Table 4.17 Artisanal catches of tunny declared by country in the study zone from 1971 to 1983. (N.B.: from 1950 to 1970, only data from Angola represented in table 4.16 are available.)
Table 4.18 Artisanal catches of auxids declared by country in the study zone from 1965 to 1983. (N.B. from 1950 to 1964, only data from Angola represented in table 4.16 are available.)
Table 4.19 Artisanal catches of bonito declared by country in the study zone from 198 to 1983. (N.B.: from 1950 to 1967, only data from Angola represented in table 4.16 are available.)
Table 4.20 Artisanal and industrial catches of Spanish mackerel in the study zone from 1968 to 1983. (N.B.: The only available industrial catches are those of the USSR.)
Table 4.21 Industrial catches of spotted tunny declared by country in the study zone from 1971 to 1983. (N.B.: from 1950 to 1970, only data from Angola represented in table 4.16 are available.)
|YEAR||ANGOLA||GHANA||F I S||SPAIN||U S S R||OTHERS||TOTAL||GENERAL TOTAL|
The industrial catches of auxids are significant from 1965 – 1970, on the order of 3,000 tonnes per year. They decrease with the departure of Japanese purse seiners from the Gulf of Guinea zone in 1977 (figure 4.14b). During recent years, 1980 to 1983, they increase regularly mostly from FIS and Spanish purse seiners: the catches exceeded 5,000 tonnes per year in 1982 and 1983 (table 4.22). Figure 1.15.b gives the geographical distribution of auxid catches from 1980 to 1983 for which positions are known from French and Spanish purse seiners.
The Atlantic bonito is the major target of Soviet purse seiners (6,000 tonnes per year from 1980 to 1982) (table 4.23), while apparently this is not the case for French and Spanish purse seiners for which there is no landing or discard data for this species in the open seas of the eastern Atlantic (figure 4.14.c).
The industrial fishing for Spanish mackerel is carried out only by USSR fleets using surface gear and reaches some thousands of tonnes in certain years (table 4.20 and figure 4.14.d).
A fleet of Dutch purse seiners search preferentially for plain bonito in the open sea of Cape Blanc in Mauritania in May-June and caught some 1,000 tonnes in 1972 (Maigret et al., 1979).
The bulk of the industrial fisheries data is obtained from USSR fleets for which it is often difficult to distinguish zones and fishing gear.
Other species, notably wahoo, a high seas species, are probably caught with the plain bonito by industrial ships but not declared.
Small tuna catches increase regularly in the zone. They show strong interannual variations, notably during recent years. The tendency is the same in industrial fisheries where the producers show an increasing interest in these species. Efforts to improve the quality of statistics have been made in Senegal (Diouf, 1985) and in the Côte d'Ivoire (Amon Kothias and Bard, 1986). Wise (1985) attempted, from declared catch data, to estimate the local captures on the entire west African coast; he concluded that around 10,000 to 21,000 tonnes per year of small tuna could be captured without being recorded.
Figure 4.15 Schematic map of spotted tunny (a) and auxid (b) catches made in 1980 to 1983 by French and Spanish purse seiners. (Each circle represents one seine set with a capture of the species recorde in the log book; the area of the circle is proportional to the catch.)
Table 4.22 Industrial catches of auxids declared by country in the study zone from 1965 to 1983. (N.B.: from 1950 to 1964, only data from Angola represented in table 4.16 are available.)
|YEAR||ANGOLA||GHANA JAPAN||F I S||SPAIN||U S S R||TOTAL||GENERAL TOTAL|
Table 4.23 Industrial catches of bonito declared by country in the study zone from 1968 to 1983. (N.B.: from 1950 to 1967, only data from Angola represented in table 4.16 are available.)
|YEAR||ANGOLA||U S S R||OTHERS||TOTAL||GENERAL TOTAL|
The size structure of catch of different species is known from measurements taken in Angola and Senegal. Elsewhere in the eastern tropical Atlantic, there is only a small amount of size frequency data.
In Senegal, the artisanal fisheries exploit two size classes: individuals measuring between 30 – 50 cm and those between 60 – 70 cm. Larger sized individuals are exploited from January to June by trolling, while those of sizes smaller than 50 cm are exploited in the warm season and also throughout the year not far from the coast, by trolling (Diouf, 1980). In Angola during 1978–1983, the baitboats unloaded individuals of size mostly between 40–50 cm and the sizes exploited range from 30–60 cm (figure 4.16.a). Catches in the East Atlantic by FIS and Spanish industrial fleets show that from 1977 –1984, the individuals unloaded have a size between 35 and 70 cm in fork length; the most numerous sizes are between 45 – 55 cm (figure 4.16.b). Data obtained elsewhere are again insufficient, but it seems that the intermediate sizes, weakly represented in Senegal are present in a larger proportion in the high seas off Sierra Leone and Côte d'Ivoire (Marchal, 1963; Chur, 1975).
Figure 4.16 Size-frequency distributions of spotted tunny landed by Angolan baitboats from 1978 to 1983 (a) and by French and Spanish purse seiners (b) at Dakar from 1979 to 1983.
Figure 4.17 Size-frequency distributions of auxids landed by Angolan baitboats from 1978 to 1983 (a) and by French and Spanish purse seiners (b) at Dakar from 1979 to 1983.
Very little data on auxid measurements are available in the study zone. In Angola, the size frequency distributions of the baitboat landings from 1979 to 1983 are represented in figure 4.17.a. The catches consist principally of individuals under 40 cm. The smallest individuals appear in this fishery in November-December and are less than 30 cm. The size distribution of auxids fished by FIS and Spanish purse seiners from 1979 to 1983 show slightly larger sizes, from 35 to 50 cm. However, individuals of greater size (50 – 60 cm) have been measured in Spanish purse seiner landings in 1977 (figure 4.17.b).
In Senegal, the majority of fish caught by line consists of individuals of 50 cm modal size from January to June. Beginning May-June, the catches become insignificant in this zone, the rare captures made are of small sized individuals. Trollers capture individual of less than 45 cm near the coasts from January to June. The Atlantic bonito captured by Angolian baitboats range in size from 30 – 60 cm.
Figure 4.18 Changes in “standardized” fishing effort directed to small tunas of the coast of Senegal.
The more numerous classes are between 40 and 50 cm in fork length. The smallest classes arrive in the zone in the fourth quarter.
The size data concerning this species is only available from Senegalese artisanal fisheries. The majority of individuals exploited have sizes varying from 40–70 cm. The largest are captured by passive gillnets and hand lines. Beach seines capture small sizes under 20 cm from June to October. Trollers exploit individuals from 40–60 cm in coastal zones during the same period. The ringnet catches consist of sizes between 40–50 cm in fork length.
Small tuna and related species are not specifically targetted. They are fished at the same time as the large tuna in the industrial fisheries or with other pelagic species targeted by the artisanal fisheries. The apparent spatio-temporal variations of catches are in fact biased by variations in target species catch. Furthermore, their preferential distribution area is often different than that of the principal target species. It is therefor difficult to define with precision fishing seasons for these species from catch data alone. Globaly small tuna are fished in the more coastal zones.
The total effort applied on small tuna is difficult to evaluate because the interest of the fishermen in these species is very variable. An estimation of the effort of the Senegalese tunny fishery has nevertheless been attempted by Diouf (1985). A standard effort, expressed in numbers of trips by trolling canoe (a trip lasts around 3 hours), has been utilized. The effort, higher from January to May, with an average of 42,000 trips per month, decreases from June and stabilizes around 10,000 trips per month (figure 4.18). The trend of this effort reflects the variation in relative abundance of tunny in the Senegalese zone.
The present examination is confined for obvious reasons to the tuna fisheries for which there is a minimum of information available. Catches of coastal countries that exploit tuna are not taken into account the when these catches are not declared. The changes by gear of the size of the fleets active during the study period will be examined. Depending on the fleet, the official figures declared by ICCAT, or when these appear false or improbable, estimates from varied sources will be used. Trends in fishing zone and season by country will next be analyzed. Species and sizes caught by gear will be examined. Finally gross annual catch rates by various gears will be presented for the principal fleets where data are available. These catch rates are generally calculated by dividing the annual catch in the region by the corresponding fishing effort (number of fishing days, number of hooks deployed, etc. …), and therefore do not constitute, in general, a biological indicator of the state of stocks, but rather an index of an economic character. Catch rate indices intended to measure trends in stock abundance will be presented in chapter 8.
It appears that this seasonal artisanal tuna fishery is among the oldest of the East Atlantic as the first data concerning it date from 1948 (Campos Rosado, 1971). The published fishing statistics in the literature and those submitted to the ICCAT enable trends in catch by species, fishing efforts, and captured sizes to be followed in the Angolan fishery.
These are described by Vilela and Monteiro (1959), Costa (1961) and Campos Rosado (1971). Costa and Gill (1965) cite experimental purse seine fishing carried out from 1964 – 1966 along the coasts of Angola. These did not lead to the development of commercial purse seine fishing. Only the pole and line fishing was thus developed; the boats are small decked units with an average length of 12 meters and equipped with a motor of around 100 hp; the crew consists of 10 – 12 men. The fishing is done with live bait which is captured at night very near the coasts, then conserved in a reservoir placed on deck. The boats generally stay at sea a dozen hours and do not go further than 40 miles from port.
The fishing zones were, according to Campos Rosada (1971), principally the Benguela zone and to a lesser degree those of Namibia (Moçâmedes) and Lucira. The area exploited by this fleet is estimated by this author to be 11 one degree squares. No detailed information is available on recent fishing zones, apart from the statistics given to ICCAT by month and 5° geographical square since 1977. Examination of these results indicates a certain stability of fishing zones in the recent period relative to previous fisheries.
The catch numbers published in different ICCAT statistics bulletins permit a close examination of catch trends (tables 4.9 and 4.10). It must be noted that bigeye catches are generally confused in the statistics with those of yellowfin (Campos Rosado, 1971) and are therefore probably underestimated (to an unknown degree). The prominent fact in the specific composition is the marked scarcity of yellowfin catches during 1950 – 1970 and its replacement in catches by skipjack. (This change is unique in the Atlantic and does not seem to have been observed in the symmetric zone of the northeast tropical Atlantic.)
Table 4.24 Mean weight of yellowfin caught by the principal fleets operating in the study area.
|F I S||ANGOLA||TEMA||CAPE VERDE||F I S||U S A||SPAIN||JAPAN||ALL COUNTRIES|
* Fishery, but data unavailable
- No fishery
The yellowfin fished are small: 35–70 cm; the skipjack are between 35–52 cm; the bigeye present generally three modes around 40 cm, 60 cm and 90 cm. For the small tunas, the catches by all fleets in other sectors of the region can be compared for the three principal species (tables 4.24, 4.25 and 4.26).