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

4.3.3. Exploitation of large tuna (yellowfin, skipjack and bigeye), in the eastern tropical Atlantic

4.3.3.1. Yellowfin

Yellowfin is historically the most sought after species in the zone. From 1955 to 1982 the catches manifest a trend with a strong and regular growth to exceeding 100,000 tonnes/yr since 1975 (table 4.6). The longline has been, since the start of the industrial fisheries development (1957 to 1962), the principal fishery device (table 4.7). The majority of yellowfin catches were made since 1969 by purse seiners (table 4.8); these supplied on the average 80% of yellowfin catches during the recent period (1979 to 1983). The yellowfin captures by baitboats are relatively stable since 1957, with average catches of 17,500 tonnes/yr for the period 1957 to 1983 (table 4.9).

4.3.3.2. Skipjack

The expansion of skipjack fisheries in the Gulf of Guinea is relatively recent. The skipjack catches were under 20,000 tonnes in 1967; they were just over 40,000 tonnes until 1970. The real expansion of the skipjack fishery started in 1970 where, as many baitboats as purse seiners captured increasing quantities of skipjack from 1970 to 1982. The skipjack catch reached nearly 100,000 tonnes per year since 1974. During this period, the purse seiners captured the majority of this tonnage (60% from 1974 to 1983), but the percentage of skipjack captured by pole and line stayed very significant, around 40% (table 4.10 and 4.11).

4.3.3.3. Bigeye

The bigeye fisheries only started significant development in 1960; the catches in weight of this species however in the east tropical Atlantic remained well below those of yellowfin or skipjack. The total catches of bigeye in the region only passed 20,000 tonnes in 1971; they have regularly gone over 30,000 tonnes since 1981 (table 4.3.).

The bigeye fishery has always been dominated by the longliners that capture the majority: 92% of total catches in 1960 to 1969, 62% from 1970 to 1982. The bigeye catches by longline in the region were relatively stable since 1970 (average 1970 to 1982 = 16,000 tonnes, table 4.12). (The development of longline fishing observed during the recent years has taken place to the north and south of the study zone). The bigeye captures by purse seiners have increased regularly since 1970, reaching on average 11,400 tonnes during the period 1977 to 1983; these numbers remain, however, subject to caution because of the difficulty identifying small bigeye frequently in the purse seine catches (table 4.13). The pole and line catches are relatively weak and approach 3,000 to 5,000 tonnes per year (table 4.14) for all fleets combined.

Table 4.6 Total catches of yellowfin, skipjack and bigeye in the study area.

4.3.4. Assessment of average catches by zone and device in the zone and by device in the study zone during the recent period for yellowfin, skipjack and bigeye

4.3.4.1. Generalities

During the recent period, good fishing statistics for all industrial fleets, for the surface fisheries as well as longline, are available from ICCAT. This data can be grouped by 5° geographic squares and are interesting to consider as they give a good global view of the average geographic distribution of tuna catches (figure 4.11) and of the relative richness of different fishing zones.

Table 4.7 Annual catch of yellowfin by longliners in the study area, by country. The figures marked by an asterisk are not calculated from statistics by 5° month, but are estimated hypothetically considering that 91.1 % 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 4.8 Total catch of yellowfin by purse seiners in the study area.

4.3.4.2 Data

The annual average catches by species and all gear by 5° geographic squares, during the recent period 1978 to 1982 have been calculated from the ICCAT data base. Table 4.15 gives the values which correspond to the men catch per 5 degree sector in the zone from 1978 to 1982 (a period chosen for the good quality of its statistics and judged as representative of current fisheries).

4.3.4.3. Average catches 1978 – 1982

a) All gears

The zones where tuna catches are the largest are the coastal zones and those of the islands (figure 4.11, table 4.15), in particular the fishing regions situated along the coasts of Cape Lopez, the Côte d'Ivoire and Ghana, Senegal and the islands of the Gulf of Guinea (Sao Thome, Anno Bon, etc. …); four 5° geographical squares supply extremely high average catches of more than 20,000 annual tonnes. Similar large sustained captures are rare on a world scale and only seem to be observed in some very active fishing zones like certain regions of the eastern Pacific. These large catches are explained both by the high rates of tuna exploitation for the region and by the particular abundance of tuna along the coast of west Africa in relation to various active enrichment mechanisms in the region (upwellings, domes, fronts, etc. …chapter 3).

Table 4.9 Total catch of yellowfin by baitboats in the study area.

b) Pole and line

During the recent period, the pole and liners are based in two coastal zones (figure 4.12.a); the north region (Senegal, Mauritania, Cape Verde Islands), and especially in the central zone of the Gulf of Guinea (off Ghana). The baitboat catches in the south zone of Angola remain small. Pole and line vessels capture on the average of 17% of large tuna fished in the region.

c) Purse seine

The purse seiners exploit all of the Gulf of Guinea zone (figure 4.12.b), the catches being especially high in the zones situated in relative proximity to the coasts. The zone situated between the African continent, latitude 5°S and longitude 20°W supplies an average of 95% of the east Atlantic purse seine captures. Purse seiners are the primary fishing gear in the region, and capture an average of 61% of the catches of the 3 major tuna species (yellowfin, skipjack, bigeye) during the recent period.

d) Longliners

The longliners exploit all of the study zone, where they capture an average of 24,000 tonnes of yellowfin and bigeye, being 22% of total catches of the three major tunas during the recent period. The current fishing zone of the longliners (figure4.0) is more vast and less coastal than of the purse seiners. This recent situation of the longline fishery is different than that of the historic fishery (1958 to 1969) during which the Japanese longliners fished a zone very close to that which is currently exploited by purse seiners (figure 4.8).

Table 4.10 Total catch of bigeye by baitboats in the study area.

Table 4.11 Total catch of skipjack by purse seiners in the study area.