4. Tuna Fisheries in the Eastern Tropical Atlantic (Contd.)

Table 4.12 Total catch of bigeye by longliners in the zone. Korea + Panama, by country. The figures marked by and asterisk are not calculated from statistics by 5° month, but are estimated hypothetically considering that 50.6% of the Atlantic catch is taken in the study area. (Mean percentage calculated from the location of catches by Cuba, Korea, Taiwan and Japan during the period 1975 to 1982.)

Table 4.13 Total catch of bigeye by purse seiners in the study area.

Table 4.14 Total catch of bigeye by bait boats in the study area.

Figure 4.11 Average catch (in tonnes) by 5° square by all industrial fisheries (pole and line, purse seine and longline) of yellowfin, skipjack, and bigeye; average for the period 1978 to 1982.

Table 4.15 Mean catches of yellowfin, skipjack and bigeye by 5° square in the study area, all gear (surface and longline), 1978 to 1982 period.

Figure 4.12 Average catch 5° square of yellowfin, skipjack, and bigeye; average over the period 1978 to 1982 by pole and line and by purse seine.

4.4. THE EXPLOITATION OF SMALL TUNA AND RELATED SPECIES IN THE EAST TROPICAL ATLANTIC

4.4.1. Generalities

The small tuna (spotted tunny, Atlantic bonito, frigate tuna) are essentially exploited by artisanal fisheries, in view of the coastal nature of these resources, but also by certain industrial fleets. The collection and treatment of statistical data are carried out in a systematic way in Senegal (Diouf, 1985). For other countries, the data come from the ICCAT or FAO bulletins and are usually not monitored by scientists. In other countries, such as Gambia, Togo, Nigeria and Cameroon, the artisanal fisheries catch statistics are not declared. These fisheries, often multispecific and utilizing varied fishing devices, are very dispersed along the coasts and difficult to sample.

The industrial fisheries are not specifically targetting small tunas. They discard them at sea or combine them in the “others” category in log books and often sell them in local markets which are difficult to monitor.

The possibilities of confusion between species are frequent and exist in all the fisheries whether industrial or artisanal. The French name “bonite” is, for example, often used not only for Sarda sarda but also for all the small tuna including skipjack, especially in the historical period. As well, the auxids and tunny are often confused in the landing statistics. In spite of these gaps, it is possible to estimate the landings in the study zone at 40,000 tonnes per year of all species combined. (table 4.16).

4.4.2 Changes in catch

4.4.2.1. Artisanal fisheries

Artisanal fisheries for small tuna are very ancient on the eastern Atlantic coast. Villaut de Bellefond cited in 1669 “bonite” captures by the artisanal fisheries of Senegal and Sierra Leone. Monod (1977) made reference to some large catches of “small tuna” at the beginning of the 18^th century in the Gulf of Guinea and at Cape Blanc (Ras Nouadhibou) in Mauritania. The artisanal fisheries statistics are available since 1950 in Angola and from the 1970's in the eastern tropical Atlantic (Senegal and Ghana). The changes in the total catch of small tuna is presented in figure 4.13 for the period 1950 to 1983.

Table 4.16 Catches of small tunas declared by artisanal (A) and industrial (B) fisheries in the study zone from 1950 to 1983 and total of the various fisheries. (Landings from the industrial fishery exist since 1955.)

Spotted tunny

During the historic period (1950 to 1970), the tunny catches varied little (table 4.17). They are in the order or 3,000 tonnes per year. They decrease from 1970 to 1977 then increase significantly in recent years and reach 10,000 tonnes in 1983 (figure 4.14a).

The largest artisanal catches are declared by Ghana but the specific break down of captures often seems doubtful; the total catch is attributed to the dominant species in the landings. In Senegal, artisanal catches are made by hand line and purse seines. The landings by trolling are in the order of 500 to 600 tonnes per year in recent years. Elsewhere, the catches are essentially carried out by surface nets.

Figure 4.13 Total declared catch of small tunas by fisheries in the study area from 1950 to 1983.

Auxids (frigate tuna and bullet tuna)

The auxid catch, in the order of 5,000 tonnes per year from 1950 to 1955, decreases and stabilizes to 1,000 to 1500 tonnes per year until 1966, the year it starts to increase, presenting a strong interannual variability (table 4.18). The insufficient data concerning these species in the zone is linked in part to a confusion between auxids and tunny, and because frigate tuna is the most oceanic of small tuna and therefore lends itself less to a generally coastal artisanal exploitation (figure 4.14.b).

Atlantic bonito

The artisanal catches of bonito show weak variations from 1950 to 1983, under 1,000 tonnes per year (table 4.19). Only data from Angola (1950–83) and Senegal (1974–83) are available in the zone (figure 4.14.c). The catches in the Senegalese artisanal fisheries represent more than 80% of the total artisanal catches declared in the zone. These catches are made by baited hand lines and trolling.

Spanish mackerel

The catch statistics of Spanish mackerel exist only since 1968 (table 4.20), but the artisanal fishery have always exploited this species in the zone notably in Ghana, Angola and along the west African coast: in Benin (30 tonnes in 1964, 70 tonnes in 1965), off Senegal, Gambia and Mauritania. The catches are on the order of 2,000 to 3,000 tonnes until 1970 then they stabilize around 1,000 to 1,500 tonnes until 1979 and augment significantly in the 1980's reaching 3,500 tonnes per year (figure 4.14.d). The bulk of the landings of this species are according to the current statistics collected in Senegal and Ghana. The captures were made by gillnet, ringnets, and hand line.

Other species

Other species of small tuna are also exploited in the zone, notably the wahoo and the plain bonito (palomette). The data concerning these species are very scarce. However, the artisanal line fishery of the Cape Verde Islands land some 1500 to 2000 tonnes per year of wahoo. In Mauritania, the abundance period of the plain bonito is from July to December, the catches taken principally in the Bay of Levrier with a particular concentration in the area surrounding Cape Blanc (Maigret, 1973). The quantities landed are not estimated. To the north of Senegal, the artisanal line fisheries captures the plain bonito from June to November. The catches here are low, on the order of 50 to 100 tonnes per year.

For all small tuna and related species, it appears globally that catches increase noticeably during recent years following an improvement of the statistical coverage, better species identification, and also an increase in fishing effort.

Figure 4.14 Declared catches of small tuna by species and fishery in the study area from 1950 to 1983: (a) tunny; (b) auxids; (c) bonito; (d) Spanish mackerel.

4.4.2.2. Industrial Fisheries

Industrial fishing data is relatively recent in the zone. It is essentially supplied by the Soviet vessels, FIS and Spanish purse seiners and Angolan and Ghanian baitboats. The Japanese purse seiners have had large catches of small tuna from 1965 – 1970. The changes in catch by species and by year from 1950 – 1983 are represented in figure 4.14.

Spotted tunny

The industrial catches of tunny have regularly increased from 1970 – 1983 as a result of a large increase in declared catches by USSR vessels (table 4.21). From 1000 tonnes in 1970 catches surpassed 10,000 tonnes in 1983 (figure 4.14a). The distribution of catches by French and Spanish purse seiners is represented in figure 4.15a. (It must be emphaized that catches recorded in log books are probably severely underestimated.)