore strata north of Cape Lopez, an extremely sparse zone which is ignored by trawlers.

(c) Lastly, these considerations, drawn from immediate biomasses, accept that all species taken into account in these biomasses can be fully exploited and used, i.e., that none will be discarded and that effort will be exerted to the maximum at the right times and places.

It is impossible to reach more exact conclusions with the data presently available. The weakest link in this chain of reasoning is obviously the appraisal of rates of exploitation undergone by the various fishing-grounds in 1963-64. Analyzing catch trends from the Congolese shelf alone as a function of the increased fishing intensity on the part of the Congolese fleet, Fontana observed that under present conditions of distribution of the fishing effort and without Brachydeuterus which at that time was discarded, the annual total catch tended to peak at around 8 000 t for a total engine power figure of about 3 000 hp. Let us interpolate GTS biomass potentials in the foregoing tables proportionally to the areas of the continental shelf, and accept the assumption that as in Ivory Coast the deep layer (50-200 m) cannot in practice yield more than 40 percent of the theoretical potential. We then get, for the Congolese shelf alone, an MSY of 6 000-12 000 t (excluding Brachydeuterus) according to whether we base the interpolation on the potential of the southern Gabon-Congo-Cabinda-Zaïre sector or on the potential of the Congo-Cabinda-Zaïre sector alone. These two methods, though rather approximative, do produce results of the same kind. We may therefore assume that the order of magnitude of the foregoing estimates, especially those for the different relative wealth of the various grounds, are correct.