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Species Synopsis No, 7FAO Fisheries Biology Synopsis No, 50 FIb/S50(Distribution restricted) SAST - Tuna
SYNOPSIS ON THE BIOLOGY OF LITTLE TUNA Euthynnus yaito Kishinouye 1923
Exposé synoptique sur la biologie, de la thonine Euthynnus yaito Kishinouye 1923
Sinopsis sobre la biologfa del ati.mito Euthynnus yaito Kishinouye 1923
Prepared bySHOJI KIKAWA and STAFF of the
Nankai Regional Fisheries Research LaboratoryKochi, Japan
FISHERIES DIVISION, BIOLOGY BRANCHFOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONSRome, 1963
218
FIb/S 50 Tuna 1:1
IDENTITY
1.1 Taxonomy
1.1.1 Definition
Phylum VERTEBRATASuperclass Gnathostomata
Class OsteichthyesSubclass Teleostomi
Superorder TeleosteiOrder Percida
Suborder ScombrinaFamily Scombridae
Subfamily KatsuwoninaeGenus Euthynnus Jordan andGilbert 1882
Species Euthynnus yaitoKishinouye 1923
1.1.2 Description
- Genus Euthynnus Jordan andGilbert
"Body plump, rounded and naked outside ofthe corselet. Mouth rather large, maxillaryreaching the vertical from the centre of the eye.Teeth more developed in size and number thanin the other genera of the Katsuwonidae. Theyare found not only in both jaws, but also on thepalatines and sometimes on the vomer too.Teeth on the palatines are in single row. Theright lobe of the liver much elongated as in thegenus Auxis. It is remarkable that the chiefcutaneous artery runs along the dorsal externalside of the chief cutaneous vein, quite contraryto the case in all the other forms of the plecos-tean fishes, and the dorsal segmental branchesof the chief cutaneous vein pass over the accom-panying artery, which is a little more or lessdeeply imbedded in the muscle. The degenera-ted hypaxial cutaneous artery lies ventral, thatis external, and similarly the accompanyingvein. Hypaxial, cutaneous blood-vessels arebent in a zigzag line. They have no connectionwíth the vascular plexus, nourishing the darkred portion of the lateral muscle. The sub-spinal rod of the vascular plexus is also welldeveloped, but the rod is separated from thevertebral column by the development of theepihaemal process, between the vertebral columnand haemalcanal. Thus the inferior foramen isremarkably larger than in the genus Katsuwonus.
219
"Dark marking in the naked part of the back,and generally some greyish spots in the pectoralregion above the ventral.
'P'ishes of this genus attaind about the samesize as the striped bonito. They are degenera-ted forms, derived from the genus Katsuwonus.Voracious fishes of temperate and tropical seas,not forming large schools, and often approachingthe coast. Until recently only one species wasknown, but I have found other two species in thePacific, quite different from the Atlanticspecies" (Kishinouye 1923).
- Euthynnus yaito Kishinouye 1923
"D. 15-16, 12-13, 8. A. 13, 7. Gill rakers8-10 + 22-24. Vomerine teeth present. Thischaracter clearly separates this species fromthe allied species of the Atlantic, with which ithas been hitherto confounded, as the presenceof the vomerine teeth in this species had beenoverlooked. Vomerine teeth are arranged inone row on a longitudinal ridge. Palatine teethare also on one row only. The upper jaw has27-30 teeth, while the lower has 24-27. Gillrakers in this species are fewer in number thanin the allied species of the Atlantic. The latterhas il-28.
'The cutaneous artery send arterioles fromthe inner and lower side in one row, while thevenules to the cutaneous vein are arranged in tworows, alternate on the inner side. To the epaxialcutaneous blood vessels both the upper and lowersegmentary branches are connected.
"Skull broad, its breadth is contained 11/3 inits length. The alisphenoid and prootic meet,and form a bridge over the groove of the prootic.Two pairs of the auxilary intermuscular bones arefound on the dorsal surface of the exoccipitals, onepair of which is situated just above the foramen ofthe spinal cord, and the other at the lower end ofpaired vertical ridges continued from the top ofthe epiotic. The supraoccipital crest is verybroad, and its vertical side meets the fusedmedian ridge of the exoccipitals.
"Black blueishblack with many dark obliquebands. Belly silvery with three or more greyishspots below the pectorals. Fins black or greyish,the ventrals are partly black and fringed with halk-white. Iris silvery with beautiful reflection, a
1:2 FIb/S50 Tuna
black spot under each eye.
Found chiefly in the southern part of ourempire. The northern limit of distributionseems to be near Chiba-ken on the Pacific coast.Lately Mr. K. Nomura sent me a specimen ofthis species, caught near Tsuruga, Fukui-ken,inOctober 1921. This is the first specimen fromthe Japan Sea. Among specimens of scombroidfishes from the Dutch Indies, kindly sent byMr. Gobee, I found three immature forms ofthis species, but the southern limit of distri-bution is riot yet determined.
This species is rather rare, and is not foundin schools, As it approaches the shore, a fewexamples are sometimes caught in drag seinesand pound-nets. Also caught with rods and linesassociated with bonito and immature tunnies#!(Kishinouye 1923)
1. 2 Nomenclature
1.2.1 Valid scientific name(s)
Euthynnus yaito Kishinouye 1923
1. 2. 2 Synonyms (according to Fraser-Brunner 1949)
Thynnus thumiina Temminck and Schlegel1850 Fauna JaponicaPoiss., p.95, pl.48.
Euthynnus alleteratus Jordan, Tanaka andSnyder 1913. Jour. Coil. Sci. Tokyo, xxxiii, p. 348fig. 88.
Gymnosarda alleteratus Meek and Hildebrand1923. Pubi. Field Mus. Nat. Hist. Zool., xv, pt. ip. 311.
Euthynnus alleteratus Fowler 1928. Mem.B.P. Bishop Mus. x. pl. ix, A.
Euthynnus affinis affinis (Cantor 1850).Fraser-Brunner 1949. Ann. Mag. Nat, Hist.,2( 20):622-628.
Fraser-Bruriner regards E. affinis affinisas the valid name and E. yaito as its synonym.However, the latter has been taken as the validname here, the reason for which will be explainedlater (see section 1.3.1).
1. 2. 3 Standard common names,vernacular names
220
TABLE I
Common and vernacular names
1. 3 General variability
1. 3.1 Subspecific fragmentation (races,varieties, hybrids)
- Varieties
There are still some questions regarding theclassification of the genus Euthynnus, andopinions about the subspeciations differ with thescientists.
Fraser-Brunner (1949) reviewed the fisheso the genus Euthynnus and established a newsystem. He placed yaito and lineatus as thesubspecies of affinis. Therefore, accordingto him, Euthynnus from the seas of the EastIndies' and southwest Pacific are E. affinis affinis
Country Standardorimonname
Vernacular name(s)
AustraliaCeylon
ChinaHawaii
India
Indochina
Indonesia
Japan
New Guinea
Philippines
mackerel tuna
lesser bonito
little tuna
yai.to
kababida
oceanicbonito
atevaila, raguduva,sureya (Sinhalese);shurai (Tamil)Tow chungbonito, kawakawa.,kawakawa kinauchoori miri, suraly(Tamil); ohaman,chuki (Marathi)
Ca ngu
Tongol, diverg-bonito, tongol komo,poetilaisuma, hiragatwuwo,obosogatsuwo,s egatsuwo,sumagatsuwo,watanabe
katsarita, turingan,yaito bonito.
Fig. i Euthynnus yaito Kishinouye
221
(Cantor) and E. affinis yaito (Kishinouye).
Information on Euthynnus is given below.
Herre (1940) says that; "Whether Euthynnusalleterata, the Atlantic or striped bonito,occurs in the Pacific, is not clear to me.Jenkins, Jordan, Fowler, Gdnther, and othershave recorded it from the tropical Pacific, allthe way from Tahiti, Australia, and Hawaii toNew Guinea and Japan, but the very few speci-me ns I have been able to examine criticallyhave proved to be Euthynnus yaito. However,I have not examined Polynesian or Australianspecimens, but have seen in Polynesia fisheswhich I thought were probably E. alleterata,but which may well have been E. yaito like thePhilippine examples.
Fraser-Brunner (1949) stated that "Formany years it has been customary for themajority of authors to consider the genusEuthynnus (Luetken) Jordan and Gilbert, toinclude a single variable, widely distributedspecies. A study of the material preserved inthe British Museum (Nat. Hist.), however,com-bined with an examinationof numerous freshlycaught specimens in the Gulf of Aden, providedgood reason to believe that there are at leasttwo species, 'each with local subspecies or race.tt
Serventy (1950) reported that: "This widelydistributed species appears to have differentiatedinto two geographical forms, E. a. alleteratusfrom the Atlantic Ocean and Mediterranean Seaand E. a. yaito from the Pacific Ocean. Thetwo main characteristics by which theseraces can be distingiished are color pattern andgillraker counts. The black skipjack,Euthynnus lineatus, from the tropical Pacificcoast of central America is a near ally but showssome striking differences.
LaMonte (1950) stated thal "Confusion occursin reports of the presence of these fish becausein many cases the species have been undeter-mined There appears to be at least threedistinct species, looking very much alike. TT
As the result of the detailed comparativeanatomy on E. lineatus E. yaito andE. alleteratus, Godsil (1954) stated that in hisopinion Kishinouyes classification was the mostreasonable.
222
As to Thynnus affinis (Cantor), Cantor1soriginal description is the only authority, andhas never compared anatomically with E. yaito.Matsumoto (l959),who made identification ofEuthynnus larvae from the East Indies, heldthe following view:
"The author believes that Kishinouyerssuspicion concerning the similarity of E. yaitoand T. affinis may be correct. If so, thespecific name affinis would have priority overyaito. However, because the description ofaffinis is inadequate, and because neither thetype specimen nor other adult specimens ofthis species were available for examination, nodefinite statement concerning the nomenclaturecan be made. Therefore, the I.ndo-Pacificspecies is designed as yaito in this paper."
1:4 FIb/S50 Tuna
FIb! S50 Tuna 1:5
- Meristic counts
TABLE IIMeristic counts of Euthynnus yaito
1/ Adult specimens from Japan (Kishinouye 1923)J Juveniles, smaller than 178 mm from the Phillipines (Wade 1950)J Young fish, between 159 and 235 mm, from Japan (Yabe et al. 1953)
4f Adults from Hawaii (Godsil 1954)
TABLE III
A tabular comparison of skeletal differences
223
ist dorsal rays2nd dorsal rays
15-1612-13
1612-14
15
9-1214-1512-13
Dorsal finlets 8 8 8 8Anal rays 13 13-14 10-13 13-14Anal finlets 7 7 6-7 7Gill rakers:
upper limb 8-10 7-8 7-11 7-9angle 1 01lower limb 22-24 22-23 21-23 22-24
Character E. lineatus E. alleteratus E. yaitoNature of
difference
Vertebralcolumn
Protuberance
Caudal keelsmainly on
Cranium
37 vertebrae
4 large lobes oneach 31st and 32ndvèrtebrae
31st and 32ndvertebrae
1. Dentigenousplate on vomer, asin yaito
39 vertebrae
Incipient lobe on33rd and 34thvertebrae33rd and 34thvertebrae
Sharp longitudinalridge on vomer
Supraoccipitalcrest atlowestpoint, appreciablyhigher thantemporal crest
Longitudinal bridgeof bone present inorbit
39 vertebrae
No trace
33rd and 34thvertebrae
Dentigenous plateon vomer as inlineatus
Positive
Positive
Positive
Positive
Relative
Relative
2. Supraoccipitalcrest at lowestpoint in same hori-zontal planeas tem-poral crests, as inyaito
Supraoccipitalcrest at lowestpoint in samehorizontal planeas temporal crests.as in lineatus
3. Intermediate be-tween yaito and
Longitudinalbridge of bone inorbit, absentalletteratus
2 DISTRIBUTION
¿.1 Delimitation of the total area of distri-bution and ecological characterizationof this area
The occurrence of yaito has been reportedfrom: Hawaiian archipelago, Japan, Ryukyu,Taiwan, Indochina, Philippines, East Indiesarchipelago, East and West Caroline Is.,Marshall Is. and California.
According to Kishinouye (1923), the northernlimit of its distribution, in the adjacent seas ofJapan, is considered to be around 34°N on thePacific coast, and the occurrence is quite rarein the Japan Sea (Kishinouye 1922). Yabe etal(1953) reported that the fishing grounds in theSouthern Seas of Kagoshima are within 10 milesfrom the coast and the fish is caught togetherwith bluefin, yellowfin and skipjack. It occursalso in the offing of To sa. According toKawasaki (1960), thìs species is caught in theshallows of Izu Is land.
In September 1952, a specimen of E. yaitowas caught in the port of Los Angeles, California.Up to that time E. yaito had not been recordedon the Pacific coast of U.S.A; however, exam-ination showed the fish to be E. yaito (Fitch 1953).
The main distribution of E. yaito is in thewestern Pacific, west of the Hawaiian Islandsand near the East Indies. It extends from themiddle of Japan through Taiwan, Indochina andthe Philippines as far as Australia. E. yaitoalso lives near islets in the ocean, but has notyet been recorded in pelagic basins far distantfrom islands or continents.
2. 2 Differential distribution
2.2.1 Areas occupied by eggs, larvaeand other junior stages: annualvariations in these patterns, andseasonal variations for stagespersisting over two or moreseasons. Areas occupied byadult stages: seasonal and annualvariations of these
- Eggs
No information on planktonic eggs isavailable.
2214
- Larval stages
Postlarvae of E. yaito have been obtainedaround Kyushu, near the Philippine Islands, inthe Sulu Sea, the Celebes Sea (Wade 1951) and inwaters around the East Indies (Matsumoto 1958,1959). They have not been caught in the IndianOcean except on the west coast of Sumatra.According to the reports of the Dana trans-Pacific survey, the postlarvae were collectedexclusively near coasts, but not in the centralparts of the Ocean. The water temperatureranged from 24. 5 C to 29. 0 C in the pointswhere collections were obtained. Matsumoto(1959) says, "Although there is no assurancethat these values truly represent the lower andupper limit of temperatures tolerated by theyoung of Euthynnus.. they at least give us an ideaof the wide temperature range throughout whichthese larvae can exist".
Wade (1951) reports postlarva collection nearthe Philippine Islands as shown in Table IV.
- Juvenile stages
Kishinouye (1923) reported a 115 mm specimenof E. yaito from the port Keelung of Taiwan.Wade (1950) collected specimens of 33. 5 to175 mm near Batangas of Luzon Island, Manadoof Celebes Ïsland and Cape Maragabato ofMindanao Island. According to Wade, many ofthese juveniles enter into the large bays of-Batangas and Balayan, which face the open sea.
- Young fish
According to Yabe et al (1953), from Augustto October many young ofTi5O to 250 mm length,which are presumed to be in their first year,come to the waters near Aburatsu in southernXyushu and are fished. The catch of yaito youngreported by Yabe et al is given in Table V.
- Adult
Adult E. yaito are fished along the Pacificcoast of Jpa?m Ryuku through Satsunanwaters as far as Chiba Prefecture. Fishingnear Izu Islands begins in May and continuestill November. The catch is reported to be ofschools striking the shallows (Kawasaki 1960).
In the middle Pacific they are caught nearthe Hawaiian Islands, Line Islands and Phoenix
FIb/S50 Tuna 2:1
60 go 40 30 20 t0 o to 20 30 40 50
Fig.
ZA
reas
of
capt
ure
of E
. yai
to, b
oth
adul
ts a
nd la
rvaeIT
O16
0I0
¡4-0
¡30
¡20
140
150
160
TO
¡80
3040
5060
90lo
ol'o
12.0
130
roso
¡50
¡40
¡30
¡20
7080
0lO
OliD
IZO
¡30
140
Iff0
160
ITO
00¡7
0¡6
040
5030
60
TABLE IV
Monthly catch of larval Euthynnus yaitotaken in Philippine waters, 1947 - 1949
Note: A specimen in Wade's collection, 10.5 mm long, has only 8 first dorsalspines and has three rows of chromatophores on the side of the caudal region.By these, Matsumoto (1959) presumes this to be Auxis . If his presumption iscorrect, it is possible that Auxis is mixed in the collection given in Table IV.
226
MonthTotal number.
JTows with larvae Larvae
ofPlankton tows Number % Number
Average catchper tow
1947October 21 3 14.2 6 0.28December 15 3 20.0 49 3.27
1948February 10 2 20.0 16 1.67March 18 5 27.8 12 0.67April 27 3 11. 1 17 0.63June 27 - - - -July 25 2 8.0 8 0.32August 17 3 17.6 35 2.06September 28 6 21.4 25 0.89
1949January 30 11 36.7 141 4.70February 43 11 25.6 681 15.70May 38 3 7.9 5 0.13June 25 - - - -July 18 - - - -August 50 4 8.0 152 3.40September 35 2 5.7 3 0.09October 20 - - - -November 16 2 12. 5 14 0. 87
TOTALS 463 60 13.0 1,164 2.51
FIh/S5O Tuna 2:3
2 :
Islands (Eckles 1949 Smith and Schaefer, 1949).They are fished in the waters within two milesof the east coast of Oahu Island and MokumanuIsland from May to October (Tester 1952).
Around the Philippine Islands they are caughtwith trolling all the year round (Wade 1950).
2. 3 Behavioristic and ecological determi-nants of the general limits of distri-bution and of the variations of theselimits and of differential distribution
Kishinouye (1923) says that E. yaito is foundwithin the temperature range of 18°C to 28°C,the lower limit being a little higher than that forother fishes of Katsuwonidae and Thunnidae.Near Izu Islands, where the E. yaito is caughtduring May to November, the surface tempera-ture in May 1960 was 210 - 23°C and in November22° - 24°C, the highest being attained in Augustat 29°C. From this, the water temperature oftheir habitat is reckoned as being 20°C to 29°C,which nearly coincides with Kishinouyersresults.
TABLE V
Catch of E. yaito young atAburatsuFish Market (1950)
227
FIb/S50 Tuna
According to Yabe et al (1953), it issuggested that E. yaito has a great adaptabilityto salinity. In 1950 the salinity of surface seawater near Aburatsu was 31. 22 to 33. 80 o/oo,and many young were caught in a salinity rangeof 31. 22 to 31. 31 0/00. Near Aburatsu, heavyautumnal rains are supposed to make turbid thewater- belt one or two miles from shore and tocause a considerable low salinity of surfacewater, but these conditions do not greatly affectthe catch.
Date1950
No. of vessels No. of fish
August 30 9 19731 4 123
September 1 5 425z 5 5194 4 955 4 4586 15 5587 2 2648 19 1, 120
9 17 1, 16420 7 335
October 25 i 13
6 4 5611 3 913 5 24
TOTAL: 104 5, 360
3 BIONOMICS AND LIFE HISTORY
3.1 Reproduction
3.1.1 Sexuality (hermaphroditism,heterosexuality, intersexuality)
Heterosexual.
3.1.2 Maturity (age and size)
According to the survey made by Wade (1950)near the Philippines, females of the spawningstage are found among individuals more than450 mm in body length, and males at the corre s-ponding stage among those over 400 mm. Again,females and males in the "spent" stage are seenamong those over 430 mm and 510 mm in bodylength respectively. From these figures theirbiological minimum can be presumed, whichnearly coincides with the results obtained byBuag (1956). Buíag stated that females of themature and the spent stage are found among indi-viduals over 490 mm and 500 mm in body length,respectively. The maturity stages that theseinvestigators give may not correspond perfectlywith each other, but it can be supposed that manyfemales attain spawning ability when they are400 to 500 mm in body length. As for maturitybody lengths for male and female, Wade (1950)states: "The immature females averaged largerthan the males, which may have been the resultof an inadequate sample. Again, it may indicatethat the females grow more rapidly or reachmaturity at a greater age and a larger size thanthe males, if the growth rate of the two sexes isequal".
3.1. 3 Mating (monogamous, polygamous,promiscuous)
Mating of E. yaito has not been observed,but it is supposed to be promiscuous.
3.1.4 Fertilization (internal, external)
External.
3.1.5 Fecundity
Buag (1956) studied the frequency distributionof egg diameter for many ovaries at differentstages of maturity and showed the development ofthe matured egg lump which is to be spawned.The total number of eggs which a female spawns
228
during the spawning period is about 100, 000, butso far no attempt has been made to estimate thenumber of eggs to be spawned.
3.1.6 Spawning
- Spawning seasons (beginning,end, peak)
Kishinouye (1923) states that near Taiwan thespawning of E. yaito takes place about May. Butin some areas individuals at different stages ofmaturity can be found all the year round.According to Wade (1951), the collection of larvaearound the Philippines suggests that spawningactivity is great during December to Februaryand declines somewhat in summer, although inAugust the number of collected larvae hasincreased a little. Wade, however, does notthink that such results of larva collection showthe general tendency of spawning activity in thewaters near the Philippines. According to BnagE. yaito inthis area has no well-defined spawn-
season, but the individual fish has a distinctspawning periodicity with a well-establishedrest period.
- Number of spawnings per year,frequency
Bufag(l956), based on his study of the changeof egg-diameter composition accompanied bymaturation, presumes that an individual spawnsmore than once during the spawning period, buthe does not define the period of a spawning cycle.
3.1.7 Spawning grounds
- Coastal (surface, vegetation,shore, shoal, sand, shelter);bottom
Spawning grounds of E, yaito, if we assumethem roughly to coincide with the distributionareas of the larvae, extend from the tropical tothe subtropical zone, but are restricted to nearshore. Wade (1950) gives the main spawninggrounds as being near the Philippines, Manadoof Celebes Island, Marigabats point of MindanaoIsland, Batangas Bay and Balagan Bay of LuzonIsland.
3.1.8 Eggs: structure, size, hatchingtype, parasites and predators
Forms and structures of a fully matured
FIb/S50 Tuna 3:1
ovarian egg or of a spawned fertilized egg arenot known. Buag (195&) has described residualeggs which are found in the ovary after spawning:"The largest measurable degenerating eggs ofEuthynnus yaito were 33 micrometer units(1. 07 mm) in diameter, translucent, with asingle oil globule measuring from 7 to 8 micro-meter units (0. 23 - 0. 26 mm), and a granular,light orange yolk. " He also mentions: "It isprobable, therefore, that mature eggs begin toripen when they have grown to 27 micrometerunits (0. 88 mm) in diameter. " From these data,it is supposed that mature eggs of this speciesmay be floating eggs about 1 mm in diam eter andwith an oil globule whose diameter is about one-fourth of that of the egg itself.
3.2 Larval history
3.2.1 Account of embryonic and juvenilelife (prelarva, larva, Po stiarva,juvenile)
- Postlarva
The postlarva of yaito has a comparativelylarge head, mouth and eyes, short and triangularabdominal sac, and anus located underneath andanteriorly of the body. The general body formresembles that of other fishes of Katsuwonidae orThunnidae.
According to Matsumoto (1959), the morpho-logical features of postlarva can be recognizedmainly in the pigmentation of the body. First,in individuals of less than lO mm body length,there are one to several chromatophores at themiddle lower jaw. These chromatophores arewell separated from each other and found in theanterior one-half to two-thirds of the lower jaw.Second, there are three to five chromatophoresalong the ventral edge of the body near the caudalregion. In a specimen 12. 2 mm, four additionalchromatophores are found along the base of theposterior end of the second dorsal fin and thefirst three dorsal finlets, which are not presentin s malle r individuals.
As to pigmentation by which larvae of genusEuthynnus may be separated from each other,
22
Matsumoto (1959) writes: "Specifically, speci-mens of yaito and lineatus consistently bearfewer chromatophores along the lower jaw thancomparable specimens of alleteratus.Another difference in pigmentation is seen intherow of chromatophores along the ventral marginof body in the region of the anal fin and finlets.L Lo and lineatus have three to five chroma-tophores, whereas alletteratus has from six toeleven."
Yaito and lineatus resemble each other inpigmentation, but yaito and ailette ratus have40 or 41 myomeres, while lineatus has 38 or 39.The number of vertebrae reaches the adult'sconstant (20 + 19 = 39) in yalto over 9 to 10 mmin body length. Yaito has ordinarily 13 seconddorsal rays and l3il4 anal rays (Matsumoto1959).
- Juvenile
Wade (1950) describes juveniles of 33. 5 to178 mm. Wade shows that yaito differs fromlineatus (Schaefer and Marr 1948) in vertebraland gill raker counts, color pattern and thenumber of dorsal and anal rays. A differenceis also seen in their pigmentation. Schaeferand Marr stated that the entire first dorsal washeavily pigntented, but Wade says: "InPhilippine specimens of E. yaf to the heavy pig-ment is confined to the anteipart of the finand to the outer ends of the posterior spines;and the basal part of the remainder of the fin iseither almost colorless or lighter in color thanthe distal. The basal half of the second dorsalis less heavily pigmented, and there is no con-spicuous series of dark spots along the analbase or on the following finlets. The ventralbars are more numerous in E. yalto and persistuntil the fish reaches a larger size. There is apronounced pale, crescent-shaped area behindthe eye of which is not evident in lineatus."
- Feeding
Kishinouye (1924) reported the fact thatyaito of 36 mm body length had eaten one
10 mm and two 6 mm young of unidentified fish.
3:2 FIb/S50 T.rna
E. yaito, 4.6 mm
E. yalto, 7.6 mm
E. yaito, 9.6 mm
Fig. 3 Euthynnus yaito - larval stages (after Matsumoto 1959)
230
FIb/S50 Tuna 3:3
3.3 Adult history
3.3.1 Longevity
According to Kishinouye (1923), yaito attainsa total length of 60 cm and a body wTjt of3.5 kg, and that on rare occasions individualsover 100 cm long and 10 kg in weight are found.Wade (1950) reported the body length distributionof catch in Philippine waters during October 1947to November 1948 and showed that the largest was66 cm long. Little is known about the life spanof yaito.
3.3.2 Hardiness
Tester (1952), who reared yaito in tanks andponds, recognized that small yit, one to threepounds in weight were more readily establishedthan larger individuals. Judging from theswimming speed, it is difficult for a confinedfish to give the gills enough water to supplyabundant oxygen. Skipjack and frigate mackerelare said to need a strong water current, butTester found that a great swimming speed is notnecessary for the establishment of small yaitothough he also pointed out the possibility oflowering mortality by supplying an adequatewater current and thus causing an optimumswimming rate. These facts are considered toshow the hardiness of yaito, especially of thesmaller individuals
Tester (1952 a, b) reported that these yaitoreared were in good condition for a few months,but that they gradually weakened, failed to feedand eventually died. The causes of this declinewere not clear but it was observed that manydeaths followed a period of cold, wet weather.
3. 3. 5 Parasites and diseases
Little has been reported on parasites of yaito.In skipjack, closely related to yaito, gill filament,pseudo gill, first dorsal membrane and mouthcavity are the main sites of external parasitism.As internal sites the ventral muscles (Cestodalarva), digestive system (Distoma,Acanthocephala), ovary, and liver (Nematoda),can be cited (Yoshisaki, unpublished).
231
Kishinouye (1923) found a very long Nematoda inthe cutaneous artery of yaito. Kishinouye recog-nized Nematoda also in the dorsal aorta of yellow-fin tuna and attributed to it the hardening anddiscoloration of blood vessels ïn the yellowfin tunabut for yalto little is known on this point.
3.4 Nutrition and growth
3.4.1 Feeding (time, place, manner,season)
Van Weel (1952), in his rearing experimentswith yaito, observed the fishest response toaqueous extracts of various diets. The speciesyaito showed little response to extracts ofnatural diets such as squid or ordinary baitfish,but clearly responded to extracts from tuna flesh.From this experiment Van Wed concluded:
the effective component of extracts whichattracts the fish is not fat, but protein;
yaito is more sensitive than yellowfin;and
in feeding, the sense of smell or tasteplays a more important part than the sense ofsight.
In a pond test by Tester et al (1954), yaitowas shown to be attracted tooe strong smell-ing aromatics and to avoid some highly-coloreddyes. In this test, yaito were given extractsfrom flesh, viscera and blood of skipjack,dolphin, anchovy and other fish and from the sub-stance of squid and shrimp, and they respondedto all of these various extracts with active feed-ing. However, their response to extracts ofthe same kind as a subsistence diet was betterthan to other types of diet and it was suggestedthat the response to extracts was entirely con-ditioned by the nature of the food.
Sea tests using various kinds of extractswere made on a skipjack group, but the resultswere negative. From these tests it was de-.duced that also in feeding of yaito the sense ofsmell played little part underiral conditions(Tester et al 1954)
3:4 FIb/S50 Tuna
E. yaito, 33. 5 mm fork length
232
N
E yaito, 67 mm fork length
E. yaito, 110 mm fork length
E. yaito, 136 mm fork length
Fig. 4 Euthynnus yaito - juvenile stages (after Wade 1950)
FIb/S50 Tuna 3:5
3:6 FIbIS5O Tuna
Tester et al describe the manner of feeding,as observed in a tank, as follows: "While feed-ing, they swim with great rapidity, taking foodat or near the surface. They enter into activecompetition for food both with each other andwith the yellowfin tuna which is also present inthe tank.
3.4.2 Food (type, volume)
Yabe et al (1953) examined the digestive tractof yaito jiieiiles near Aburatsu and found thatits main content was Eng raulis japonicus, theremainder being larvae of various kinds ofArthropods. However, in the waters nearayukyu where E. japonicus are not found, fishescomprise a very small portion of the digestivetract content of yaito (0. 8 - 2. 5 kg), which con-sists mainly of larvae of various Arthropoda,squids and Pteropoda (Yabe et al 1953).
Ronquilo (1953) studied the stomach contentof yaito near the Philippines, and found that themain diet was Stomatopoda and Decapoda,e specially their megaiopa larvae, and that fishesmade up only a small portion. It may be con-cluded that in waters where pelagic predominantspecies such as E. japonica are not present thefood of yaito depends to a greater extent onvarious kinds of planktonic crustacean larvae.
3.4. 3 Relative and absolute growthpatterns and rates
No detailed studies have been made on thegrowth of yaito. Some estimations of growthrate during the larval and young periods have,however, been attempted. For example,Kishinouye (1923) supposed that a 115 mm longjuvenile caught in Port Keelung in August mighthave been spawned in May, namely three monthsbefore. Wade (1950) also presumed a juvenile115 mm long from the Celebes Sea to be three orfour months old. Again, Wade estimates33,5 mm and 35 mm young from Marigabato Pointto be one month old and specimens 40 to 175 mmlong from Batangas to be spawned about sixmoñths before. That is, fish 40 to 175 mm longare estimated to be 1. 2 to 6 months old.
Yabe et al (1953) presumed yaito about150 to 250mì fished near Aburatsu to be in
233
their first year of life, and showed the changein the body-length composition of these catchesthrough the fishing season. As the appearanceof young is roughly simultaneous in differentparts of southern Japan and as their activeappetite suggests a large growth rate per indi-vidual, the figure shown could well designate thegrowth pattern of yaito, though for a very shortperiod.
3.4.4 Relation of growth to feeding, toother activities, and to environ-mental factors
The relation between feeding and growth hasnot intentionally been studied either in tank orpond tests. In a concrete tank built on the sea-shore, to give an example, 5 yaito weighingabout 5 lb which were transfeto the tankfrom late August to early November 1951,increased by 1 to 2 lb until they all died in earlyDecember (Tester 1952). The relation betweenthe amount of food eaten and body weight increasewas not defined.
3.5 Behavior
3.5.1 Migration and local movement
Young yaito in the waters near Japan indifferent parts of the Pacific coast west ofSuruga Bay appear over a comparatively shortperiod from late summer to early autumn.Yabe et al (1953) suppose that these young areprobaljin the first year of life, that they migratefrom the southern waters, stay near Japan forseveral months of higher water temperatures,grow feeding on Engraulis japanicus, and leavefor the south when the water temperature becomeslower.
From collection of the larvae near the Philip-pines, Wade (1951) shows the diurnal verticalmigration of yaito. (Table VI). Of 250 net tow-ings during daytime, the larvae were capturedin 16 towings (6.4%), with an average catch of0. 39, and a total of 97 larvae. At night, on theother hand, out of 212 towings 43 (20. 2%) weresuccessful, the catch per trial averaged 5. 05(total 1, 070), being far greater than in the day.This shows that the fish migrate to deeperlayers in the daytime and move upward to sur-face water at night.
FIb/S50 Tuna 3:7
234
Fig. 5 Euthynnus yaito Change of body-length composition of yaito youngduring the 1950 fishing season (Aburatsu Fish Market, Kyüshû; seeYabe et al 1953)
r 26
30r
31
2
5
6
7
8
9
20
2 2so !'2Q5
3:8 FIb/S50 Tuna
TABLE VI
Hourly summary of larval Euthynnus yaito collectedin Philippine waters, 1947 to 1949
3.5.2 SchoolingYoung fish of yaito have the habit of forming
a shoal, as do many other species (Yabe et al1953).
Tominaga (1943) describes the behavior ofyaito as follows: "Though yaito live mainly inrapid tidal currents near sIary islands, theyavoid the farthest flow and hold the body in a
235
comparatively slow current. When they find asuitable place they stay there as a nest and dartat crustaceans (for the most part the megalopalarvae), squids and young fishes which happen todrift near them and capture these. They pursuethe food animals obliquely anteriorly but do notfollow them posteriorly for a long distance andreturn to the nest. So we must arrange so thatthe imitation bait may come within the sight fieldanterior to them near their nest and at the same
HourTotal number of
plankton towsTows w..th larvae larvae
Avera8ecatcI1Number Perçent Number
0600 - 0659 26 2 7.7 13 0.50
0700 - 0759 18 1 5.5 7 0.390800 - 0859 19 2 10.5 11 0.58
0900 - 0959 23 1 4.4 2 0.091000 -1059 15 1 6.8 24 1.60
1100-1159 18 - - - -
1200 - 1259 22 - - - -
1300 - 1359 19 1 5.3 8 0.42
1400 -1459 21 3 14.3 13 0.621500-1559 16 1 6.2 1 0.06
1600 -1659 31 2 6.5 5 0.16
1700 -1759 22 2 9.1 13 0.591800- 1859 16 3 18.7 9 0.56
1900 - 1959 21 7 33.3 619 29. 50
2000 - 2059 16 2 12.5 49 3.06
2100 - 2159 23 4 17.4 55 2.38
2200 - 2259 21 2 9.5 9 0.43
2300 - 2359 17 - - - -
0000 - 0059 18 4 22. 2 25 1. 39
0100 - 0159 13 5 38.4 14 1.07
0200 - 0259 21 6 28.5 67 3:19
0300 - 0359 15 5 33.3 56 3.730400 - 0459 13 3 23.0 3 0.23
0500- 0559 18 2 22.2 164 9.12
time to simulate the respiratory movement ofsquid. With a mere straight hauling we cannotget a good result. This relation between thebehavior of fish and the fishing technique is veryinteresting. In dim light of morning and eveningthey leave the nest and eat plankton and smallfishes, forming small schools and splashingwater at the surface. Then we can angle themwith scattering bait, although in this way theycannot be fished so abundantly as in the case ofskipjack which leap at the surface much morewildly. In particular, as yaito feed on megalopalarvae which are carried by upwelling currentscaused by the tide, they do not leave islands fora long distance, and only very rarely migrateinto an open ocean, and,moreover, the schoolsthey form are very small, So they cannot be theobjective of skipjack fishery. Their jaw teethare small in size and their gill rakers also thoseof plankton-feeders. But they have vomer teethwhich permit them to catch slightly larger foodsand this, as described above, causes themnot to form a large school but to seek for a nestin an appropriate site.
Hiatt and Brock (1948) in the northernMarshall Islands observed three medium-sizedyaito herding a closely packed school of severalhundred scad (Decapterus sanctaehelenae(Cuvier) ) over a large coral head in Rongeriklagoon. The authors reported that: "The threetuna usually followed the school of scads ratherclosely, with one tuna at each rear flank of theschool and the third lagging behind them. Nowand then the scads wouldturn off to one side, atwhich time the tuna oiithat side would moveforward swiftly and herd them back into line. Itbecame obvious that the school of scads was pre-vented from leaving the area over the top of thecoral head, being herded back whenever it movedover.deep water. On one occasion a laggardscad was swiftly picked off by the rearmost blackskipjack; however, except for this incident,thetuna made no attempt to prey upon the scads dur-ing our period of observation. Scads are un-common in the lagoons in the northern Marshallsand probably do not constitute a large proportionof the diet of this species of tuna, "
Based on his rearing test in a tank, Van Weel(1952) reporta as follows: "When two yellowfinand five tunny (yaito) were present, the twospecies tended to school separately and to exhibitdifferent reaction patterns. After one yellowfindied, the other joined the tunny, and although a
236
slower swimmer, it attempted to keep up withthe school." Tester (1952) observed that:"Occasionally, the large fish would bump thesides of the wall; the small ones seemed ableto keep clear. When both large and small fishwere present, they tended to swim as a school,the large one keeping to the outside and the smallones to the inside of the circular course. "
FIb! S50 Tuna 3:9
FIb/S5O Tun.',
4 POPULATION (STOCK)
4.1 Structure
4.1.1 Sex ratio
Wade (1950) gives data on the body lengthand sex composition of yaito caught by trollingin Philippine waters during November 1947 toNovember 1948. Table VII was extracted fromWade's data to indicate the sex ratio of thecatch for every month. This Table shows thatthe sex ratio for the whole sample is nearly 1:1,and that in autumn and winter the proportion ofmales is greater while in spring and summerthe ratio is 1: 1 or rather that the number offemales is larger.
4.2 Size and density
4.2.1 Average size
Yabe et al (1953) showed the average sizeyaito in tii aters near Aburatsu as given. inTable VIII.
237
4.4 Mortality morbidity
4.4.1 Rates of mortality
No information is available on naturalmortality.
The mortality rate in live wells according toTester (1952), is.7.5 % forlarger fish and17. 9 % for smaller ones. It was also suggestedthat the mortality during transportation dependslargely on the design of the live well, namely,its capacity, pumping ability to renew the water,establishment of corner baffles, and so on.Smaller yaito are comparatively easilyestablished in tanks or ponds, and Tester (1952)reported case where 18 out of 23 wereestablished, with a mortality of 21. 7 %.
4.4.2 Factors or conditions affectingmortality
I41
of No information is available on factors affect-ing mortality under natural conditions.
TABLE VII
Sex ratio of E. yaito caught by trolling in Philippine waters
Month No.of Male Female M +F x 100
1947November 1 0 1 100December 26 19 7 26. 9
1948January 42 22 20 47.6February 108 58 50 46.3March 13+1 4 9 69.2April 38+1 12 26 68.4May 86+2 48 38 44.2June 23 12 lI 47.8July 18 9 9 50.0August 1 1 0 0
September - - - -October 14 9 5 357November 86+1 49 37 43.0
TOTAL: 456 + 5 243 213 46.7
Deaths under rearing experiments can besaid to occur in the following three periods:
Deaths in live wells during transportation;Deaths during pre-feeding period in tankor pond; andDeaths occurring after establishment intank or pond.
Deaths in live wells and during pre-feeding,according to Tester et al (1954), can be attri-buted to the restricte ace, but the physio-logical incompleteness due to unsuitability ofoxygen or temperature is, probably, of secon-dary importance. White corner baffles, ceilingcover wire netting and illumination by night haveproved considerably effective in lowering mor-tality. Deaths after establishment are supposedto result mainly from the physiological incom-pleteness, but the cause is not yet clear althoughlowering of temperature is recognized to havesome effect (See section 3. 3. 2).
TABLE VIII
Average size of E. yaito landed at Aburatsu Fish Market (1950)
238
MonthNo. of fishmeasured
Body length (mm)Ranges Mean sizes
August 26 107 183 - 240 20030 51 201 - 225 21331 98 203 - 239 217
September 1 53 194 - ¿40 2142 19 199 - 268 2184 92 209 - 245 2255 78 204 - 290 2246 91 200 - 309 2217 79 203 - 307 2358 132 196 - 325 2349 149 200 - 271 22220 115 213 - 274 23625 13 223 - 255 ¿36
October 6 5 230 - 295 24111 9 252 - 278 26213 24 254 - 286 ¿6516 138 244 - 313 27017 111 220 -332 272
L+:2 FIb/S50 Tuna
5 EXPLOITATION
5.1 Fishing equipment
5.1.1 Fishing gear
Fishing of yaito in the neighboring waters ofJapan is effected mainly with trolling from small-sized sailing boats and, on some occasions, withtrolling from small motorized vessels or anglingwith Engraulis japonicus as bait. Tbe speciesyaito which are caught in skipjack and tunafishery (pole and line) are mostly about 800gror more in body weight.
Traditional Japanese fishing gear for yaitoand young of bigeye and bluefin is describedbelow. Bamboo poles 1 to Z m in length, andthree-yarned hemp twine, are used. The totallength of line, with a gut about 50 cm long joinedto the end, is equal to that of the pole. A small-sized hook is used for skipjack angling. At firstthe Engraulis juvenile is hooked and used as baitwhile little shrimp are continuously scattered;the fish then become active in taking bait, andimitation bait can be used. For skipjack anglingsmall-sized imitation bait, or sometimes alsoahook with a tin lever and a plume, is used. Withimitation bait, the length of pole is about 5 to 7 ni,the line being as long as the pole.
In Taiwan, the Philippines and also Hawaii,yaito are fished mainly by surface trolling, andalso with corrals in the Philippines (Herre 1940).Concerning trolling in Hawaii, Tester (1952) re -ported: '
FIb/S53 Tina
difference in selectivity between pole and lines,and trolling.
5.4.3 Catches
Owing to incomplete fishery statistics, theannual catches in different parts are uncertain
5. 5 Fisheries management and regulations
Neither management nor regulation ispractised.
5. 6 Fish faiiìiing, transplanting and otherintervention
Except for rearing experiments for ecologicalstudies, nothing has been undertaken.
2140
