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

Table 4.30 Bigeye catch rates by different fleets in the study area.

- no fleet
* yleld unknown

4.5.8.3. Fishing zones and seasons

Fishing zones and seasons of the FIS fleet have changed considerably in different periods. In order to facilitate the discussion of this change, distinguish four characteristic periods of the development of the fishery can be distinguished:

1) 1953 – 1960:

The initial phase of exploitation of the northern zone (region of Dakar), essentially by ice carrying baitboats based in Dakar. Fishing only took place during the winter months (November to May) alternating with albacore fishing in the Gulf of Gascogne;

2) 1961 – 1968:

Major development of small pole and line boat fleets and small freezer purse seiners (less than 200 tonnes carrying capacity) that rapidly exploited all of the coastal zone between Mauritania and Angola (the equator was crossed starting in 1962), in a fishing zone comparable to that exploited until 1974 (figure 4.10.a). The first attempts at purse seine fishing took place in 1962 (purse seiner DANGUY). Commercial purse seine fishing developed rapidly beginning in 1964 as a result of tuna boats specially constructed for this purpose. These fleets were based in the ports of Dakar, Abidjan and Point Noir. The catches were marketed by the French company SOVETCO (Société de Vente du Thon Congelé) that assured complete statistics of all catches by freezer tuna boats. Fishing remained seasonal, with reduced activity from June to September although boats tend to fish all year.

Table 4.31 Skipjack catch rates by different fleets in the study area.

3) 1969 – 1974:

Rapid development of the large purse seine fleet and decrease in the number of freezer baitboats and small purse seiners. The fishing zone of purse seiners remained coastal (figures 4.22 and 4.23d). As during the preceding period, the fishery was characterized by the existence of geographically and temporally localized fishing seasons; Cape of Three Point in August and September, Cape Lopez in June and July, etc…

4) 1975 – 1983:

The freezer baitboats and small FIS purse seiners almost completely disappearance, while effort by the large purse seiners increased, extending their fishing zone toward the open sea of (figure 4.25). The pole and line boats only exploit the northern zone between Mauritania and Guinea from May to November (figure 4.24). The new principal fishing season that developed by large FIS purse seiners exploiting large yellowfin in the equatorial zone during the first quarter of each year. It is noted however that the FIS fleet seems to target more and more for skipjack, especially from the year 1981 (Fonteneau, 1986). This change in fishing strategy is apparent seasonally when the FIS fleet exploits the zone situated to the north of the equator and the offshore areas off Liberia during the last quarter (figure 4.25.b).

Table 4.32 Numbers of tuna boats by category and fishing mode for the FIS fleet from 1953 to 1983 (by year of landing).

4.5.8.4. Species caught

The changes in annual FIS catch by species and by gear are given in tables 4.8 to 4.14. The specific composition used results, not from log books, but from estimates made by scientists (CICTA, 1984). The historical series of figures differs slightly from those published by ICCAT; the present figures result from recent corrections made on the FIS statistics bulletins. One notes that during the “historic” period, 1953 – 68, yellowfin was almost the only species caught by the FIS fleet. During 1969 – 1980, skipjack became progressively more and more significant in the catches. Since 1981, one can consider the skipjack is really a target species for the FIS fleet in the same way as yellowfin.

4.5.8.5. Sizes caught

Tables 4.24 to 4.26 give the change in average weight of yellowfin, skipjack and bigeye caught by pole and line boats and purse seiners of the FIS fleet from 1965 to 1983. Skipjack and bigeye sizes are relatively stable during the entire fishery. Yellowfin are, on the other hand, small in the historical fishery (1953 – 1968). Catches of large yellowfin (+ 30 kg) increased significantly from 1969, and especially from 1975, principally following the expansion of fishing zones in the sector of the equator where these large yellowfin are frequent. The catches of small yellowfin (for example, less than 5 kg) also increase in the FIS purse seine catches where there leads to certain stability in average weight, in spite of increased catches of large yellowfin. Figure 4.26 and 4.27 give several examples of size frequencies characteristic of FIS purse seiners and pole and line boats.

4.5.8.6. Catch Rates

Catch rates of the FIS fleet have been presented to ICCAT from 1969; these results are given in tables 4.29 to 4.31 and in figures 4.28, 4.29 and 4.30. Catch rates from 1955 – 1968 have been published by various authors in publications from the centers in Dakar, Abidjan and Pointe Noire to which reference can be made (Champagnat, 1968; Champagnat and Lhomme, 1970; Baudin Laurencin et al., 1970; Le Guen et al., 1968; Marcille et al., 1969). These data on catch rates are difficult to interpret as they are based on landing statistics; in addition, these fishing efforts are not standardized.

The following comments can be made for each gear:

• The FIS pole and line boats have very stable yellowfin catch rates between of 1.0 and 1.5 tonnes/day during 1969 – 85 (average 1.6 tonnes/day). Skipjack catch rates increase from a mean low level of 0.7 tonnes/day, in 1969 – 76, toward a moderate level of 1.2 tonnes/day from in 1977 – 1983. Bigeye catch rates increased regularly from less than 0.4 tonnes/day until 1975 when they surpassed 0.7 tonnes per day (average 1977 – 1983 = 1.0 tonnes/day). These changes are linked, among other factors, to changes in pole and line fishing zones and seasons. Fishing year around between Dakar and Angola until 1975, the pole and line boats exploit currently only the zone situated to the north of 10° N during summer, the region where bigeye are particularly abundant.

• The middle sized FIS purse seiners have relatively stable yellowfin catch rates, but decreasing slightly between 1969 and 1983 (average 1969 – 1983 = 2.4 tonnes/day). Skipjack catch rates, on the other hand, have a marked increase between 1969 – 1976, catch rates less than 2 tonnes/day (average = 1.3 tonnes/day), and 1977 – 1983 where these catch rates generally exceed 2 tonnes/day. Bigeye catch rates oscillate without marked trends at a moderate level of 0.12 tonnes/day (1969 – 1983).

• Large purse seiners have relatively stable yellowfin catch rates through 1979 (average 1969 – 1979 = 5.3 tonnes/day) but which have decreased to less than 3 tonnes/day since 1982. Skipjack catch rates oscillate without marked trends between 1 and 3 tonnes/day from 1969 – 1983 (average = 2.0 tonnes/day). Bigeye catch rates are also without trends, but low (average 1969–83 = 0.14 tonnes/day) and very variable from one year to another.

4.5.9. Ghana

4.5.9.1. Generalities

The artisanal fleets of Ghana are very probably among the most ancient to have exploited Atlantic intertropical tuna. For example, Irvine (1947) cites traditional catches of tuna (bigeye) and sailfish by canoes fishing with lines. With regards industrial tuna fishing there was an attempt at fishing by Ghana purse seiners in 1964 that ended in a commercial setback and a rapid half of this fleet. The current fleet of Ghana tuna boats developed in 1973 with pole and line boats and since 1980 with purse seiners. The fishing statistics pertinent to this fleet are relatively incomplete and fragmented and unfortunately do not permit the close examination of fleet activities, in particular with regard to fishing zone.

4.5.9.2. Boats and fishing methods

Three fleets can be recognized in Ghana: the artisanal fishing fleet, pole and line boats and purse seiners.

• Artisanal tuna fishing is for Ghana traditional and extremely active. Occurring in this fishery are multiple fishing gears: ringnets, gillnets, trollers, beach seines, etc. …

• The pole and line boats are classical Japanese type tuna boats. The Ghana pole and line boat fleet developed principally by transfer of flag from Japan to Ghana. The pole and line boats of Ghana started operation in 1973 (2 boats) but have especially increased their fishing effort from 1980 (10 boats) to 1983 (28 boats) (table 4.4.).

• The Ghana purse seiners are large American and Norwegian types. No description has been published on this fleet.

Figure 4.22 Quarterly catches of yellowfin FIS pole and line boats, in tonnes, by 1° square; average for the period 1969 to 1974. (The catches observed each year are superimposed on the 1° sectors).

Figure 4.23 Quarterly catches of yellowfin FIS purse seiners, in tonnes, by 1° square; average for the period 1969 to 1974. (The catches observed each year are superimposed on the 1° sectors).

Figure 4.24 Quarterly catches of yellowfin (a) and skipjack (b) by FIS baitboats, by 1° square; period 1975 to 1983, in tonnes (The catches observed each year are superimposed on the 1° sectors).

Figure 4.25.a Quarterly catches of yellowfin by FIS purse seiners, by 1° square; period 1975 to 1983, in tonnes (The catches observed each year are superimposed on the 1° sectors).

Figure 4.25.b Quarterly catches of skipjack by FIS purse seiners, by 1° square; period 1975 to 1983, in tonnes (The catches observed each year are superimposed on the 1° sectors).

Figure 4.26 Examples of size-frequency distributions of yellowfin, skipjack, and bigeye caught by FIS baitboats.

Figure 4.27 Examples of size-frequency distributions of yellowfin, skipjack, and bigeye caught by FIS purse seiners.

Figure 4.28 Catch of yellowfin per nominal unit of effort (in tonnes per day) for the principal baitboat (a), purse seine (b), and longline (c) fleets.

Figure 4.29 Catch of skipjack per nominal unit of effort (in tonnes per day) for the principal baitboat (a) and purse seine (b) fleets.

Figure 4.30 Catch of bigeye per nominal unit of effort (in tonnes per day) for the principal baitboat (a), purse seine (b), and longline (c) fleets.

4.5.9.3. Fishing zones and seasons

No precise data are available. The Ghana purse seiners seem to exploit only a relatively reduced fishing zone in the fishing grounds close to Ghana. The Ghana pole and line boats exploit fishing grounds comparable to those of the Japanese pole and line boats (figure 4.35).

4.5.9.4. Species

Estimates of catch by artisanal fisheries are submitted to ICCAT and result in a sampling system of catch and effort, stratified by gear and zone. Small tuna and sailfish, along with skipjack, are the principal species caught by the artisanal fisheries of Ghana.

Catches by species of major tuna by pole and line boats and purse seiners are available at ICCAT (tables 4.8 to 4.14). The specific composition of pole and line catches is stable and very characteristic of this fleet, with a strong dominance in skipjack catches and the presence of a small variable percentage of yellowfin and bigeye. The actual proportion of these two species, yellowfin and bigeye, has been the object of numerous divergent estimates due to diverse causes (paragraph 4.5.10.4).

The purse seine catch generally shows yellowfin and skipjack catches of equal importance with reduced bigeye catches. From 1980 to 1983 the following average specific composition was observed: yellowfin = 47%; skipjack = 45%; bigeye = 4%. These numbers are comparable to those of other purse seine fleets that operate in the zone.

4.5.9.5. Sizes caught

Sizes caught by pole and line boats of Ghana are well known thanks to routine systemic sampling of landings carried out since 1973 by the Fishery Research Unit (FRU) of Tema. The tuna are generally small (figure 4.31), less than 5 kg, with sizes of the 3 principal species (yellowfin, skipjack and bigeye) being very comparable as the fishery exploits mixed schools of small tuna.

The sampling of Ghana purse seine catches has remained mediocre since the origin of this fleet. The sizes of caught skipjack are probably comparable to those of FIS fleet catches because of low variability in skipjack sizes. With regard to yellowfin, the small number of individuals measured (300 fish measured/year on average, for a total average annual catch of 3,300 tonnes) does not allow a confident estimate of caught sizes. This samplings does indicate however that the Ghana purse seine catches may be composed of small fish (mean weight under 10 kg), a weight well under that of yellowfin catches by FIS and Spanish purse seiners.

The sizes caught by the artisanal fishery are not subject to ICCAT declarations. They remain unknown.

4.5.9.6. Catch Rates

The Ghana pole and line boat catch rates are included with the Tema fleet (combining the pole and line boats of Korea, Japan, Panama and Ghana), because of the uniformity of this fleet. The catch rates given in table 4.33 are calculated in the following manner:

• catches by species are in tables 4.9, 4.10 and 4.14.

• efforts from 1972 to 1981 are total hours at sea collected by the FRU. of Tema.

• effort from 1969 – 1971 and 1982 – 1983 are estimated assuming that tuna boat has 165 days at sea by year, average calculated from 1972–81 (excluding 1975, year of reduced activity).

• catch rates are calculated by dividing catch by estimated effort (in non standardized days at sea).

One notes that the catch rates of pole and line boats of Tema are particularly high with a strong skipjack dominance.

Figure 4.31 Examples of size-frequency distributions of yellowfin, skipjack, and bigeye caught by Tema baitboats.

Table 4.33 Catch rates estimates by species for the Tema baitboats (Japan + Ghana + Korea + Panama).