Previous Page Table of Contents Next Page


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

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.)

YEARANGOLAGHANASENEGALOTHERSTOTAL
1971174800501798
197297700501027
19737720050822
19748790537501466
19758601092501228
197602370550778
197767901540502269
197850159811446507978
197918454161697507347
198049241171921316561
1981439290026801276146
1982620152342141626519
1983453500945728410118

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.)

YEARANGOLAGHANAOTHERSTOTAL
19651718001718
19661475001475
19671205220003405
1968631180002431
1969738300003738
1970512300003512
1971920270003620
19722064512707191
1973682260708
19741299629507594
1975435600106436
19760431104311
197715513914014069
1978211104701258
197977428604363
198065756607631
1981692048642181
19821281009721209
198363357003633

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.)

YEARANGOLASENEGALOTHERSTOTAL
196841400414
196969200692
197084000840
197180500805
197246700467
197349000490
197434343350
197523402083
1976411640205
197742961491052
19784145239946
197920415913376
198033814043521
198113261532779
1982206920361162
19837956016655

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.)

YEARANGOLAGHANASENEGALOTHERSTOTALUSSR*TOTALGENERAL TOTAL
196801800001800018001800
19690250000250020027002900
19700350000350020037003900
197107000070060013001900
19720150000150060021002700
19730100000100060016002200
19743483513540391580047155515
19750598314091222811401368
197605551270018257619011977
19772072011880192864425723216
1978817711054019064810671611526
19792415691112232728143941675606
1980704412404974983049834983
19816819834891482688026882688
198213829827731003489660254986100
1983138222510988514312117054826652

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.)

YEARANGOLAGHANAF I SSPAINU S S ROTHERSTOTALGENERAL TOTAL
1971660001000508102608
19722570033401507411768
197319826028001646681490
1974408661583250039827054171
19753631308604016015172745
19761076400047018011361914
197764754431187869010338036072
1978325683806127202676014738
19794621315702184112294610293
198083617972836307125834014901
19817323681850443615320692913075
1982111461725061561085310578812307
19831179032342396528571123721355

Auxids

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.

Atlantic bonito

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).

Spanish mackerel

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).

Plain bonito

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.

4.4.2.3. Conclusion

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
Figure 4.15

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.)

YEARANGOLAGHANA JAPANF I SSPAINU S S RTOTALGENERAL TOTAL
196509020009022620
196604090004091884
19670106800010684473
19686281100028175248
196939338500034247162
19709380400038137325
19712516350008864506
1972222137400015968787
1973437301600034534161
19742374860007238317
1975100200001206556
19762714001551964507
19774289026824264114710
19781460008039492207
19792800004507305093
1980191003047694393211563
1981282072057340719824163
19823870158016055623919510404
1983149014901734165550288661

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.)

YEARANGOLAU S S ROTHERSTOTALGENERAL TOTAL
196821000102516
19694030003401032
1970300030870
1971440044849
1972812000281748
1973924033523
19748140218915991949
197515154229118481931
197679012818621572362
1977409416413947125764
197811716923518442790
1979472125021722548
19803964336465367057
19816445598147045483
198247632929266687830
198345237575231723827

4.4.3. Size structure of catches

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.

Spottecd tunny

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

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

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.

Auxids

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).

Atlantic bonito

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

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.

Spanish mackerel

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.

4.4.4. Fishing seasons

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.

4.4.5. Fishing Effort

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.

4.5. EXAMINATION BY COUNTRY OF THE CHANGES IN TUNA BOAT FLEETS EXPLOITING MAJOR TUNA IN THE EASTERN TROPICAL ATLANTIC

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.

4.5.1. Angola

4.5.1.1. Generalities

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.

4.5.1.2. Boats and fishing methods

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.

4.5.1.3. Fishing zones and seasons

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.

4.5.1.4. Species caught

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.

YEARBAITBOATSSEINERSLONG LINERS
F I SANGOLATEMACAPE VERDEF I SU S ASPAINJAPANALL COUNTRIES
1956** *----80.2
1957** *----69.0
1960** *----56.1
1961** *-*--*
1962****-*--56.4
1963*****-*-53.4
1964*****-***
1965  9.7***28.7-**51.1
1966  7.8***31.7-**53.7
196715.5***33.8-**54.9
196814.5***27.011.0**49.7
196911.0***21.122.7*40.940.9
1970  5.4***16.3  6.0*12.560.4
1971  6.1***13.414.1*13.254.8
1972  6.9***16.313.4*25.347.8
1973  8.2***20.817.4*30.645.6
1974  8.4***16.520.3*30.450.9
197512.22.93.2*20.013.6*26.051.8
1976  5.92.93.6*18.117.5*-54.4
1977  9.03.04.8*19.3  7.8*-46.6
1978  7.03.04.3*22.9  7.922.3-50.8
1979  6.12.83.5*26.119.322.4-45.6
1980  5.73.63.024.718.8  6.912.6-50.5
1981  7.03.22.217.520.611.4  9.9-46.6
1982  5.03.32.918.113.911.311.1*48.6
1983  8.1---15.9-***

* Fishery, but data unavailable
- No fishery

4.5.1.5. Sizes captured

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).


Previous Page Top of Page Next Page