Species Synopsis No. 2FAO Fisheries Biology Synopsis No, 45 FIb/S45(Distribution restricted) SAST - Tuna

SYNOPSIS OF BIOLOGICAL DATA ON BLACK SKLPJACKEuthynnus lineatus Kishinouye 1920

Exposé synoptique sur la biologie de la thonineEuthyrmus lineatus Kishinouye 1920

Sinopsis sobre la biologia del barrilete negroEuthynnus lineatus Kishinouye 1920

Preparéd byTHOMAS P, CALKINS AND WITOLD L, KLAWE

Inter-American Tropical Tuna CommissionLa Jolla, California, U, S, A,

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

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i IDENTITY

1. 1 Taxonomy

1. 1. 1 Definition

Order PerciformesSuborder ScombroideiFamily ScombridaeSubfamily ScombrinaeGenus Euthynnus Jordan ,t Gilbert 1882Species Euthynnus lineatus Kishinouye

1920

The classification of the species of the genusEuthynnus is in dispute. In this paper the organi-zation of the genus suggested by Godsil (1954),and by Matsumoto (1959), wili be accepted. Theseauthors devide the genus into three species;

Euthynnus lineatus Eastern PacificEuthynnus yaito Central Western Pacific

Euthynnus alletteratus Atlantic

The black skipjack, Jithvnnu. tue, is amarine, pelagic fish inhabiting the tropical andsubtropical waters of the Eastern Pacific. Itoccurs, roughly, from the area of the U. S. -Mexican border to Peru, in fairly close associa-tion with the mainland and offshore islands.

1.1.2 Description- Genus Euthynnus Jordan &

Gilbert 1882

The fishes of this genus have the fusiform,streamlined body and lunate caudal fin common toall tuna-like fishes. The body is devoid of scalesexcept for the corselet. Euthynnus can be distin-guishedfromKatsuwonusby the presence of aseries of oblique or longitudinal black markingsabove the lateral line (,uwonishas stripes onthe belly). The dorsal fins are contiguous ornearly so. This latter feature distinguishesEuthvnnus from ÀUXIS whose dorsal fins are widelyseparated. The distal margin of the first dorsalis concave. This character distinguishesEuthynnus fromSarda, the bonitos, whose firstdorsal has a margin that is nearly straight. Thereare usually eight dorsal and seven anal finlets.

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The intestine is straight, wjthout a fold.There are 37 or 39 vertebrae. Teeth are pre-sent on the palatines in all species and on thevomer in two of the three specie s, A detaileddescription of the generic characters 'of theskeleton is given bjr Godsil (1954).

Species Euthynnus lineatus Kishinouye 1920

The following description is taken fromGodsil (1954).

This species can usually be separated fromothers of the genus by the dorsl markings,"The general pattern consists of a series ofthree, four, or five broad continuous blackstripes running horizontally on the back from thecorselet to the caudal fin, The most ventral ofthese stripes invariably starts anteriorly belowthe lateral line and generally crossesthis in itspath to the caudal region, Variations consistmainly of interruptions in the continuity of thestripes or branchings of individual stripes withsupplementary short ir regular marking s between.However, variations can be sufficiently extremeto preclude a positive identification and suchvariations widely overlap the markings of otherdescribed species." A specimen with typicalmarkings is shown in Fig, 1. The dorsalstripes of Euthynnus yaito and E. ailette ratusare typically more broken, irregular andoblique. "These black dorsal stripes aresuperimposed upon a background of blue, deep-ening into black dorsally and shading into greyand silver below. The belly of the fish is duskyor silvery with an irregular number (usually from2 to 6) of black spots between the pectoral andpelvic fins. Such spots are not diagnostic orinvariable for specimens are occasionally seenwith no apparent spots." Specimens occasion-ally are found which have oblique stripes on thebelly,

Meristic counts are given in the followingtable.

Fig. 1 Black skipjack, Euthynnus lineatus. Fish caught by D.C. Vannat Socorro Island; total length 512 mm, weight 4 lb.lOoz. (2.14 kg).Photograph by J.C. Brown.

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* Taken from Godsil (1954)

TTThe caecal mass and the liver are thedominant organs in the ventral view of the visceraT.The liver has three lobes, the right lobe beingabout twice as long as the other two. It extendsthe full length of the body cavity.

The most important skeletal features which arediagnostic for this species are the vertebral countof 37 (36-38), as opposed to 39 in the other twospecies, and the presencè of four protuberanceswhich occur on the 31st and 32nd vertebrae.

1. 2 Nomenclature

1. 2.1 Valid scientific names

Euthynnus lineatus Kisbinouye 1920

1, 2. 2 Synonyms

Euthynnus affinis lineatus Fraser Brunner1949

'MERISTIC COUNTS OF Euthyrmus lineatus *

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1:3

1.3.1 Subspecific fragmentation (races,varieties, hybrids)

There are no recognized subspecies ofEuthynnus lineatus. Fraser-Brunner (1949>proposed that E. lineatus and E, yaito aresubspecies of . affinis. Other authors havesuggested thatE. affinis and E, yaito are thesame species iit that E, lineaLs is a separatespecies. All seem to agree that E. alletteratus

Meristic CharacterCount No.

SpecimensRange 'TÑ4ost Common

1st dorsal spines 13-15 14-15 2Q

2nd dorsal rays 11-12 II 10

Dorsal finlets 8-9 8 20

Anal rays 11-13 11-12 18

Anal finlets 7-8 7 20

Gill-rakers - 1st arch: Upper limb 7-11 9-10 30Angle 1 1 28Lower limb 23-29 26-27 30

Total: 32-41 35-38 30

Number of vertebrae 36-3 8 37 27

Precaudal vertebrae 19-20 20 24

1st complete haemal arch on Vert No. 15-17 15-16 23

1. 2. 3 Standard common names, vernacularnames

Negra, Bonito negro Ecuador

Barrilete negro Mexico, Peru

Black skipjack; cross-bredmackerel United States;

U.S. sports-fishermen

Mexican little tunny Kishinouye

1. 3 General variability

is a distinct species which may or may not havesubspecies.

Races or subpopulations of E. lineatus mayexist but none have yet been defined. Godsil(1954) reported considerable variation in theexternal markings of E. lineatus. Some of theatypical specimens which he examined had dorsalmarkings of the E. yaito or E. alletteratus type.However, there was no variation in internalfeatures in the specimens of atypical appearanceand all could be positively identified as E. lineatusThe variation in external markings was not con-fined to specific geographical areas, but seemedto be general throughout the range of the species.Considerable variation in dorsal markings betweentwo sides of the same fish sometimes occurs.

The existence of hybrids has never beenobserved or investigated.

The blood (erythrocytes) of two E. lineatuswas tested with a series of 540 bean extracts, byDr. C. W. Cotterman and the junior author, aspart of the preliminary work on a program fortyping fish blood. On the basis of agglutinations,the blood of the black skipjack can be differentiatedfrom that of six other scombroids from the EasternTropical Pacific which were tested with the sameseries of reagents

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Fig. 2 Records of adult black skipjack, Euthynnus lineatus, chartedby one-degree squares.

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2 DISTRIBUTION

2.1 Delimitation of the total area of distri-bution and ecological characterizationof this area

Black skipjack are known to occur along thePacific coast of the Americas from southernCalifornia to Peru. The present northernrecords for this species, off San Simeon,California (35°20 'N by 121040 'W), was reported byNowell (1961). There have been two other reportsfrom California waters (Roedel 1948, and Fitch1952). Black skipjack have been taken off BajaCalifornia (Godsil 1954), from the Tres MariasIslands, 2l°30 'N on the Mexican coast (Fowler1944) near Acapulco, Mexico (Mais and Jow,

'1960), off Costa Rica (Schaefer and Marr, 1948),from Colombia Bank, 2°N by 79°W. and fromGuayaquil Bank, 3°35'S by 80055tW (Godsil 1954,and Clemens, 1957). There are numerous reportsof black skipjack from the Galapagos Islands(Fowler 1938 and 1944; Seale 1940; Schmitt andSchultz, 1940; Godsil 1954). They have beenreported from northern Peru (Hildebrand 1946).

The occurrence of black skipjack is sometimes-reported in the logbooks of commercial tuna fish-ing vessels, although this species is not taken bythese vessels. A search of the tuna boat log-book records of the Inter-American TropicalTuna Commission (IATTC) revealed that thisspcies has been encountered, at various times,nearly everywhere along the coast from themiddle of Baja California to Guayaquil Bank,off northern Peru. They have frequently beenreported from the Gulf of California, theRevillagigedo Islands, all along the coast ofMexico and Central America, and the GalapagosIslands. In addition, black skipjack haveoccasionally been reported from Clipperton andCocos Islands and there is a single report fromShimada Bank (l6°52'N by 1l7°30'W), 1.80 milessouthwest of Clarion Island. Logbook reportsof black skipjack from the waters south of Guaya-quil Bank are rare. This may be due not toscarcity of the specie s in these waters but,rather, to reduced coverage by the tuna fleet.Locations where black skipjack have been foundas reported in the literature, from tuna vessellogbooks and from scientific logs of dceanographiccruises by the Scripps Institution of Oceanography,the Bureau of Commercial Fisheries of the U.S.

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Fish and Wildlife Service and the field-books ofDr, C. L, Hubbs and Dr. D. P. De Sylva, areshown in Fig. 2. Locations are shown to thenearest one-degree square of latitude andlongitude.

The major portion of the waters in whichblack skipjack occur is tropical, The EquatorialPacific Water mass extends approximately from23° - 24°N to northern Peru and the GalapagosIslands. To the north and south of this tropicalwater mass are transition regions which areinfluenced by the cool California Current in thenorth and by the cool Peru Current in the south.The principal warm currents are the NorthEquatorial, the Equatorial Countercurrent, andthe South Equatorial. Near the coast, whereblack skipjack are most frequently found, thesecurrents are usually weak and variable.

The well-developed thermocline, which isa characteristic of most tropical waters, tendsto hinder the renewal of nutrients in the surfacelayer and thereby limits biological productivity.However, in the area which comprises the rangeof the black skipjack the thermocline is relative-ly shallow (10-50) meters. In general, thethermocline shoals from west to east and thereare areas near shore where the therrnocline issufficiently shallow to permit the exchange ofnutrients between the deep layer and the euphoticzone. There is a thermal dome located offCentral America (9°N by 90°W) where thethermocline is frequently less than 10 m andsometimes reaches the surface. In the Gulfof Panama and in the Gulf of Tehuantepec sea-sonal high winds bring about upwelling whichcontributes to productivity. The Gulf of Guaya-quil and the Galapagos are in the transition areabetween tropical water and the cool rich waterof the Peru Current. Upwelling enriches theinshore waters off Baja California but the watersfarther offshore in this area are very poor innutrients and standing crop. The waters of theGulf of California are warmer and more salinethan adjacent waters. Upwelling alzo occurs in theGulf of California and the thermocline is general-ly not deeper that 20 m. This description wa com-piled from the following references: Brandhorst1958; Cromwell and Bennett, 1959; Cromwell1958; Holmes, Schaefer and Shimada, 1957;Sund and Renner, 1959.

Z 2 Differential distribution

2, 2,1 Areas occupied by eggs, larvaeand othèrjunior stages: annualvariations in jhese patterns, andseasonal variation for stagespersisting over two or moreseasons, Areas occupied byadult stages; seasonal and annualvariations of these,

The eggs of black skipjack are pelagic but theyhave never been specifically identified in planktoncollections. Mature and running ripe adults haveoccasionally been encountered and larvae andjuveniles have been collected and identified.

Larval and post-larval black skipjack have beencollected from Point Antonio, Baja California(29°45!N) to the vicinity of Malpelo Island (4°00'Nby 81°351W), In the Gulf of California they havebeen collected both from near the head and from themouth, The sites of collection form a continuousband from the Tres Marias Islands, near theentrance of the Gulf of California to the Gulf ofPanama. Larvae and juveniles of black skipjackhave, to date only been encountered withinapprpximately, 150 miles of the mainland. Theonly exception is one individual captured nearMalpelo laland (Klawe, in press). The locationof capture of larval and juvenile black skipjackis shown in Fig. 3.

Collections have not been extensive enough toreveal annual variation in the distribution oflarvae and juveniles.

The areas occupied by adult stages have beendiscussed in section 2.1. Since black skipjackare not sought by commercial fishing vessels,it is difficult to detect seasonal and annual varia-tions in the distribution of adult stages. However,when the logboòk reports of occurrence and thosefrom other sources are analyzed by quarters ofthe year, there are some noticeable differencesin distribution. In the first quarter of the year,black skipjack have been encountered most oftenoff the coast of Central America and near thesouthern tip of Baja California. In the secondquarter there have been fewer reports from Cen-tral American waters and a great increase ofreports from tue lower Gulf of California and fromthe Revillagigedo Islands. In third quarters themajority of reports has been from the banks off the

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bution and of the variations of these

west coast of Baja California. The reports ofblack skipjack from California waters have allcome in third quarters. In fourth quarters the re-ports have been scattered from Baja Californiato central Peru with not much concentration in anyarea. There, have been more reports from theGalapagos Islands during fourth quarters than atother times of the year.

It should be noted that these apparent changesin seasonal distribution may not be due to actualshifts in the population but may have resulted fromseasonal changes in the concentration of thecommercial tuna fleet which is largely governedby the regional availability of yellowfin and oceanicskipjack. The reports of black skipjack from log-books and from the literature indicate only thepresence of black skipjack in a particular areaand furnish no accurate information as to quantity.It is a certainty that only a part of black skipjacksightings are reported in the logbooks, since thesefish are, at the present time, of no commercialvalue and there has been no program for collectinginformation on this species.

2.3 Behavioristic and ecological determi-nants of the general limits of distri-

limits and of differential distribution

Temperature, as for related species, is pro-bably an important determining factor for thelimits of distribution of black skipjack. Blackskipjack have been reported from waters rang-ing from 60°F to 87°F (IATTC logbook records).They have been reported most frequently fromwaters with surface temperatures between 75°Fand 85°F. There have not been any experimentsperformed to determine the absolute range oftemperature which can be tolerated by thisspecies.

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cocos is.

Fig. 3 Localities of capture of larvae and juveniles of black skipjack,Euthynnus lineatus.

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BIONOMICS AND LIFE HISTORY

3. 1 Reproduction

3.1. 1 Sexuality (hermaphroditism, hetero-sexuality, intersexuality)

Black skipjack are heterosexual. Males andfemales do mot differ in external appearance.

3. 1. 2 Maturity (age and size)

The number of mature black skipjack that havebeen examined and reported on is very small.Mead (1951) examined two females, taken off Gen -tral America, whose ovaries were swollen andturgid. The lengths were 54. 4 and 55. 0 cm(fork length). It seems probable that some indi-viduals would reach maturity at a smaller size.De Sylva and Rathjen (1961), in their study ofE. alletteratus, the Atlantic species, give theisual length at first spawning as 350 mm, Theage of black skipjack (E. lineatus) at maturity isunknown.

3.1. 3 Mating (monogamous, polygamous,promiscuous)

The spawning of black skipjack has never beenobserved. However, from examination of theeggs and from what is known of its life history andthat of closely related species, it is certain thatspawning is pelagic.

3. 1.4 Fertilization (Internal, external)

Fertilization is exterra1.

3.1.5 Fecundity

The fecundity of black skipjack has neverbeen investigated. It seems logical that thenumber of eggs produced and the relation ofthis to size of fish is in the same order ofmagnitude as that of related species. Forexample, Vildoso (1960) estimated that thefecundity of the bonito Sarda chilensis is 288. 2thousand for 500 mm fork length; 413. 3 for550 mm; 754, 2 for 600 mm; 771. 1 for 650 mm.In mature oceanic skipjack, Katsuwonus pelamis,between 61Ú and 700 mm, the number of ova inthe maturing group has been found to range be-tween 330.0 thousand and 1, 000.0 thousand(J. Joseph, unpublished data).

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3.1. 6 Spawning

- Spawning seasons (beginning,end, peak)

In the waters off Central America there isconsiderable evidence that spawning takes placein winter and spring. Schaefer and Marr (1948)reported taldng two adults in an advanced stageof maturity in the Gulf of Nicoya, Costa Rica, InFebruary 1947. Ripe adults were taken in Aprilof the same year from the inshore waters ofCosta Rica. Juveniles were collected furtheroffshore in March. Clemens (1956) collected'post larvae" black skipjack from CentralAmerica in January 1955. Mead (1951) reportedtaking larvae and ripening adults off CentralAmerica in May 1949. Although these collec -tions were all made in the winter and spring itis probable that black skipjack spawn all year inthis area with the peak In early spring. Kiawe(in press) compiled the following table of theaverage number of larvae caught off Cape Blanco,Costa Rica, per hoùr of surface tow during diffe-rent months:

Spawning in the Gulf of Panama may ex-tend over a considerable part of the year.Matsumoto\ (1959) identified black skipjacklarvae and juveniles in material from the"Dana Expeditions", collected in the gulf inJanuary 1922 and in September 1928. Thespecimens ranged in size down to 5. 0 mm,indicating that spawning could not have beenfar removed in time and location.

Larvae collected during Scripps Insti-tution of Oceanography and CaliforniaCooperative Oceanic Fisheries Investi-gations cruises indicate that spawning alsotakes place all year of the southern Mexi-can coast with a peak in the early springbut in more northern waters, off BajaCalifornia, spawning is limited to summermonths (Kiawe, in press).

Month No. oflarvae

Month No. oflarv

I 1.5 vI 0. 1II 0.1 vt 0. 2m 3. 2 IX 1. 4Iv 14. 9 X o. oV 0. 6 XI 0. 2VI 0.0 x-Ï' 0. 7

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- Number of spawnings per year,frequency

It ìs not known whether individual black skip-jack spawn more than once a year0 Most likelythe black skipjack, like other tropical tunas havean extended spawning season in warmer waters,and individual fish do not spawn all ova at once0

- Spawning time of day.

The time of day in which spawning takes placeis unknown

- Induction of spawning, artificialfertilization

The factors which induce spawning are unknown,

The junior author collected riirming ripe blackskipjack of both sexes off Central America inApril 1957 and attempted to fertilize the eggs0 Nodevelopment resulted, however.

3,1. 7 Spawning grounds

Since black skipjack eggs are pelagic and theadults are encountered near the surface it seemslikely that spawning takes place in open water nearthe surface. The spawning grounds appear to be inthe open ocean but in close proximity to land mas seFor the location of probable spawning areas seesection 2.2.1,

3.10 8 Egg: structure, sizé, hatching type,parasites and predators

The egg of black skipjack is spherical andcontains a single oil globule. In appearance andstructure it is similar to those of other tunasThe junior author measured a small sample ofpreserved eggs taken from a running ripe female,and found that the egg diameter was between 0. 90and 0. 95 mm. The diameter of the oil globuleranged from 0. 23 to 0.26 mm.

There is no information on parasites andpredators of black skipjack eggs. The eggs areundoubtedly eaten by a wide variety of organisms.

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3. 2 Larval history

3 201 Account of embryonic andjuvenile life (prelarva, larva,postlarva, juvenile>

Young blaclç skipjack, like other tuna-like fishes, do not pass through a sharplydefined metamorphosis marking transitionfrom larva to postlarva to juvenile. Thedefinition of each stage, therefore, must besomewhat arbitrary0 Matsumoto (1959) de-fines larvae as individuals with less than afull fin-ray count, especially in the first dorsal.These fish are usually li mm or less in totallength. The size range of postlarvae is approx-imately 12 to 18 mm0 In this size range theanal opening moves back from mid-way be-tween the pelvic and amai fins to the origin ofthe anal fin0 Young tunas larger than 18 mmare classed by Matsumoto as juveniles.

The larva of E. lineatus is similar inappearance to thaT of other tunas (see Fig. 4).It would be difficult to separate the larvae ofthis species from that of other related species,unless a size-series is available0

Matsumoto (1959) described a series ofyoung black skipjack ranging in size from 5.0to 27. 0 mm. The following description isbased largely on his work.

At 5, 0 mm the head is large - about37 percent of total length and the mouth isaIs proportionately large, The anal open-ing is near the middle of the body. There isno spine or ray development, Between6 and 12 mm the body elongates .nd increasesin size in relation to the head0 The dorsalspines develop and pigment appears on thefirst dorsal fin. When length has reached 10-12 mm all of the unpaired fins are almost fullydeveloed. The fins and finlets become moreclearly defined, The number of rays in thepectoral, fins comes within the adult range.By length 22 mm the dorsal half of the body isdark as far back as the caudal peduncle (Mead,1951). In the larger juveniles, vertical barsappear faintly along the dorsal region.

- Schaefer and Marr (1948) state that juveiilesof black skipjack are less deep bodied than yellow-fin tuna of the same size. Below 44 mm in lengtF,

Fig. 4 7.1 mm larva of black skipjack, Euthynnus lineatus. FromMatsumoto (1959).

Fig. 5 45 mm juvenile black skipjack, Euthynnus lineatus. Drawingby Sheila K. Hillsdon.

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the juveniles of black skipjack can be distinguishedfrom those of oceanic skipjack by the heavier pig-mentation of the dorsal fin in the former. At86 mm the second dorsal is pigmented from theorigin of the rays to half-way to their distal end.The head and body are more darkly pigmented thanyellowfin and oceanic skipjack of the same size,A juvenile black skipjack is shown in Fig. 5.

- Feeding

Clemens (1956) held juvenile black skipjack inship-board aquaria and offered them various fooditems such as ground fish flesh, planktonic fisheggs and living zooplankton, The subjects dis-played more interest in the live zooplankton thanin the non-living material, but did not feed untilactively swimming larval blennies were offered.After feeding on larval blennies, the subjects wereinduced to feed on planktonic fish eggs and groundfish flesh by gradually substituting the non-livingmatter in the diet. The juvenile black skipjackfed until their stomachs were greatly distended.Feeding periods were adjusted so that the subjectswere fed again when their stomachs had retúrnedto normalin five to six hours.

- Rates of: development and survival

Clemens (1956) was able tO hold some juvenileblack skipjack (see above) for as long as ten days.In that period their weight increased eleven timesand length increased approximately 1,4 times overthat at the beginning of the holding period. Theauthor felt that this growth rate would have repre-senteda minimum in nature because of the un-natural conditions of confinement.

Nothing is known as to the rates of developmentor survival of the larval or postlarval stages.

- Parental care

None.

- Parasites and predators

There is no ìnformation on the parasites ofblack skipjack larvae, postlarvae or juveniles.There is no specific information on predators ofthese stages, however, it would be surprisingif, like almost everything in the sea, they werenot preyed upon by almost every organism whichis able to catch and alIow them.

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3. 3 Adult history

3, 3, 2 Hardiness

The hardiness of black skipjack is similarto that of other tuna-like fishes. They arehardy enough to be successful in their naturalenvironment, but they can only stand a minimumof handling.

3. 3. 3 Competitors

Black skipjack are found in close associa-tion with yellowfin tuna, oceanic skipjack, anddolphin fish, among others. All of these speciesundoubtedly compete with the black skipjack forfood. Whether this competition is in any wayharmful to any of the species concerned is un-known.

3,3.4 Predators

The large bill-fishes undoubtedly prey onblack skipjack, and they have been found in thestomach contents of yellowfin tuna (Alverson, inpress),

3. 3. 5 Parasites and diseases

The parasitic copepods Caligus coryphaene(see Shiino, l959a) and Caligus macarovi(see Shiino, l959b) are known external parasitesof black skipjack. The trematodeHirudinella marina was found to be parasitic inthe stomachs of several black skipjack examinedby the authors.

There is no information available on diseasesdf black skipjack,

3.3.6 Greatest size

The largest black skipjack collected by thejunior author was a running ripe male of forklength 636 mm and weighing 10 lb. 8oz.(4. 75 kg). Since only few individuals havebeen examined for length and weight (comparedto other tuna-like fishes), it is likely thatindividuals larger than the above occur.

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3. 4 Nutrition and growth

3. 4. 1 Feeding (time, place, manner,season)

Black skipjack feed throughout their lives andin all seasons of the year. Like related species,feeding is probably by sight and therefore confinedto the daylight hours and to the surface layer.

3, 4, 2 Food (type, volume)

According to Walford (1937), the black skipjackis a feeder of "voracious habits and insatiableappetite. " The stomach contents he examined con-sisted of small fish and squid. One stomach con-tained fourteen small fish and thirty s quid. Oneof the species of fish consumed by black skipjackis the frigate mackerel (Auxis thazard). SixteenAuxis three to five. inches long were found byF. G. Alverson in the stomach of a black skipjacktaken in the Gulf of Tehuantepec (Kiawe, in press).The displacement volume was 127 ml. The pelagiccrab (Pleuroncodes planipes) has been found in thestomachs of black skipjack from the banks off BajaCalifortha. A juvenile sierra mackerel(Scomberomorus sierra) was identified in thestomach contents of a black skipjack collected inthe Gulf of Panama by IATTC personnel.

3.4. 3. Relative and absolute growthpatterns and rates.

The growth rate of adult black skipjack hasnever been investigated. Some information onthe growth of juveniles is given in section 3. 2, 1.

3.5 Behavior

3. 5, 1 Migration and local movements

There have been no tagging programs conductedwith this species and therefore there is no exactinformation on migrations. Possible seasonalmovements have been discussed in section 2. 2, 1.

3.5.2 Schooling

Black skipjack, like other tuna-like fishes,tend to school by size. Bini (1952) stated that,in Peruvian.waters, black skipjack are frequentlyfound schooled with yellowfin tuna and oceanicskipjack of the same size. They were present insmaller quantities than the commercially

1143

important specie s, however.

Commercial tuna fishermen have observedthat black skipjack will frequently gather arounddrifting or anchored tuna boats. Yellowfin andoceanic skipjack also display thís type of be-havior, but it is believed to be more pronouncedwith black skipjack.

3.5.3 Reproductive habits

Spawning is pelagic; outside of that nothingis known of the reproductive habits of thisspecies.

4 POPULATION (STOCK)

4.1 Structure

4.1.1 Sex ratio

There is no information on.the sex ratio ofE. lineatus, De Sylva and Rathjen (1961) foundthat the sex ratio of E, alletteratus landed inFlorida did not differ significantly from 1:1.

FIb/S'45 Tuna 41

EXPLOITATION

5.1 Fishing equipment

5.1.1 Fishing gear

There is no fishing gear specifically used forE. lineatus; however, this species is incidentallytaken by gear engaged in exploiting other species.Black skipjack have been taken by tuna purse-seines, live-bait pole and line gear, commercial.trolling gear, and sport fishing gear. In variousplaces in Latin America black skipjack are takenby trolled hand lines. Walford (1937) stated that"they will strike any lure trolled at any speed upto eight or ten miles an hour.

5.1. 2 Fishing boats

There are no boats specifically designed forfishing for black skipjack. This species isoccasionally taken by all lypes and sizes ofcommercial tuna vessels from small albacoretrollers to the largest live-bait and purse-seinevessels. In Ecuador and Peru, black skipjackare taken by canoe and raft fishermen as well asby the other types of boats mentioned above.

5. 2 Fishing areas

5. 2.1 General geographical distribution

Black skipjack are distributed over a widearea in the Eastern Pacific (see section Z. 1).; how-ever, they are not fished commercially anywhereexcept for a few caught in Latin American waterswhich are sold fresh in the local markets.Virtually all of the black skipjack landed arecaught within a few miles of the coast.

5. 2. 2 Geographical ranges (latitudes,distances from coast, etc.)

See sections 2.1 and 5.2.1.

5. 2. 3 Depth ranges

Black skipjack are taken in the surface layer.

5.3 Fishing seasons

5.3.1 General pattern of fishing season

There is no fishing season as such for blackskipjack. They are encountered more by chance

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than by design.

5. 3. 2 Duration of fishing season

Black skipjack are caught all year through-out most of their range.

5. 3.3 Dates of beginning, peak andend of season

The landed catch is probably very smalland very little of what is landed is reported.Therefore, it is impossible to designate abeginning, peak or end of the fishing seasonexcept in very localized areas. In 1954 theRepublic of Ecuador made a census of fishermenin the course of which, among other things, thefishermen were questioned on the seasonal abun-dance of the various species landed. The blackskipjack was listed as being most abundant fromMay to September in the open coastal areas andfrom January to March in the Gulf of Guayaquil(Ecuador, n.d.).

5.3.4 Variation in time or duration offishing season

The fishery is not sufficiently developed forvariations in time or duration of fishing seasonto 'be apparent.

5.4 Fishing operations and results

5.4.1 Effort and intensity

Negligible.

5.4.2 Selectivity

Black skipjack are taken by non-selectivegear.

5.4.3 Catches

Catch statistics for black skipjack are spa rs'e.Most of the landings are probably made by smallboats in Latin America and go unrecorded.Landings have occasionally been recorded atMancora, Peru, but it is unknown what propor-tion they are of the total landings even in thatimmediate area.

Tuna purse-seiners sometimes set on blackskipjack by mistake. These catches are dumpedoverboard but estimates of the tonnage have' been

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recorded in the logbooks. These references showthat some fairly large schools of black skipjack areoccasionally encountered and accidental catchesfrom i - loo tons have been recorded.

5.44 Past and present factors of effecton operations and results

The principal factor, past and present, affect-ing fishing operations and results is the lack of amarket for black skipjack.

5. 5 Fisheries management and regulations

None.

5. 6 Fish farming, transplanting and otherintervention.

None

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