CHAPTER 5. BRIEF SUMMARY OF FINDINGS

The contribution made by the DR FRIDTJOF NANSEN programme between August and December 1986 to the CECAF COOPERATIVE SURVEY 1986 consisted in: (i) development work with reference to acoustic methods, (ii) participation in various acoustic inter-calibration experiments conducted jointly with the other survey vessels, and (iii) acoustic cum trawling surveys covering various parts of the shelf between Agadir in Morocco and Freetown in Sierra Leone. The field work was undertaken in two periods, August 19 to October 4, and November 6 to December 12.

The acoustic development work consisted mainly in repeated survey coverages of assumed identical biomasses of fish over a time interval of a few days in order to test the consistency of the resulting abundance estimates. The experiments included sardinellas and scads off Senegal, and sardine off Morocco. The main finding was that the consistency of the results is high. The data also support previous findings in demonstrating that surveys during night will give higher estimates than those from daytime. This bias seems to be related to a diurnal change in schooling behaviour. The experiments and their results are described in a paper submitted to the Acoustic Symposium in Seattle, June 1987.

The programme of intercalibrations and other comparisons of the acoustic systems included all of the four vessels which participated in the survey. It consisted of intership calibrations using the sea bottom as a common target, intership calibrations on fish layers, and simultaneous mini-surveys of defined areas, the last for the purpose of comparing the entire systems. The results demonstrated the importance of such inter-calibration programmes by revealing differences in both the performance of the instruments and in the working procedures which would affect the abundance estimates to a significant degree.

The acoustic surveys provide data from major parts of the distributional area of the main stocks of small pelagic fish between Agadir and Cape Bojador in Morocco and from Cape Blanc in Mauritania to Freetown in Sierra Leone. Estimates of the total standing biomass of these fish are presented by areas for each survey coverage. Frequent trawl sampling provides a basis for allocating these total biomass figures on main species or species groups. When account is taken of survey coverage and the distribution of the various stocks the following findings can be presented:

For the sardine (Sardina pilchardus), both the September and the November surveys gave estimates of biomass between Agadir and Cape Juby of 1 million tonnes. A coverage between Cape Juby and Cape Bojador in November demonstrated the presence also in this area of approximately 1 million tonnes of this fish. The species was furthermore located in a small area off Cape Blanc in Morocco in December in a dense aggregation estimated at about 540 000 tonnes. Comparisons with previous survey findings indicate the stock(s) of sardine must be in a state of recovery.

The two sardinella species, Sardinella aurita and S. maderensis can not be properly separated in the assessments, but they appeared with about the same incidence in the catches. Their joint total biomass is estimated at about 650 000 tonnes, the main part located on the Senegambia and Mauritania shelf. A school area of sardinella found off Freetown in August could, however, not be properly assessed. The distribution of the sardinellas is very similar to that found in previous surveys, but biomass estimates are about 30 per cent higher than those from surveys in 1981/82.

Also the distribution of the horse- and jack mackerels and scombrids, hairtails and barracudas was as previously described with the horse mackerel Trachurus trecae having the highest abundance in Mauritania and southwards in Senegambia, overlapping with the false scad Decapterus rhonchus which was found in highest concentrations from the Gambia southwards past the Bijagos Islands. The hairtail Trichiurus lepturus was an important part of this group in Mauritania, but further south the bumper Chloroscombrus chrysurus dominated especially in inshore waters together with the lookdown Selene dorsalis, the Spanish mackerel Scomeromorus tritor and other species. A somewhat rough allocation of the total biomass estimates of these fish on species or species groups gives stock biomasses as follows: close to 500 000 tonnes for horse mackerel; about 180 000 tonnes for scad and 200 000 - 250 000 tonnes for bumper, hairtails, Spanish mackerel and barracudas, a total of about 900 000 tonnes for these types of fish. This is about the same level as found in a 1981/82 survey, where however the allocation on species was somewhat different.

The trigger fish Balistes capriscus was as previously found mainly off Guinea and Guinea Bissau. The biomass of this western stock was estimated at 220 000 tonnes, a considerable decline from a level exceeding 1 million tonnes in surveys in 1981/82. This species is known to undergo long term changes in stock size, but there is also the possibility that an industrial fishery which started in 1980 and reached abt. 100 000 tonnes per year in 1981/82 can have affected the state of the stock. An evaluation of this could be made with access to more detailed data.

The demersal stocks in the region Freetown - Cape Roxo and off Gambia have been separately assessed by trawl surveys. The demersal fish Freetown - Cape Roxo were estimated to 340 thousand tonnes of which 90 thousand tonnes were classified as commercially valuable and the remaining 250 thousand tonnes as low market value fish, such as flying gurnards and silverside grunts. Off Gambia the demersal fish was estimated to abt 30 thousand and 25 thousand tonnes from the two coverages. High market value fish was assessed to 2100 and 5600 tonnes from the two surveys. The silverside grunt is the dominating species in the fish community, making up abt. 44 and 60% of the total biomass during the repective surveys in Gambia.

Fig. 2.1. Integrator values: DR. FRIDTJOF NANSEN versus CORNIDE DE SAAVEDRA. Units: 0.001 m²/nm².