SYNOPSIS OF BIOLOGICAL DATA ON BLUEFIN TUNA Thunnus thynnus orientalis(Temminck and Schlegel) 1842, LONGFIN TUNA Thunnus alalunga (Bonnaterre)

1788, YELLOWFIN TUNA Thunnus albacares (Bonnaterre) 1788 and BIGEYETUNA Thunnus obesus (Lowe) 1839 (SOUTH AFRICA)

Exposé synoptique sur la biologie du thon rouge Thunnus thynnus orientalis(Temminck et Schlegel) 1842, germon Thunnus alalunga (Bonnaterre) 1788,

thon à nageoires jaunes Thunnus albacares (Bonnaterre) 1788 et patudoThunnus obesus (Lowe) 1839 (Afrique du Sud)

Sinopsis sobre la biología del atan rojo Thunnus thynnus orientalis(Temminck y Schlegel) 1842, albacora Thunnus alalunga (Bonnaterre)

1788, atn de aleta amarilla Thunnus albacares (]onnaterre) 1788 y patudoThunnus obesus (Lowe) 1839 (Sudûrica)

P'repared byB,. DE JAEGER

Division of Sea FisheriesCape Town, South Africa

FISHERIES DIVISION, BIOLOGY BRANCHFOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONSRome, 1963

588

Species Synopsis No. 18FAO Fisheries Biology Synopsis No. 61 Flb/S61(Distribution restricted) SAST - Tuna

FIb/S61 Tuna 1:1

yi IDENTITY

1.1 Taxonomy

1.1.1 Definition

Phylum CHORDATASubphylum Vertebrata

Superclass GnathostomataClass Osteichthyes

Subclass ActinopterygilSuperorder Teleostei

Order Perc-iforrnesSuborder Scombroidea

Family ScombridaeGenus Thunnus

Spp. Thunnus thynnus, T. alalunga, T. aibacaresT. obesus

var. T. T. orientaljs

1.1. 2 DescriptionSee Talbot and Penrith (1963)

1. 2 Nomenclature

See Talbot and Penrith (196 3)

y The data used in compiling this synopsis are based on one years observations, viz, winter 1960to autumn 1961; all the findings and conclusions must therefore be regarded as preliminary.

589

FIb/S61 Tuna 2:1

2 DIS TRIBU TION

2. 1 Delimitation of the total area of distri-bution, and ecological characterizationof this area

The total area in which the four tuna specieswere recorded by the author 's institution is situa-ted between Cape Point in the south and theOrange River in the north, along the westernseaboard of South Africa (See Fig. 1),

The ecological characterization of the area(St. Helena Bay - Cape Hangklip) in which thesurvey was conducted (which is also the mainarea of fishing activities) is briefly as follows:(See Fig. 2).

Hydrological

The waters of the above-mentioned areaconsist of the upper part of the Antarctic Inter-mediate current which has upwelled along thecoast to form the Benguela current. Tempera-tures of this (inshore) water vary from 90 to13° C and salinities from 34,70 to 35.00°/oo,The influence of solar radiation is, however,responsible for an increase of the temperaturerange from 90 to 17°C with the same range insalinities. The inorganic phosphate coiltent isof the order of i mg atoms/M3,

Biological

Phytoplankton is most abundant duringspring, summer and early autumn, in whichseasons the content of the inorganic phosphatein the surface layer is relatively high comparedwith that in winter when phytoplankton abun-dance is at a minimum. During the seasonswhen phytoplanicton is relatively abundant, thesurface winds in this area are from a southerlydirection with the result that upwelling is in-creased and heavy, cold, nutrient-rich wateris found near the coast. This upwelled wateris ideal for the florescence of phytoplanktonicorganisms, provided other physical conditionsare favorable,

Diatom blooms are maximal in spring,summer and early autumn,

The heaviest blooms of dinoflagellates arerecorded during autumn,

590

Over the period in which the tuna survey wasconducted, namely 1960 to 1961, the general trendin the abundance of phytoplankton was similar tothe general conditions described above.

Approximately 100 phytoplankton speciesoccur in the area; of these, some occur through-out the year in small numbers, others disappearfrom the area and reappear later in a suddenbloom, Fourteen species predominate in the areabecoming very abundant at times (as much as 14million cells/litre), and constitute the major por-tion of the phytoplankton standing crop,

In the colder inshore area (see section 2. 3)zooplankton is most abundant during late spring,summer and early autumn; the volumes are re-latively low during winter.

The seasonal variation of zooplankton roughlyfollows that of the phytoplankton, the peaks of thezooplankton production occurring either a fewweeks later than the phytoplankton blooms orsimultaneously, and usually in an area situatedsome dIstance from the dense patches of phyto-plankton,

Real "blooms" of zooplankton can occur andare most often found in the zones of mixed water,situated in the vicinity of the areas where upwel-led water breaks through to the surface (or to thewaters adjacent to the surface). These "blooms"are characterized by enormous quantities of twoor three species of Copepoda and sometimes alsoof Chaetognatha.

A bloom hardly ever contains more than onepredominant species at the same place or time.The degree of mixing between upwelled and sur-face water seems to be a factor on which theabundance of one or another species of copepodsdepends.

Each of the species which at times occurs inlarge swarms is preyed upon by pelagic fish,

In the warm Atlantic water (see section 2. 3)no large volumes of zooplankton are found,except for swarms of Tunicata, Despite smallzooplankton volumes, a much larger variety ofspecies is usually present in this water than isfound within the zone of upwelled water.

7°12°

15' -

-'- \.ÇE

Fig. i Areas in which tuna occur

591

50" 12°

15°

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200: o

SOUTH WEST J

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.125° r 0

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i, / REPUBLIC OF

30g - LLOTHSOUTH AFRICA 30'

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CAPEKLIP

OSSEL BAY3° 35.

CAPEOF LEGEND:

GOOD HOPE A S OF REPORTEDOCCURRENCE

COMMERCIAL FISHGA S

40° L I . I I t u I_ J J .i I I k I_&i i I I I I I I I i I ¡ i I i100 15° 20° 250 300 350

2:2 FIb/S61 Tuna

200 30°15

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R000EWAL Bay

O Q Q

, CAFE INFANTA

DANGER POINT O

i I I I I

4° 15° 16° 17° 18° 19° 20° 21° 22° 23°

FIG. 2 AREAS OF INVESTIGATION

592

24°

300

Q Q O O Q

31°

320

O O Q

33°

34°

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LEGEND:

AREA IN WHICH TUNA SURVEY35°

WAS UNDERTAKEN

AREA FROM WHCH HYDROLOGICALAND BIOLOGICAL ENVIRONMENTAL O

DATA WERE OBTAINED36°

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38° I i I I I I I

FIb/S61 Tuna 2:3

O O

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2:4 FIb/S61 Tuna

More than 100 species of Copepoda are knownto occur regularly in the warm surface water,whereas only about 24 species are usually foundinshore in the O to 100 m layer.

No definite seasonal trend in zooplanktonvolumes has been ascertained in the warm off-shore waters; this is due to the irregularoccurrence of big swarms of tunicates whichobliterate the volume of smaller plankton orga-nisms in the catches.

2. 2 Differential distribution

2. 2. 1 Areas occupied by adult stages;seasonal and annual variationsof these

Table I shows the seasonal occurrence ofthe four species of commercially important tuna,For each specie s, the catch per season is indi-cated as a percentage of the annual catch of thatspecies.

From the table it can be seen that bluefintuna were most abundant during winter, in whichseason 75. 2 percent of the annual catch of thatspecies was taken. This species occurred to alesser degree during spring (when 21. 2 percentof the annual catch was taken) and hardly at allduring summer and autumn during which seasonsonly 1. 8 percent.of the total annual catch wastaken. For the year as a whole, bluefin tunaoccurred more frequently than any of the otherthree species.

Table ISeasonal occurrence of commercially important tunas

593

Longfin tuna occurred throughout the year butwas most abundant in autumn and spring when43. 1 percent and 38. 9 percent respectively ofthe annual catch of that species was taken.During winter and summer smaller numbers offish were taken and the catches during each ofthese seasons amounted to9 percent of theannual total for that species. - For the year asa whole this species constituted the secondlargest group taken.

Yellowfin tuna occurred most frequentlyduring spring and autumn, when 51, 2 percentand 41.8 percent respe ctively of the total annualcatch was taken. This species occurred to alesserextent during summer (when 7.0 percentof the catch was taken), and not at all duringwinter when no catches were recorded.

Bigeye tuna occurred in relatively largernumbers during spring and winter when 52. 3percent and 25, 0 percent respectively of theannual catch was taken, and in relatively fewernumbers during summer and autumn when 18, 2percent and 4, 5 percent of the annual catch wastaken.

In the above discussion no indication hasbeen given of the fishing effort requiredto takethe number of fish actually caught. It isnecessary to qualify the above remarks toallow for the apparent fluctuation in sea onalabundance as a result of fluctuation in fishngeffort.

SeasonBluefin

No. %

LongfinNo. %

YellowfinNo, %

BigeyeNo.

Winter 161 75.2 13 9.0 - - 11 25.0

Spring 45 21, 2 56 38. 9 22 51, 2 23 52. 3

Summer 4 1, 8 13 9, 0 3 7, 0 8 18, 2

Autumn 4 1,, 8 62 43,1 18 41, 8 2 4, 5

Total for year 214 100, 0 144 100, 0 43 100,0 44 100, 0

An analysis of the fishing effort (number ofhooks used) indicates that the effort was slightlyhigher in spring and autumn than in winter andsummer, the ratio of effort in the former twoseasons to that in the latter two seasons beingin the order of 3:2.

Making allowance for this variation in fishingeffort does not, however, seem to dialurbundulythe pattern of seasonal occurrence obtained,

2, 3 Behavioristic and ecological determinantsof the general limits of distribution andof the variations of these limits and ofdifferential distribution

See Fig» L)

General: In the Cape Point region, water ofeither Indian Ocean or Atlantic Ocean origin ispresent intermittently, Although the reasonfor the dominant occurrence of a certain typeof water during certain times of the year is notquite clear, it is evident that meteorologicalconditions are directly concerned,

Thus, it is usually found that during thewinter months in the area south of Cape Point,water of Atlantic origin penetrates the area fromwest to east, resulting in the subsequent de-flection of the winter-weakened Agulhas systemto the south and east, During spring a reversalof these conditions occurs and it is found thatAtlantic water is steadily replaced by Agulhaswater, In the regions where the different watermas ses meet, a varying area of mixing is en-countered.,

Off the Cape west coast area various stagesof mixed water occur, consisting of Atlanticsurface water and the upper portion of AntarcticIntermediate water, It is almost certain thatduring certain seasons a component of Agulhaswater is also present in this area,

However, during normal winter conditionththe waters are exclusively Atlantic in origin,This ismainly due to the transportation ofsurface water from the west towards the coàstas a result of the prevalence of strong westerlywinds, During this season the process of up-welling is temporarily slowed down in this area,

With the advent of spring and the prevalenceof strong southerly winds, increased upwelling

594

It would appear that tuna shoals congregateon the oceanic side of the Benguela currentalong the south west Cape Coast.

The hydrological data collected in the above-mentioned area during the tuna survey (winter1960 to autumn 1961) (De Jager, Nepgen andVan Wyk 1962), revealed the following trends:

Winter 1960

occurs, These physical conditions create ahorizontal temperature transitional area in theproximity of the coast; to the coastal side ofthis area, cold, heavy and nutrient-rich wateris found while on the oceanic side warur, light -er and relatively nutrient-poor water is present,

It is acknowledged that the boundary zone inwhich rapid temperature changes occur withina short horizontal distance, is the border ofthe Benguela current which is generally regardedas a cold current, The area of transition wherewarm and cold water meet is often characterizedby the presence of vast quantities of phyto- andzooplankton (see section 2,1), and large shoalsof pelagic fish particularly pilchards (Sardinopsocellata) and maasbanker (Trachurus trachurus),

The horizontal sections of temperature andsa1inity show little or no upwelling. Thei!1ackt of isotherms is very prominent and thewide spacing of the isotherms is indicative ofuniformity, A portion of Agulhas water hasbeen deflected to the south of Mossel Bay butthe remainder has moved in a westerlydirection and has persisted as a 100 rn thicksurface layer,

The main feature illustrated by the verticalsection is the almost complete absence of up-welling, Upwelling seems to have occurredto a limited degree to levels below 50 m,Above this cold water, the O to 50 m larerseems particularly uniform, The strikingpart of this section is the 100 m thick homogene-ous surface layer extending almost the entirewidth of the section (T°C 16°C; S°/oo 35,44to 35,48°/oo), The narrow salinity range indi-cates the uniform, well'mixed properties of thelayer,

It appears that m{xed water with a large com-ponent of Agulhas water has been pushed againstthe coast by westerly winds, The Aguihas water

FIb/S61 Tuna 2:5

2:6 FIb/S61 Tuna

also appears to have exerted an effect below100m as shown by the presence of warmerwater between the two 160 C isotherms on theloo M T°C chart.

It is interesting to note that bluefin tuna werefound in abundance during this season (see sec-tion 2. 2) when the above-mentioned conditionsprevailed in the area surveyed. No yellowfintuna were recorded during winter and bigeye andlongfin were found in small numbers only.

Spring 1960

Appeciable upwelling has taken place to asubsurface level in the ins hore regi9n and theappearance is that of a decaying upweUingsystem. A tongue of Agulhas water exthndingto the nbrth'ha$ apparently isolated a "pool" ofcool water possibly of upwelled origin. Howevera homogeneous. 70 to 80 m thick layer lies to theseaward side of the upwelled water having tem-peratures of 16. 8°C to 16. 9°C and salinities of35, 5 10/00 to 35. 54°/oo, rhermoclines are notevident, except in the 50 to 100 m layer at thestations situated do ser inshore.

The cool water at low salinity lying offshorehas apparently been pushed seawards by intru-ding water of Agulhas origin.

Bluefin tuna catches were lower this seasonthan during the preceding otie (see section 2. 2)when little or no upwelling occurred. Thecatches were triade in the 70 to 80 m thickhomogeneous layer (16. 8°C to 16. 9°C) whichlay to the seaward side of the upwelled zone.Longfin, yellowfin and bigeye tuna were record-ed in appreciable numbers during spring.

Summer 1960 to 1961

Strong upwelling appears near the coast, inconjunction with a marked thermocline closeinshore. A much greater horizontal gradientis apparent. Again most of the Aguihas wateris deflected. southwards of Mos sel Bay and onlya + 100 m thick layer penetrates to the south-west of Cape Point; it then moves north to mixwith the cobi water and its identity is rapidlylost. It can, however, be traced as a maximumtemperature layer containing two 21°C isotherms.This water appears as a "pool" on the verticalsection offshore, and the "dip" in the isothermshere also points to the intrusion of Aguihas water.The 15°C isotherm here lies at much the same

595

depth as in spring; it appears that only thesurface water has changed and warmed.Little vertical stratification is evident.

During this season of marked upwellingbluefin tuna were caught in minimal numbers(see section 2, 2) and it appears that thisspecies prefers water where conditions areuniform, Longfin, yellowfin and bigeye tunawere recorded during summer in smallnumbers only.

Autumn 1961

Aguihas water of relatively very hightemperature (24°C) turns back south of Capeipíanta; an upper loo m layer appears toproceed as far as south of Cape Point.Upwelling is not vigorous but is still presentto a considerable degree. The offshore sta-tions are charcterized by a 30 m thick .layerincreasing to 50 m seawards in the area ofmixed water, The thermocline is mar.kedinshore but is less evident offshore.

Bluefin catches were again minimal duringthis season (see section 2, 2) but longfin andyellowfin tuna were recorded in quantity duringautumn. Bigeye tuna were caught in minimalquantities only, Surface temperature datataken over the whole year (winter 1960 toautumn 1961) and plotted against the percen-tage of times the lines were successfully setin water of a specific temperature range,indicate that bluefin tuna were caught between15°C and 200 C, The catch rates werehighest between 16°C and 17°C (See Fig. 3).

Bluefin tuna occurred in abundance off thesouth west Cape coast in 1960 to 1961 duringwinter and spring when little or no upwellingwas in evidence, and when a homogeneouswater layer (100 m thick) was present.Conditions were uniform during these seasonsand a horizontal gradient was not evident.Catches were made to the seaward side ofupwelled waters when upwelling took place.Bluefin tuna catches were minimal duringsummer and autumn when marked upwelling,coupled with large horizontal gradients, tookplace in the area.

Long fin tuna were recorded within thetemperature range 15° C to 23° C; the catchrates were highest between 17° C and 19° C,(See Fig, 4), Longfin occurred in abundance

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¿:8 FIb/S6l Tuna

during spring (1960) and autumn (1961) (See sec-tion 2. 2). In spring appreciable uprel1ing occur-red in the inshore region but the longfin catcheswere made to the seaward side of upwelledwaters. During the summer when upwelling wasmarked, longfin catches were much lower thanduring spring. In autumn longfin tuna occurredin abundance when upwelling was less vigorous,'

Yellowfin tuna were caught within the tem-perature range of 17°C to 21°C; the catchrates were highest between 2OC and 21° C

597

(see Fig. 5). Although yellowfin occurred infar smaller numbers than longfin, their maxi-mum occurrence was recorded in the sameseasons, viz, spring and autumn. Duringwinter yellowfin were not recorded within thesurveyed area.

Bigeye tuna were reçorded within thetemperature range 16°C to 21° C; the highestcatches were recorded betwéen 180 C to 19°C(See Fig. 6). This species occurred in rela-tively large numbers during winter and spring.

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FIb/S6l Tuna

3 BIONOMICS AND LIFE HISTORY

3. 1 Reproduction

3.1.1 Sexuality (hermaphroditism,heterosexuality, intersexuality)

All species appear to be heterosexual; noinstance of intersexuality was recorded.

3.1,2 Maturity (age and size)No breeding fish were encountered

3.1.8 Egg: structure, size,hatchingtype, parasites and predators

Eggs were not present in the plankton catchesin the surveyed area.

3. 3 Adult history3.3.1 Longevity

Bluefin bina; the length range varied between88 cm to 172 cm for the period winter 1960 toautumn 1961. The corresponding weight rangewas 35 to 220 lb, According to Sella (1929),age/weight relalionship for the Mediterraneanbluefin a South African bluefin of 172 cm wouldbe 8 years old if the growth rates of the twospecies are comparable,

Longfin tuna; the lengths recorded for theperiod under review were 68 to 118 cm with.a corresponding weight range of 15 to 65 lb.

Yellowfin tuna; the length range variedbetween 82 and 156 cm with a correspondingweight range of 25 to 160 lb. Pacific yellowfin(Migdalski 1958) of 50 lb weight are + 2. 5 yearsof age; of 100 lb weight they are ± 3. 5 yearsof age; in 6 years the tuna reaches approxi-mately 200 lb If the growth rates of theSouth African yellowfin and the Pacific yellow-fin were comparable, then our biggest yellow-fin will be 5 years old.

Bigeye tuna; the length ranged between 104to 160 cm and the weight range was between50 and 200 lb,

3,3.2 Hardiness

All four species are robust and firm and arenot easily damaged.

3. 3, 3 Competitors

The following are the possible competitorsoccurring in association with the four tuna

599

species in the surveyed area:

Blue shark Glyphis glaucusMako shark - Isurus glaucusBrown shark - Carcharinus obscurusThrasher shark Alopias vulpinusMoonfish Lampris regiusAngelfish Brama rauFrigate mackerel Auxi tiardMackerel Scomber japonicusLancetfis h Alepisaurus feroxStockfish Merluccius capensisSnoek Thyrsites atunYellowtail Seriola lalandiSkipjack Euthynnus pelamisGannet Morue capensisCormorant Phalacrocorax capensisCape Penguin Spheniscus demersusWandering

Albatros s - Diomedea exulans

3. 3.4 Predators

The following species are considered pre-dators of the four tuna species:

Blue shark - Glyphis glaucusMako shark - Isurus glaucusThrasher shark Alopias vulpinusBrown shark - Carcharinus obscurus

3.3.6 Greatest size

Bluefin tuna 172 cmLongfin tuna 118 cmYellowfin tuna 156 cmBigeye tuna 160 cm

3.4 Nutrition and growth

3:1

3.4,1 Feeding (time, place, manner,season)

All four species are all selective feedersof a voracious nature.

3,4. 2 Food (type, volume)

The food of the tuna may be broadly classi-fied in invertebrates and vertebrates; of thesethe following were important items in the diet:

Cape Fur Seal Arctocephalus pusillusSperm Whale - Physeter catodonSei whale - Balaenoptera borealis

Crustaceans: Funchalia woodwardilPhronirnaPuerulus stages of

Ja sus lalandiiMegalopa crab larvae

Cephalopods: Cranchia scabraLoligo reynaudiiOctopoteuthis sicula

Myctophum humboldtiMyctophurn coccoCentropholoide s falcatusScomber japonicusOreos orna atlanticurnAlepisaurus feroxBrama rauLepidopus caudatusGempylus serpens

Table IIStomachs analysed

600

3.5 Behavior

3. 5.1 Migration and local movements(See section 2. 2).

3, 5, 2 Schooling

Longfin and yellowfin tuna were observed inextensive schools at the surface. At no timewere bluefin and bigeye observed in surfaceschools.

BluefinNo,

LongfinNo, %

YellowfinNo, %

BigeyeNo,

Stomachs con-taming food. 50 52,6 54 79,4 22 84,6 10 90, 9

Emptystomachs 45 474 14 20,6 4 15,4 1 9,1

Total 95 100,0 68 100,0 26 100,0 11 100, 0

3:2 FIb/S61 Tuna

Argo nodosaFish: Sardinops ocellata

Merluccius capensisAuxis thazard

FIb/S61 Tuna 3:3

Table III

Percentage of stomachs in which various food types occur

Occurred in percentage of:

Table IV

Percentage of weight of various food types found in tuna stomachs

601

Type of food

Percentage (by weight) of food types in:

BluefinStomachs

LongfinStomachs

YellowfinStomachs

BigeyeStomachs

% To %

Fish 40 55 85 26

Crustaceans 20 22 4 18

Cephalopods 40 21 7 56

Tunicates - 2 1 -

Other - 3 -

Total 100 100 100 100

Type of food BluefinStomachs

LongfinStomachs

YellowfinStomachs

BigeyeStomachs

Fis h 12 54 65 27

Crustaceans 24 44 23 18

Cephalopods 18 34 19 73

Tunicates 1 4 4

Other - 4

FIb! S61 Tuna

4 POPULATION (STOCK) Thunnus thynnus orientalis

4.1 Structure

4,1.1 Sex ratio

The ratios of male to female tuna caughtfrom June 1960 to May 1961 were as follows:

Males Females

4.1.2 Age composition

See section 3.3.1.

4.1. 3 Size composition

Figs, 7(a) to 10(b) show length frequencydistribution, separately for males and femalesfor the four species, caught from June 1960 toMay 1961,

Length-weight relationship; the length-weight relationships, expressed in exponentialform, are given below for the various species,In each case "W" represents weight in lb, and"L" length in cm,

Thunnus alalunga

W 5.088 x l0 x L2' 98malesW = 3,094 x l0 x L3 °9females

Number of fish examined:

males 96females = 47

Ratiomale si

females

1/ In the case of the male yellowfin only seventeen specimens were examined, Some modificationof the length-weight relationship obtained might, therefore, possibly occur if more fish were tobe included in the sample.

602

W = 6.394 x l0 x L2' 92 malesW = 4,83lxl0 xL2'98 females

Number of fish examined:

males 131females = 81

Thunnus albacares

W = 1.8172 x l0' x L2' 72 malesW 1, 2810 x 10 x L2' ''9 females

Number of fish examined:

males = 22females = 22

4.6 Relation of population to communityand ecosystem, biological production,etc.

See sections 2, 1, 2, 2 and 2, 3,

4:1

Thunnus thynnus131 81 1.6:1

W = 1.796 x l0W = 8,800 x

x L3'18 malesx L' 85 femalesorientalis

Thunnus alalunga 96 48 2:1 Number of fish examined:

Thunnus albacares 26 17 1, 5:1 males = 17females = 26

Thunnus obesus 22 22 1:1Thunnus obosus

25

20

NUMBER

OF

FISH

lo

50 55 60 66 70 75 80 85 qo 9$ too ¡os (0 ¡5 (20 IZO ¡o ¡35 (4(0 145 ¡50 lG5/60i65,ioLENGTH IN CENTIMETRES

Fig. 7(a) Bluefin tuna males length-frequency distribution

NUMBER

OF 15

FISH

IO

N 133

603

N= 81

Fig. 7(b) Bluefin tuna females length-frequency distribution

4:2 FIb/S6l Tuna

#55 ¡60 ¡65 ¡70505560657016 9o5 9095100/051(0 ¡5l20l25/3O/35l,.5,LENGTH IN CENTIMETRES

25

20

NUMBER

0F 15

FI 5H

Io

25

20

NUMBER

0F IS

FISH

Io

N 96

50 55 60 65 10 75 ea 85 90 95 too ¿oS io iS 120 55 I& 3E ¡40 J45 go ¡59 60 5 ITO

LENGTH IN CENTIMETRES

Fig. 8(a) Longfin tuna males length-frequency distribution

6O

N 48

50 SS 60 6 70 5 80 85 90 95 /°5,iio .s ,aO ¡5$ (0 ¡35 /40 ¡45 50 55 lf ¡5 ¡70LENGTH IN CENTIMETRES

Fig, 8(b) Longfin tuna females lengthfrequency distribution

FIb/S61 Tuna 4:3

4:4

25

20

NUMBER

OF IS

FISH

IO

O

605

FIb! S61 Tuna

N 17

50 55 (.0 (.5 70 75 O 85 90 95 /00 no sS 12.0 25 So 35 14.0 545 50 55 55,0 ¡5,5

LENGTH IN CENTIMETRES

Fig. 9(a) Yellowfin tuna males lengthfrequency distribution

25

20

NUMBER

OF 15

FISH

IO

N 26

50336065707580 559095 f00/(IØ//5/5/360 /70LENGTH IN CENTIMETRES

Fig. 9(b) Yellowfin tuna females lengthfrequency distribution

FIb/S61 Tuna 4:5

25

20

NUMBER

OF IS

FISH

¡O

O

25

20

NUMBER

0F 15

FISH

Io

o

u

N 22

556065707550559095100 f05110 IS ¡50135130 3SoI4s5o S5éO/SSflO

LENGTH N CENTIMETRES

Fig. 10(a) Bigeye tuna males length-frequency distribution

_I_I. _...i.

606

N = 22

50 55 Sa 65 To 75 80 8 90 95 i /05 I O iie /20 f55 150 155 /40 166 ISo ¡55 l6o 65 ¡70

LENGTH IN CENTIMETRES

Fig. 10(b) Bigeye tuna females length-frequency distribution

5 EXPLOITATION

As the South African commercial tuna fisheryis still in its infancy very little can be said. Upto the presents tuna have been caught only by long-lining, and the pattern of the fishery with regardto boats, equipment, seasons and areas is not yetclear.

607

FIb, s61 Tuna 5:1