4.1 Review and discussion of biomass estimates
4.2 Estimate of resource potentials
Table 4.1 shows the biomass estimates described above presented by areas and types of resources. For the small pelagics two sets of estimates are given, a “seasonal” estimate representing the September survey and a mean value representing the other three surveys. The estimate for trigger-fish represents the March survey since this may also be a seasonal occurrence. The acoustic biomass estimates of demersal fish are believed to be gross underestimates and we have more confidence in the swept area results.
|
Table 4.1 Estimates of fish biomass by types of resources
and areas. 1 000 tonnes, rounded numbers. |
|||
|
|
North of Cape Lopez |
South of Cape Lopez |
Totals |
|
Small pelagic |
10 |
30 |
40 |
|
“ “ seasonal |
|
160 |
170 |
|
Triggerfish, seasonal |
50 |
|
50 |
|
Demersal |
|
|
|
|
(acoustic estimate) |
|
|
30 |
|
(swept area estim.) |
20 |
120 |
140 |
|
Totals |
80 (30) |
280 (150) |
360 (180) |
Pelagic fish
For this group there is clearly a seasonal pattern in abundance which must be considered when comparing different data. Three sets of survey data are available from recent years. The Gabon shelf was covered briefly with DR FRIDTJOF NANSEN at the end of August 1981, the season of assumed high biomass of pelagics. The biomass was estimated at 360 000 tonnes and samples gave small sized sardinellas, mostly S. aurita and some carangids. The extensive two-vessel surveys by NIZERY/CAPRICORN in 1982 gave biomass estimates for the shelf south of Cape Lopez of 80 000 tonnes in April and May and abt. 330 000 tonnes in August. North of Cape Lopez the estimate was 38 000 tonnes in May and 76 000 tonnes in August. The Oceanographic Institute of Spain, Coastal Centre of the Baleares surveyed the Gabon shelf with the R/V CORNIDE DE SAAVEDRA 8-17 June 1983 and found an estimated biomass of abt 175 000 tonnes, the major components being round sardinella and horse mackerel.
The estimates based on our 1985 survey and made at the corresponding seasons of the year are one to two times lower than these previous findings. Although all of these surveys are based on acoustic techniques there may still be a problem of comparability of methods related to the difficulties of distinguishing echo-records of fish in records of dense plankton. This will, however, remain unresolved. There occur on the other hand very often large scale variations in the size of small pelagic stocks between periods. These may be related to natural fluctuations in recruitment or to fishing pressure and the Gabon stocks would seem to be vulnerable to both of these effects. They consist to a large extent of immature fish deriving from one years spawning only. Since the Gabon shelf forms nursery grounds for the northern part of the sardinella - and perhaps also horse mackerel stocks of the Congo and Angola the fisheries in these southern areas may well affect the Gabon parts of the stocks. The 1985 Angolan catch of sardinella was abt. 240 000 tonnes out of an estimated total biomass of abt. 400 000 tonnes. This represents a high rate of exploitation which caused a reduction in stock size in Angola and may have affected also the Gabon area. The lower biomass estimate of small pelagic fish in Gabon in 1985 as compared with previous surveys may thus at least in part have been caused by a declining stock. There is also a decline in the reported landings of carangids and sardinellas in Gabon from 3 200 tonnes in 1983 to 2 600 tonnes in 1984 and 1 800 tonnes in 1985.
Demersal fish
There are no reasons to expect fluctuations in biomass of demersal fish caused by migrations in and out of the area, but there could still be seasonal changes of availability caused by inshore and offshore movements. There are no previous swept area surveys. The acoustic estimates of demersal fish were as follows: DR FRIDTJOF NANSEN, 1981, 130 000 tonnes; NIZERY/CAPRICORN 1982, April and May 40-50 000 tonnes, August 212 000 tonnes; CORNIDE DE SAAVEDRA 1983, (“other fish”), June 55 000 tonnes. Also these estimates are considerably higher than our acoustic biomass estimates. It seems unlikely that any great change can have occurred in the stocks of demersal fish and we think it justified to have more confidence in the swept area method for assessment of demersal stocks.
There is, however, a need to evaluate also this estimate. In our application of the method it is based on an assumption of a 100 per cent efficiency of the trawl gear i.e. all the fish located in the path of the trawl will be caught. This is perhaps unlikely to be the case and lower figures for gear efficiency down to 50 per cent are some times used in these calculations. Our assumption of a high gear efficiency may thus tend to produce conservative estimates of biomass. Another problem is as mentioned under 3.4 above that the shallow water fauna of croakers, catfishes, threadfins and others are not well represented in our sampling. The fish densities in these shallow waters is probably higher than the mean densities which have been used to estimate the total biomass, and although the shallow water areas which represent some 15% of the shelf are included in calculation the effect will again be a tendency of underestimation of the total biomass of demersal fish in Gabon. Taking these various effects into account we propose that our figure of 140 000 tonnes can be used as a likely estimate of the demersal stocks less the part representing the shallow water community of croakers etc.
Trigger fish
The trigger fish is reported as occurring only sporadically in previous surveys. The significance of the findings of aggregations of the species off northern Gabon in the March and June surveys is thus uncertain. The fish was of small size and could represent recruitment in an increasing stock.
In order to maintain the fish stocks as a lasting resource only a part of the standing biomass should be fished. The proportion of this long term yield varies between types of fish, and in the following we will use 40 per cent for small pelagic fish and 25 per cent for demersal species. Existing fisheries must also be taken into account in calculating the total potentials.
Pelagic fish
There is a need to distinguish between the exclusive resource found throughout the year in Gabon and that shared with Congo and Angola which is available only during part of the year. The exclusive resource consists mostly of carangids, bumper, horse mackerel and scads, some sardinellas and ilisha and anchovy. With a standing biomass of 40 000 tonnes the potential yield is abt. 16 000 tonnes.
Another 130 000 tonnes of mainly round sardinella and perhaps some horse mackerel is according to our findings available in Gabon waters during part of the year, probably over some months. This biomass seems, however, to represent mainly juvenile and immature fish of stocks shared with Congo and Angola and the exploitation and management of these resources should be a matter for regional deliberations. In theory the potential yield from this biomass might be of the order of 50 000 tonnes, but the development of such a fishery on these resources may not be commercially feasable, nor would it seem to be desirable from the point of view of good biological management. There is, however, a basis for Gabon to claim an access to this resource which corresponds to the biomass found within Gabons EEZ.
The reported annual landings of small pelagic fish in Gabons present fisheries is according to FAO’s Yearbook of Fishery Statistics (Vol. 60) abt. 2 000-3 000 tonnes. Some further quantities of these stocks are no doubt taken as bycatch in the shrimp fisheries and discarded, but there exists clearly a considerable biological potential for increased yield from these resources.
Demersal fish
Of the estimated biomass of 140 000 tonnes (shallow water community excluded), 50-60 per cent represent fish of commercial interest. With a yield proportion of 25 per cent, the potential long term catch with no existing fishery would approach 20 000 tonnes. Present catches of groupers, grunts, seabreams and barracudas are reported to be abt. 5 000 tonnes and the total potential would thus exceed 20 000 tonnes.
Trigger fish
The uncertainties regarding the future existence of this stock have been mentioned several times above. This is a slow growing fish and the long term potential should be estimated at 12 000 tonnes from a biomass of 50 000 tonnes.
General development considerations
The estimated biomass for the various types of resources and the calculated long term yields can be summarized as follows, (1 000 tonnes):
|
|
Biomass |
Yield |
Present landings |
|
Small pelagics, Gabon stocks |
40 |
16 |
2 |
|
Small pelagics, seasonal |
130 |
50 |
|
|
Demersal, excluding shallow water |
140 |
>20 |
5 |
|
Trigger fish, uncertain |
50 |
12 |
|
We would, however, still like to offer a few comments of a general nature concerning the feasability of utilizing these resources. The only resource of a magnitude which might warrant a fishery on an industrial scale is the seasonal sardinella stock. The behaviour of this generally small sized fish does not, however, seem favourable for industrial purse seining and there is also the problem of a restricted availability during the year. A use of this resource on a regional basis should perhaps be considered. An alternative or complementary use in a semi-industrial fishery by a technically advanced artisan fleet could also be a possibility.
The other resources, Gabon’s exclusive stocks of small pelagics and the demersal fish out on the shelf have relatively low abundance and the fish densities are nowhere very high in their rather extensive distributions along the coast. These are not favourable characteristics for the development of fishing on a full industrial scale. In order to utilize these resources, and in particular the valuable demersal fish considerations should also be given to an exploitation at an intermediate level of technology by artisan units fishing with long lines and gillnets. The inshore components of the small pelagics, ilisha, bumper, anchovy and others probably already form pan of the bycatch of the shrimp- and trawler fleets. If market outlets exist or can be developed for these types of fish the use of this bycatch should be attempted and the direct exploitation of these resources by the fleets could be considered. Lastly we want to point out that the stocks of silverside grunt, Atlantic bigeye, driftfish and picarel represent a considerable source of cheap fish for internal consumption in Gabon if simple arrangements for exploitation and marketing could be made.
