A. Fonteneau, T. Diouf and M. Mensah
The object of this chapter is to present a synthetic assessment of changes in tuna fisheries of the study zone. The various fishing methods active in the region will first be described (paragraph 4.2), then the catch trends in the region will be analyzed by species, vessel type and zone for the major tuna (albacore, skipjack and bigeye) in paragraph 4.3 and for the smaller tuna and related species in paragraph 4.4. A summary of the changes in various fisheries will be presented by country in paragraph 4.5.
Most of the statistical data in this chapter, unless otherwise stated, are from the ICCAT data base. The numbers employed are the best statistical estimations available at the time of writing this chapter; they are sometimes different from certain numbers published by the ICCAT as a result of frequent corrections to the data base. Two important points to be noted at this stage:
the grouping of fishing statistics from French, Côte d'Ivoire and Senegalese vessels into a fleet historically called FIS.
the specific composition of small yellowfin and small bigeye captures has been the object of numerous revisions and remains problematic. This is due in part to significant difficulties distinguishing these two species when they are small and also to size limits of 3.2 kg imposed by the ICCAT on yellowfin in 1972 and on bigeye in 1978.
The study region is without exception that defined in the introduction (figure 1.1.). The catch/effort data tables in this chapter correspond to this zone; in order to facilitate the understanding of various fisheries, in particular those by longline and those for bigeye, the maps intentionally cover a zone more vast than that accepted for the assessment of fisheries. (In particular the zone situated at the south of 5°S and to the west of 5°W).
Although the statistical catch tables for all fleets and all years supply the best current estimates, the cartographic representations of each fleet are, on the other hand, based on variable periods depending on the active periods of boats, changes in their fishing strategy, and on the existence of fishing statistics by month and 1° geographic square (ICCAT standard) permitting the making of these maps.
Figure 4.1 Schematic representation of the three principal modes of tuna fishing in the Eastern Atlantic: (a) French type pole and line boat; (b) large purse seine; (c) longliner.
Fishing tuna by pole and line or baitboat consists of attracting and retaining the fish from a school with the aid of live bait thrown in the sea. The technique has been described by several authors; Postel (1969), Gobert (1983), Portais (1986). The most desirable bait consists of clupeids (gilt sardinees, sardines, anchovies …) or small carangids. It is generally caught with the aid of a small seine or ringnet, then conserved live in tanks of variable dimensions. Some bait tanks are included in the boat's construction and are fed by pumps that sustain a total renewal of water 4 to 6 times per hour; others are embarked and placed on the deck for each trip, notably on the small sized pole and line vessels; the water is then renewed manually by decanting. If the reactions of the school after the baiting are judged to be satisfactory, the boat is stopped and the water jets are activated to excite the fish and form a screen between the men and the fish; then the fishing commences.
The bamboo or fiberglass rods are 3 to 5 m in length with nylon lines and are rigged with feathered or bare hooks. Each hook has a live fish pierced through the middle of the body, in which the wriggling attracts the attention of the tuna and makes it bite. During the period of fishing, baiting continues in order to maintain tuna activity. When the bite is on, one lure alone is sufficient to trick the fish; the rate of active fishing can accelerate. Certain boats are provided with halyard rods run through a pulley. The halyard serves to hoist the large fish: it is pulled by two fishermen while the rod is retained by a third fisherman.
The size of pole and line vessel is variable: from a dozen meters to around 45 meters. Figure 4.1 shows a diagram of a French pole and line vessel with its fishing accessories.
After having located a sufficiently compact tuna school, purse seine fishing consists of encircling the school with a closing net which can be progressively reduced in volume. The technique has been described by several authors: Neely (1962), Postel (1969), Marcille (1969), Stequert and Marsac (1983). The purse seine vessels and their equipment had evolved rapidly. The old purse seiners were boats of 30 to 45 m in length carrying purse seines with a length of 600 m to 800 m and a drop of 75m to 100 m. Some of these boats seined with live bait until the mid 70's, the fishing technique consisting of encircling tuna schools after having stopped them with bait. Temporary associations between the bait boats and purse seiners can still be observed at certain periods in Senegal in particular: the bait boat stops the school with bait and permits the purse seiner to encircle it with his net; the catch is then shared between the two boats.
Marcille (1969) described the fishing method practiced by Japanese purse seiners during 1965 – 1975. Fishing was done by three groups of purse seiners each comprised of 3 to 4 vessels. The two principal boats of each group navigated in pairs transporting in halves a purse seine of 2000 to 2200 m long and 240 m in drop. They separated only at the moment of encircling the school, assisted by the towing of the 2 accompanying vessels. The transfer is done at sea to freezer boat that accompanies each group. Because it is difficult in areas of strong currents and requires a large crew, this type of fishing has been abandoned in favor of large classical purse seiners fishing individually.
The tuna purse seiners, currently used in the Atlantic, are boats of 45m to 75m. They use a nylon purse seine net approaching 1500 m in length and 185 m in depth.
On the fishing grounds, the most important activity of the crew consists of searching for signs at the sea surface signaling the presence of tuna. The usual signs are birds, cetaceans, flotsam and jetsam, or white foam generated by feeding. The watch is assured by means of powerful binoculars installed on the bridge or on the crows nest that often rises more than 15 m above the sea. Searching can also be done by helicopter or airplane. Sounders and sonars are frequently used to detect schools and to help guide their encirclement.
As soon as a school of tuna is spotted, the purse seiner approaches; the skipper evaluates the size and behavior of the school, then takes note of the meteorological conditions (wind, swell, current) and casts off a small boat or skiff that holds the net. The purse seiner circles the school and eventually returns to the position of the skiff where it is recovered at the extremity of the net; this operation lasts around 15 minutes. Once circled, the purse line is hauled which closes the net. The purse seine then forms an immense pocket with a bottom situated around 70 – 90 m in depth. The skiff keeps the vessel outside of the net during its recovery. This phase lasts around 30 minutes. These maneuvers are sketched in figure 4.2.
The fish concentrated in the pocket are loaded by a brail, a type of large landing net 2 to 3 tonnes in capacity, handled by the aid of a hoist and special boom. The duration of these operations of brailing depends on the size of the catch; the rate of a brailing is around every three minutes. The contents of the brail are poured in a collector that leads to the vessel interior where the first sorting takes place. Next the fish is diverted into freezer wells that contain brine previously cooled to -18°C. Once frozen, the tuna will be stored dry in the same wells. A light circulation of brine is continuously maintained in the tanks in order to avoid the “sticking” of the fish.
All of these operations together constitutes a set or haul. The duration of sets is variable according to the category of purse seiner, the type of set (positive or empty) and the fleet. Fonteneau et al. (1986) have analyzed the relations between the durations of sets and the catch per set of FIS and Spanish purse seiners. The following relations have been obtained (figure 4.3):
T = 1.34 + 0.0205p for Spanish purse seiners
T = 2.67 + 0.0310p for FIS purse seiners
(T is the duration in hours and hundredths of an hour and p the catch in tonnes); these relations are calculated excluding the empty sets.
The duration of positive (non-empty) sets was on average 2 hours and 15 minutes for the Spanish fleet in 1980–1981 and 3 hours and 14 minutes for the French purse seiners. The null (empty) sets on Spanish purse seiners lasted an average of 1 hour and 40 minutes, and 2 hours and 37 minutes on the French. These large differences in the duration purse seine sets are due to differences in equipment used to recover and stack the net on board the tuna boat (opening or closing rings).
Figure 4.2 Different phases in the deployment of a purse seine (Stéquert and Marsac, 1983). a, b, c, d: launching the skiff and encirclement of the school; e, f: closing the net and formation of the purse.
Longline fishing consists of mooring, in open water, passive longlines carrying hooks distributed in a regular fashion and baited with dead baitfish. The technique was developed by the Japanese and is used mainly by Asiatian fishermen, and also by several countries bordering the Atlantic (Cuba, Venezuela). It is also described by Postel (1969), Woo Il Choo (1976), and Weeb (1973). A longline consists of a collection of elements, each contained in a basket. Each basket of the longline contains 360 meters of main line, 20 meters of float line and 7 hook lines of 22.5 meters each. The number of baskets placed end to end at each set varies from 220 to 500. Given 7 hooks per basket, there are about 1500 to 3500 hooks for a length of line from 80 to 180 kilometears (figure 4.4). These lines are set and retrieved every day. The bait almost universally employed is a belonid, Cololabis saira (saury), a species found only in the north Pacific which the fishermen carry frozen world wide. The hooks of the traditional longline are moored at a depth of 50 to 120 m. A longline set generally begins at about 0200 hours and is finished by about 0800 hours. It is totally manual and done while the boat is moving at a speed of eight knots. The boat moors 2 to 3 hours near the last buoy placed in the water.
The raising of the longline is semi-mechanical. It starts with the last basket set at about 100 hours and continues until the next day. The speed of the boat is about 3 knots. The main lines are raised by a longline winch and then the hook lines are hauled by the fishermen. The fish caught are
Figure 4.3 Relation between catch and duration of positive sets for FIS and Spanish purse seiners based on a sample of purse seiners observed from 1980 to 1983.
Figure 4.4 Schematic representation of a longline (Suzuki, et al., 1977): (a) ordinary longline, (b) deep longline and a branch line of a longline.
brought aboard with a harpoon or a crane. The duration of a longline haul is a function of the size of the catch.
Another variety of this device is the deep longline. It is described by Suzuki et al. (1977), and was introduced by the Japanese in the Atlantic starting in 1980 (Suzuki and Kume, 1982). It is not fundamentally different of the classic longline, but is comprised of 10 hooklines or more per basket, and 2 times less buoys. The hooks are set at a depth of 50 to 250 m. The deep longline is targeted to capture the large bigeye abundant in deep waters. Figure 4.1.c gives the profile of a modern longliner.
The artisanal fisheries, localized along the west African coast, use different surface methods. In Senegal, Mauritania and in the Cape Verde Islands, baited hand lines, trolling, ringnets, beach seines and gillnets are the most used. In the Côte d'Ivoire and Ghana the artisanal fisheries utilize gillnets, shore nets and small sardine ringnets (Marchal, 1963). In Angola and the Sao Thome and Principe Islands, gillnets and fixed surface nets are also used.
These fisheries use small craft for the most part equipped with outboard motors and at times with oars or sails. The fishing techniques are classic, but can present variations in accordance with the countries. In Senegal trolling is performed on canoes equipped with a motor of 8 hp, with 1 to 2 people on board simultaneously trolling two lines of 8 to 12 meters, rigged with 60 gm. metallic spoons or hooks with lures specially made by the fishermen (Diouf, 1981).
The tables 4.1 to 4.3, and the figures 4.5 to 4.7 give the annual catch by gear of yellowfin, skipjack and bigeye since the beginning of industrial tuna fisheries in the study zone. The activities of different gear in operation in the region (paragraph 4.3.2) will first be summarized, then the catch trends by species and gear during the period of 1950 to 1983 (paragraph 4.3.3) will be described. The geographic distribution of mean catches by species and region of yellowfin, skipjack and bigeye, during the recent period will be examined next in paragraph 4.3.4.
The exploitation of tropical tuna on an artisanal scale on the west African coast is probably a very ancient phenomenon, the origin of which is difficult to determine. The principal canoe fisheries, for example in Ghana and Senegal, have probably captured small quantities of tuna for several centuries. Villaut de Bellefond cited for example in 1669 captures of “bonites” (skipjack) by these fisheries of Senegal and Sierra Leone.
These artisanal and semi-industrial fisheries of the former Portugese colonies, Cape Verde Islands, Sao Thome and Angola, also possess subsistence tuna fisheries, probably very ancient, making their origin difficult to determinate because of lack of statistics and available references. The global volume of these historical captures was probably always very modest, at the maximum several thousand tonnes.
Table 4.1 Total catch of yellowfin by gear in the study area.
|YEAR||BAIT-BOATS||SEINERS||LONG-LINERS||VARIOUS AND NON-IDENT.||TOTAL|
Table 4.2 Total catch of skipjack by gear in the study area.
|YEAR||BAIT BOATS||SEINERS||VARIOUS GEAR||TOTAL|
The first pole and line boats, above all the live bait boats, exploited tropical tuna during the years 1953 to 1956 (table 4.4) in the Dakar-Senegal zone. It was French (Bask and Breton) and Spanish (Canary Islands and Bask) tuna ice boats that practiced seasonal fishing during winter alternating with summer albacore fishing in the North East Atlantic.
The catch rates obtained at that time were in general excellent in spite of the modest means employed, these fisheries rapidly developed and around 50 French baitboats operated seasonally in Dakar since 1957. The first fishing with freezer baitboats of the American “clipper” type started in 1954 in Dakar; However, it was necessary to wait until the beginning of the 1960's in order to see the French introduce a large fleet of 30 American style freezer-baitboats that practiced fishing in all of the eastern tropical Atlantic (the equator was crossed at the end of 1962), while the ice baitboats, especially the French and Spanish, were stationed in the coastal region between Cape Palmas and Mauritania.
Almost simultaneously in 1962, the Japanese freezer-baitboats began to exploit the end of the Gulf of Guinea searching principally for skipjack, while the other fleets of baitboats searched principally for yellowfin. The effort of the baitboat fishing fleet based in Ghana, at Tema, reached a high level from 1970 (around 30 boats) with a progressive transfer of flag to Korea and Ghana. Simultaneously, a progressive decline of most of the other French and Senegalese freezer-baitboats was observed while the French ice-baitboats stayed at a relatively stable level.
Table 4.3 Total catch of bigeye by gear in the study area.
|YEAR||BAIT BOATS||SEINERS||LONG LINERS||VARIOUS AND NON IDENT.||TOTAL|
Figure 4.5 Change in yellowfin catch by gear in the study area from 1950 to 1983.
Figure 4.6 Change in skipjack catch by gear in the study area from 1950 to 1983.
Figure 4.7 Change in bigeye catch by gear in the study area from 1950 to 1983.
The first Japanese longliners appeared in the Gulf of Guinea in February 1957. These boats obtained excellent catch rates, especially of large yellowfin, that brought about a very rapid growth in the fleet. Around 200 boats operated in the region at the beginning of the 1960's and reached their maximum activities during 1961 to 1965.
These Japanese longliners were progressively replaced by Taiwanese (1966) and Korean (1968) longliners, in particular by a succession of flag transfers. The longliners although they are recently in a light regression, maintain a high level of effort in the region. A new partner, Cuba, came in the mid 1970's to exploit the eastern Atlantic with longliners and have remained in the zone until the present (1985) with a high effort level.
One notes between the historical and recent period a change of fishing zones of longliners that, at the beginning, exploited the coastal zone of the Gulf of Guinea (figure 4.8), and that subsequently exploited primarily the high seas (figure 4.9) situated at the exterior of the purse seine fishing zone.
Tuna fishing by purse seine is a technique old relatively, practiced after the 1939–45 war on the coast of California. This technique was not, on the other hand, introduced until recently into the Gulf of Guinea. One finds among the first positive attempts of purse seine fishing the voyages of the American purse seiner May Flower in 1960 and those of the Spanish purse seiner Marinero in 1961 along the coast of west Africa. One had to wait for the year 1964 to see the first French and Spanish purse seiners in operation in the region in commercial exploitation. They were boats of small capacity, generally 100 tonnes and equipped with a small purse seine 800 m long and around 100 m deep. During this experimental phase of seining, several countries (Norway, Ghana, Yugoslavia) attempted test fishing without commercial success and rapidly abandoned purse seine fishing.
Table 4.4 Numbers of baitboats active in the study area for the principal tuna fleets.
|YEAR||ICE BAITBOATS||FREEZER BAITBOATS||TOTAL|
* Figure unknown, catch unknown
() Order of magnitude estimate
+ Figure unknown, catch known
- Reduced activity, figure unknown
Several purse seiner fleets rapidly joined these first boats: Japanese purse seiners at the end of 1964, American and Canadian purse seiners in 1967, Senegalese purse seiners in 1969, purse seiners from the Côte d'Ivoire in 1970, etc. …(table 4.5).
The notable fact in change in the purse seiners is, besides the regular increase in fishing effort, the considerable increase in size of tuna boats and purse seines. The first large purse seiners in operation in the zone (capacity of more than 400 tonnes) are those of the U.S.A. in 1967. Since that date, the new purse seiners are for the most tuna boats with a capacity of 400 to 1500 tonnes, derived directly from large American purse seiners.
With regard to fishing zones, one notes that the purse seiners exploited the coastal zone until 1974 (figure 4.10a), then exploited the open sea zones (in addition to the coastal zones) from 1975 (figure 4.10b). This extension toward the open sea fishing zones has had the effect of increasing the the capture of yellowfin of a weight significantly greater than those that were caught until 1974 in the coastal zone.
|Figure 4.8||Average catches by Japanese longliners, in number of individuals, of yellowfin and bigeye; mean for the period 1957 to 1969.||Figure 4.9||Average catches by Cuban, Taiwanese, Korean and Japanese longliners, in tonnes, of yellowfin and bigeye; mean for the period 1957 to 1969.|
Table 4.5 Numbers of purse seiners active in the study area.
|GHANA||MOROCCO||YUGOSLAVIA||G R. CAYMAN||USSR||NORWAY||PORTUGAL||CONGO||CUBA|
Figure 4.10 Average catch by 1° square, total of the three species, yellowfin, skipjack and bigeye, by surface fleets during the period 1969 to 1974 (a) and during the period 1975 to 1982 (b).