NOAA Technical Report NMFS Circular 451 

Synopsis of Biological 

Data on Skipjack Tuna, 

Katsuwonus pelamis 

Walter M. Matsumoto, Robert A. Skillman, and 

Andrew E. Dizon 

January 1984 

FAO Fishel ies Synopsis No. 136 

U.S. DEPARTMENT OF COMMERCE 

Malcolm Baldnge, Secretary 

National Oceanic and Atmospheric AdministratIOn 

John V Byrne, Administrator 

National Marine Fisheries Service 

William G Gordon Assistant Administrator for Fisheries 
The National Marine Fisheries Service (NMFS) does not approve, recommenrl or endorse any proprietary product or proprietary material 

mentioned in this publication. No reference shall be made to NMFS, or 

to this publication furnished by NMFS, in any advertising or sales promotion which would indicate or imply that N:YIFS approves, recommends 

or endorses any proprietary product or proprietary material mentioned 

herein, or which has as its purpose an intent to cause directly or indirectly 

the advertised product to be used or purchased because of this NMFS 

pu blica tion. 
CONTENTS 

Introduction ............... . ......... .. ....... . ......... . ............. .. .... .. .. . . . . .. ... . .. . ..... . . . ... . 

1 Identity ......... . ...... . .... . .... . ..... . ... . ........ . ......... ... . . .. . . ....... .. ... . .. . .. ... . . ... . . . . 

1.1 omenc1ature . ..... . ... ... . . .. . .. . . . . .. .. . ................. . ..... .... .. ... ... . .. .. .. .. .. . . .. . . .. . 

1.11 Valid name .. .. .... .... . .. .. .... .... . .. . . . ............ .... . . . ... . ... . ... .. .. .. .. .. . .. . .. . . . 

1.12 Objective synonymy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

1.2 Taxonomy. ... ... . . . . . .... ..... .. . . .. .. .. . .. . . . . . . .. ... .... ....... . ........ .. .. . .. . . ... . . . . . . . .. . 2 

1.21 Affinitie ...... . ....... . .. . ..... . ..... .... ... . .. . . . . . ....... . .............. . .... ..... ...... 2 

1.22 Taxonomic tatu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

1.23 Subspecies... .. . .. . .. .. . . .. ..... . ....... . ... .. . ... . ..... ............. . ... . ........ . .... . . . . 3 

1.24 Standard and common name , vernacular name . .. . .... .. ............... . .............. ...... . 3 

1.3 Morphology ... . .................. . ...... .. .. . . .. . ... . .. ................. ... ... ... .. . ...... . . .. ... 3 

1.31 External morphology (and description) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

1.32 Cytomorphology. . .. . . .. . ................ . . .. ........ . ........................... . . . .. ... .. 6 

1.33 Protein pecificity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

2 Di tribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11 

2.1 Total area .... .. .... . ....... . ................. . .......... . ............. . .............. . . .. . . . . ... 11 

2.2 Differential d. tribution ..... . ...... . ............... . ................ . ............... . .. .... .. . .... 12 

2.21 pa\\n, larvae, and ]U\enile ..................... . ........ . ................ . ........ . .. . . . ... 12 

2.22 Adult .. .. .......... . .... . . . ................ . ................................... ..... . ... . 14 

2.3 Determinant of di tribution change ............................... . ....................... . . . . . .. . . 17 

2.31 LarvaeandjuveOlle . .. . .... . ............................................................. . . 17 

2.32 Adult ........ . ....... . .. . ....................... . ............................ . ... . . . ..... 18 

3 Bionomic and life hi tory ...... . . . . . .. . ................................................................ 20 

3.1 Reproduction .. . ..... . ................................... . ................................... . ... 20 

3.11 exuality .. . ................................. . .................................... . ........ 20 

3.12 l'.latunty .. ... ............................................. . ............ . ........... . .. .. .. 20 

3.13 and 3. 14 Mating and fertilization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 22 

3.15 Gonad ................................... . ............................................. 22 

3.16 pawning ............................................................................ .. .... 23 

3. 17 pa \\ n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 26 

3.2 Preadult phase. . . . . . . . . . . ... . .. .. ...... . .. . .......................... . ........... . ... . .. .. .... 26 

3.21 Embryonic pha e ....... . . . . . ............................................ .. ................ 26 

3.22 Lan. al pha e .. .......... . ................................................................ 26 

3.23 Adole cent phase . .......................................................................... 28 

3.3 Adult pha e . . .... .. . . ... .. .... . . . .......................................... . .... . ................ 28 

3.31 \1aximum ize and longevity ..... . ........................................ . ..... . .. . ........ . 28 

3.32 Hardines ..... . . . ...... .. .......... . .. . ....... . ............... . ........... . ............ . .. 28 

3.33 Competitors ............................................................... . .. . ............ 31 

3.34 Predators ... .................. . ..................................................... ... . . . . 31 

3.35 Parasite, diseases, injuries, and abn rm it~e ............................ . .................... 31 

3.4 utrition and growth .............................................. . .......... . .. . ....... . ......... 32 

3.41 Feeding .. .. ....... . .. . .......................... . ............... . ....... ... ......... .. .... 32 

3.42 Food ................ . ..... . ... .. .... . ................. ... ..... .. .. . .......... . .. . . . ...... 35 

3.43 Growth rate ................... . ..... .. ............................ . .......... . ............ 36 

3.5 Behavior .......... . ... .. ..................... . ................ . ........ . ......... . ....... .. .. . ... 41 

3.51 Migrations and local movements . ........ . . , ......................................... . ........ 41 

3.52 Schooling .............. .. .... . .................... . ........... . ................... .. ..... . 46 

3.6 Physiology . . ... .. .. . ....... .. .................. .. ..................... . . . .. .. ................. .. . 53 

3.61 Energy transfer (metabolism) .. .... .. .. . ........................................... . ... . ..... 53 

3.62 Thermoregulation ................. . ................ . ........... . ....... .. ............. . .. . . 58 

3.63 Hypothetical habitat of skipjack tuna ................... .... .... .. ......... . . . ....... . .... . ... 60 

3.64 Responses to stimuli ..... . . . ........................ . ... .. ...... . . . . .. . ... ......... . .. .. .... 60 

4 Population .. .. .......... . ... . ............................. . ............................ .. .. .. ........ 63 

4.1 Structure ......... . ...... . ... . ......... . ......... .. .................... ... .. . .......... .. .. . . . ... 63 

4.11 Sex ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 63 

4.12 Age composition .. ... .. . . . ...... . ....................... . ..... . . . ..... ... ......... . ........ 63 

4. 13 Size composition . . ......... . .......................... . .... .. ................... .. ......... 63 

4.2 Abundance and density (of population) .... .. ........... .. .. .. .. ......... . . . . .. . ... .. .... . ... .. .. .. .. 69 

4.3 Natality and recruitment . . .. . ......... .. .. .. .... .. . . ... . . . ....... . . . .................. ... . ... ..... . 71 

4.4 Mortality and morbidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 71 

jij 
• - D ;nami of population (a a \\ hole 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 72 

.! 6 The population m the community and the eeo') tern ...... .... . ......... . ...... . ...... . ............... 73 

.l. 1 Ph} i al feature of the biotope of the community .. ............ ... ..... .. .... ... ....... . ....... 73 

4.62 'pe e compo ition of the ommunlty ................................ . ............ . .. . .... . .. 73 

.l.6 [nrerrelallon of the population of the pecles in the communit)' and eco ystem ........... . ......... 74 

4.M H} pot helical habitat of k.i PJ ad.. tuna ............... .. ... ........................ . ..... . .... .. 74 

E: plOllatlon . . . . . . . .. . .............. . .................. ... .. ... .... ...... ... ............... . ... . ..... 74 

".1 Fi hmg equipment .. . . .. . .......................... ........ ............... ....................... 74 

- ~ h hmg area .... . ........................................ . ...................................... 75 

5.3 FI hmg ea on. . ............................ . ............ ............... ... .................... 76 

':04 FI hmg operation and result ................ . .. ................... ..... ................ . ..... . .. .. 76 

5 41 Effort and inten it} ...................... . ......... ...................... . ...... . .. . ..... . .. 76 

5.42 electi\ it} ...................................... . . ......... ........ ..................... . . . 76 

5043 atche ...................... . ................. ................... . ....................... 76 

6 Pr teetlon and management ............... . ............. .. .. ......... . ............................... . . 78 

A kno\\ledgments ............................................. . . ......... ... . ..... .... . .................. 78 

l Ilerature cited ................................................... ....... ... ................. . ........... 78 
Synopsis of Biological Data on Skipjack Tuna, 

Katsuwonus pelamis 

WALTER M. MATSUMOTO,’ ROBERT A. SKILLMAN,’ and ANDREW E. DIZON’ 

INTRODUCTION 

There has been a major expansion in the world’s skipjack 

tuna, Karsuwonus pelamis, fisheries in the past decade or so 

with the catch increasing better than twofold, from 320,000 

metric tons (t) in 1969 to 699,000 t in 1979. While catches increased in both the Atlantic and Pacific Oceans, the most striking increase has been in the western central Pacific, which includes the Japanese southern water fishery and the fisheries of 

Papua New Guinea and the Solomon Islands. In this area the 

catches have risen over sevenfold. from 41,500 t in 1969 to 

316,000 t in 1979. 

The catch of skipjack tuna can probably be increased over 

present levels, but by how much is uncertain. To deal with the 

rational utilization of this resource as it approaches the level of 

full exploitation, it is imperative to have in hand all information concerning the biology, physiology, and various other 

factors relating to the fish and fishery. 

Synopses of biological and technical data and information 

on the skipjack tuna by oceans were first compiled by Waldron 

(1963) for the Pacific Ocean, Postel (1963) for the Atlantic 

Ocean and Mediterranean Sea, and Jones and Silas (1963) for 

the Indian Ocean, and were presented at the Food and 

Agriculture Organization of the United Nations (FAO)-sponsored World Scientific Meeting on the Biology of Tunas and 

Related Species, 2-14 July 1962, at La Jolla, Calif. Forsbergh 

(1980) completed a synopsis of the species for the Pacific 

Ocean. 

This paper reports current knowledge of the species in all 

oceans and reviews and evaluates, where\er appropriate, the 

results of past and recent research on the skipjack tuna 

through 1980. 

1 IDENTITY 

1. I Nomenclature 

1.1 I Valid name 

Kutsuwonus pelamis (Linnaeus 1758) 

1.12 ObjectiLe synoiiymy 

Scornberpelamis Linnaeus 1758 (original description); Osbeck 

1765; Bonnaterre 1788; Gmeliri 1788; Rafinesque 1810 

Scotnber pelaniys Bloch and Schneider 1801; Kino 1810: 

Cuvier 1829; Valenciennes 1836 

Scomher pe/uniides 1.ac;p;de 1802 

Thynnus pelamis. Risso 1826; Steindachner 1868: Hoek 1904 

T/iynnus vagans Lescon 1828 (original description). 1830 

Th-vnnus pdaniys. Cuvier and Valenciennes 183 I ; Temminck 

and Schlegel 1842: Cantor 1850; Bleeker 1856; Ci”ut1it.r 

1860, 1876; Playfair and Cibthcr 1866; Day 1878; Liitken 

1880; Macleay 1881; Eigenmann and Eigenmann 1891; 

Richard 1905; Cunningham 1910; Parona 1919 

Pelarnvs pelam.vs. Bleeker 1862, 1865 

OrcXnnu.5 pelun?.vs. Poey 1868; Goode and Bean 1879; Goode 

1884 

Eurh.vnnus pelumys. Jordan and Gilbert 1882: Carus 1893; 

Jouhin and Le Danois 1924 

Gymnosardu pelanits. Dresslar and Fesler 1889: Jordan and 

Evermann 1898; Jenkins 1904; Snodgrass and Heller 1905; 

Evermann and Seale 1907; Jordan 1907; de Miranda Ribeiro 

1915; Meek and Hildebrand 1923; Breder 1929; Hornell 

1935; Palnia 1941 

Euth.i.nnk pelumic. Smitt 1893; Stark< 1910; Tanaka 1912; 

Jordan and Jordan 1922; Bizelow and Welch 1925; Fouler 

1928, 1938; Herre 1932; Deraniyagala 1933; Nobre 1935: 

Smith 1949: de Beaufort and Chapman 1951; Bigelou and 

Schroeder 1953: Steinitz and Ben-Tuvia 1955: Fourmanoir 

1957; Bailey et al. 1960, 1970; Collette and Gibh 1963; 

1. Nakamura 1965; Robins et al. 1980 

Karwwonus pelanyc. Kishinouye 1915; Frade and de Buen 

1932; Le Gall 1934; Morro\\ 1957 

GymnosUrdu pelarnys. Thompson 1918; Barnard 1925 
harciin~onirs pr/uint\. Ki\hinouye 1923; de Buen 1926, 1930, 

1935, 1958; Jordan et ai. 1930: SoIdato\ arid Lindberg 1930: 

klirenbaurn 1936; Scale 1940; Serventy 1941; God\il and 

Bqcri 1944: Nichols and Murphy 1944: Fonler 1945; Lahlonte 1945; Clcmen\ and M'ilby 1946: Hildebrand 1946; 

Brock 1949; Ri\a\ 1949. 1951: Herre 1953; Belloc 1955: 

Munro 1955, 195X: Brigs\ 1960; Jones and Silac 1960, 1963, 

1964; ScIiuIt7 1960; Postel 1963. 1964: Talbot 1964; Whitley 1964; Williams 1964; Collette 1978 

Kur.su~vonirs i'qqunr. Jordan et al. 1930; Barnhart 1936; Ohada 

and Matsubara 1938: Fomler 1949 

Karsuwonis pelumis. Herre 1933 (typographical error?] 

Eur/?.vnnw (Kut.wwonur) prlaniis. Fraser-Brunner 1950; Mendis 1954: Bini and Tortonese 1955: Williams 1956; Scaccini 

et al. 1975 

1.2 lawnomq 

1.21 Affinities 

Phylum Vertebrata 

Subph\luni Craniata 

Series Pi\ce\ 

Superclas\ Gnatlio\tornata 

Class Teleostoini 

Sub&\\ Actinopterkgii 

Ordcr Pcrcifornie5 

Suborder Sconibroidei 

Family Sconibridac 

Tribe Thunnini 

Subfaniil) Sconibrinae 

<;cn ti? Ku/sir~wnir.s 

Species pe/uinr< 

The Family Scombridae was defined by Regan (1909) and 

further clarified by Starks (1910), uho divided it into fi\e subfamilies: Scombrinae. Scomberomorinae, Acanthocybinae, 

Sardinac, and Thunninae. Kishinouye (19 15) divided Regan's 

Sconihridae into three Families: Scombridae, Cybiidae, and 

Thunnidae. He subsequently (1917) split Thunnidae into trio 

Families. Thunnidae and Katsu\+onidae, and placed both into 

a ne\sly erected Order Plecostei on the basis of their \$elldebeloped s~bct~taiieous \. ular system. Takahashi (1324), 

however, rejected Kishinouye's new order, pointing out that 

the subcutaneous vascular syrtem was not unique to tunas, 

but \vas also found in swordfish, Xiphius pludius; dolphin, 

Cor-vphaena hippurus: stargarer, Gnathapus elonparus; goby, 

Aprocryples chmensir; and puffer, Sphoeroides ruhriceps. 

These two views have persisted to the present. 

Berg (1940) followed Kishinouye in placing the tunas into a 

single Family Thunnidae and renamed the Order Thunniformes. He relegated Katsuuonidae to subfamily \tatus as 

Auzidini. Some investigators retained the Family Thunnidae, 

uliilc rele_eating Kattuuonidae to generic status (Fomler 1945: 

Herre 1953; Morrow 1954, 1957; Munro 1955). Zharoc (1967) 

divided the Thunnidae into two subfamilies, Thunninae and 

Kat\u\voninae, \I hile others recognized the Families Thunnidae and Kat\u\\onidae (Jordan and Hubbs 1925; Barnhart 

1936: Kamohara 1941; tvlorice 1953; Godsil 1954). 

Most other investigators followed Regan and Stark's classification, maintaining the Family Scombridae as a natural unit 

(Barnard 1925: Fo\\ler 1928; Hildebrand 1946; Schultz and 

Stearn 1948: Smith 1949; Frarer-Brunner 1950; de Beaufort 

and Chapman 1951; Rivas 1951; Jones and Silas 1960, 1963, 

1964; Schultz 1960; Collette and Gibbs 1963; Waldron 1963; 

I. Nakamura 1965; Gibbs and Collettc 1967: Collette and 

Chao 1975; Collette 1978). Frater-Brunner (1950) recognized 

only the subfamilies Gasterochimatinae and Scombrinae. I. 

Nakamura (1965) added a third subfamily, Thunninae, but 

Colletre and Chao (1975) reberted to tun subfamilies, as proposed by Fraser-Brunner, and split the Scombrinae into two 

tribes, placing Scombrini (the more primitive mackerels) and 

Scomberomorini (the Spanish mackerels) in one group and 

Sardini (the bonito,) and Thunnini in the other (Fig. I). 

Opinion on the generic name also has differed over the 

years and the differences continue to this day. Jordan and 

Gilbert (1882) credit Liltken for establishing the genus Eurhynnus and separating alliterarus and pelamys from the other 

tunas largely on the basis of an important osteological character, the trellis formed on a portion of the abdominal vertebrae. 

Kishinouye (1915) established the genus Katsuwonus and 

separated it from Eurhynnirs based on differences in internal 

characters. Since then many others have recognized Katsuwonus (de Buen 1930; Jordan et al. 1930; Herre 1932; Serventy 1941; Godsil and Byers 1944; Clernens and Wilby 1946; 

Hildebrand 1946: Rivas 1951; Godsil 1954; Belloc 1955; 

Munro 1955; Briggs 1960; Gosline and Brock 1960; Jones 

and Silas 1960; Schultz 1960; Postel 1964; Talbot 1964; Whitley 1964; Zharov 1967; Collette and Chao 1975; Collette 

1978). 

Fraser-Brunner (1950), in his attempt to maintain the 

Family Scombridae as a natural unit, synonymized Katsuwonus with Euthynnu~, as did other investigators (de Beaufort 

and Chapman 1921: Bigelou and Schroeder 1953; Mendis 

1954; Bini and Tortonese 1955: Williams 1956; Bailey et al. 

Figure I.--The wbfamilir\. tribe,. and genera of the Sromhridsc tfnim C'allrllr 

2nd C'hm IY7Sl. 
1960, 1970; Collette and Gibbs 1963; I. Nakamura 1965; 

Robins et al. 1980). Of those who relied heavily upon internal 

characters, I. Nakamura (1965) supported his choice on the 

basis that in both genera the atlas was not reduced in size 

and the alisphenoids did not meet ~entrally at the median line. 

Robins et al. (1980), noting that the structure of the bony 

shelf that divides the anterior six vertebrae into dorsal and 

ventral portions is shared by Auxis, Euthynnus, and Karsuwonus only, suggested that the latter was more intimately related to Eurhynnus than to Thunnus. 

Because of the close relationship of these t%o genera, similarities in a number of characters such as noted abobe, are 

expected; however, the differences pointed out by Kishinouye 

(1923), Godsil and Byers (1944), Godsil (1954), and Collette 

(1978) seem large enough to justify their separation. These 

differences include the following: 

1) The dorsal and ventral branches of the cutaneous artery 

in Karsuwonus are about equally developed, insread of 

having the ventral branch short and dendritic as in 

Eurhynn us. 

2) The trellis on the vertebral column moves the aorta ventrally a distance slightly less than the depth of a centrum 

in Karsuwonus; less than in Airsis and Euthynnus, but 

more than in Tlricnnus. 

3) The liver in Katsuwonus resembles that of the yellowfin 

group of Thunrrus more than Euthynnus and Auxis because the right lobe doe5 not extend the length of the 

body cavity. 

4) Hepatic veins are absent from the ventral surface of the 

liver in Karsuwonus, as opposed to Auxis and Eulhynnus. 

5) The prefontals are smaller in Kursuwonus and do not 

project into the dorsal outline in dorsal view. 

6) The edge of the posterior orifice of the niyodome in 

Karsuwonus is nearly vertical, whereas it is roughly 

spherical in Euthynnus. 

7) The exoccipital condyle in Euthynnus projects beyond 

the transverse margin of the skull a distance approximately equal to the length of the first vertebra, whereas 

it is much shorter in Kutsuwonus and equals roughly 

half the length of the first vertebra. 

8) Katsuwonus has 41 vertebrae, whereas Euthynnus has 

39 or fewer. 

Attempts to determine generic relationships between Kutsuwonus and Eurhynnus by serological and immunogenetic 

techniques have also given mixed results. Electrophoretic 

analyses of hemoglobin (Sharp 1969, 1973) have indicated 

that Karsuwonus differs from Eurhynnus and Thunnus. 

Karsuwonus has only two hemoglobin bands; E. allerreruius 

and E. affinis have three identical bands; and the four species 

of Thunnus examined (albacares, alalunpa, sib;, and thynnus) 

have at least four bands. One of the bands in Karsuwonus 

and Eurhynnus seems to be identical whereas none of the 

hemoglobins in Karsuwonus or Euthynnus is identical to any 

hemoglobin in the four species of Thunnus examined. Analyses of heart, red and white muscle tissues, and enzymes, on 

the other hand, have provided conflicting evidence. Sharp 

and Pirages (1978) found that K. pelamis and E. affinis were 

the most closely related of rhe non-Thunnus tunas, that both 

species were almost equally close to T. albucares, and that 

3 

E. linearus and Ausis rhazard \bere about as close to each 

other as were K. pelamis to E. uffinis, but somewhat more 

distantly related to T. albucures, and, therefore, to thc I(. 

pelamis-E. uffinis group. The authors indicated that their results showed little reason for separating Kutsuwonus from 

Euthynnus. While this reems to be true when coniidering 

only E. affinrs, the distance between ufjrinir and lineurus and 

the latter’s close relationship with A. thoiurd suggest5 otherwise. 

We thus agree with Collette’s (1978) view that placing 

Katsuwonus in synonymy with Eurhynnus \\odd obscure the 

relationships of Eurhynnus with Auxis and Karsuwonus with 

Thunnus. 

1.22 Taxonomic status 

The name pelamis was established by Linnaeus in 1758 and 

was accepted by workers throughout the remainder of the 

18th century. Bloch and Schneider (1801) used the name 

pelamys and most workers also used this name in the 1800’s 

and early 1900’s. Some of these, notably Cuvier (l829), 

Valenciennes (I 836), Temminck and Schlegel (1842), Cantor 

(1850), Bleeker (1856), and Ghther (1860), although spelling 

the name pelamys, attributed the authorship to Linnaeus. 

Two other names, pelamides by Lac$$ede (1802) and vagans 

by Lesson (1828), were used briefly. Both names reflected 

differences in the geographic locality of capture. The name 

pelamis gained in acceptance between 1890 and 1935, and 

nearly all workers today use this name. 

Existing morphological and meristic descriptions of the 

skipjack tuna, Karsuwonus pelamis, from all oceank (Jones 

and Silas 1963; Postel 1963; Waldron 1963) and biochemical 

genetic study by Fujino (1969) indicate that there is but one 

worldwide species-Karsuwonus pelamis. 

1.23 Subspecies 

No subspecies are recognized. 

1.24 Standard common and vernacular names 

The standard common and vernacular names of the skipjack tuna, obtained from various sources (Herre and Umali 

1948; Rosa 1950; Herre 1953; Okada 1955; Van Pel 1960; 

Jones and Silas 1963; Postel 1963; Waldron 1963; Moreland 

1967; Miyake and Hayasi 1972) are listed in Table I. 

1.3 Morphology 

1.31 External morphology (and description) 

Body robust, rounded in cross-section and full in outline; 

maxillary not concealed by preorbital; about 40 teeth on jaws, 

but absent on vomer and palatines; corselet is well defined 

with hardly any scales visible on rest of body; interspace between 1st and 2d dorsal fins hardly exceeding eye diameter; 

lateral line with a decided downward curve below 2d dorsal 

fin; margin of 1st dorsal fin rtrongly concave; pectoral fin 

short and triangular reaching posteriorly to the 9th or lGth 

dorsal spine most commonly, but to only the 8th or as far as 

the 1 Ith spine in exceptional specimens (Fig. 2). 
Table I.-< onrinued. 

Standard common 

Countr\/localit! name \ ernaculai name 

Aden. CiulCol 

Alhariia 

Angd.l 

4u\tidIla 

Bralll 

Uritiih C'luiana 

Bllll\h \be51 

Indm 

Canada 

Canan I\. 

Chlk 

Cubd 

Denmark 

Ea\[ Africa arid 

France 

Germdny 

Grrecc 

India 

Indone\ia 

Laniibar 

Israel 

Ital) 

Japan 

horea 

Madagascar 

hlddeira Is. 

hlaldl\er 

Mc\,co 

Monaco 

Palamidd 

BCNlllO 

Strlpcd tuna 

Himto de barriga 

Ii\tadd 

Oceamc honiio 

Oceanic horiiiu 

Skipjack; thonine\ 

Bonito 

Atun 

At"" 

Bug\tiihct honil 

SkLpjack 

I ictao; bonite; 

Echter boiiito 

Pelam)\ 

Oceanic skipjack 

Cakalang 

\entre ray: 

bonitou; boiinicou 

Balamida 

Palometid 

Katruo 

Bonitr 

Gaiado 

Kaduma\ (\mall); 

Godhaa (large) 

Barrileie 

Bonita 

AI mu\\: ddhub; haighciha 

Uatermeli~n: skipjack 

Bonito rajado 

\\bite bonito 

Uhitc bonito; banlo; barriolet 

Skipjack tuna; oceanic honito, 

5rriped bonito 

Cachurreta; cachureta; 

cachorreta; harriiete 

hlernia 

\eheua(alw reler\to 

Bonitr5 ventre ray/ 

Bauch5treitiger 

Pelamis; tonina 

Bonito; kali-phila-nia\; 

barichoora: chooia; metti 

Tjakalang; tjakalang-lelaki; 

tjakalang-perempuan ; 

tlakalang-merah; skipjack 

Paamla; paamitun; palametto; 

palarnida; palamaru, palamid; 

tonina de dalniana; nzirru; 

paiamitu impinah; tonnetfo 

KatuHo. katauuo; magatsuao. 

mandagat\uuo: mandara; 

honeat suo 

Eirlhvnnus rp , Aurrr 5pp.) 

Gd-da-ri; )eo-da-raeng-i; 

chi; niog-maen-dungda-racng-i 

Bonitc\a \entre ray!; 

diodar). m'ba\\i 

Skipjack luna 

>lorocco 

Ue\\ Guinea 

heu Zealand 

horua) 

Peru 

Philippine I\ 

Poland 

Polynesia (except 

Tahiti and 

Hawaii) 

Portugdl 

Roumania 

Senegal 

South Africa 

Spain 

Sri Lanka 

Sweden 

Tahiti 

Taiaan 

Tunis I a 

Union of Soviet 

Socialist 

Republic3 

United Kingdom 

United Statea 

of America 

Li5tao 

Tjakalang 

Bonito 

Bonit 

Barrilete 

Gul)asan 

Bonite 

At" 

- 

Paiamida 

Kirt-kin 

S k i pjac k 

1 istado 

Bonito 

Bonit 

Auhopu 

Theng chien 

Boussenna 

- 

Striped bellied 

Skipjack tuna 

bonito 

Trup prugavac 

I 'hakoura 

Strtped bonito; \triped 

tunn); skipjack. slipper 

Striped tuna: \kiplack; 

pundahan; bankulis; sohad; 

oceanic honito; honiio; 

palanaban: puyan; tuhngan 

Bonito 

Gaiado; bonito de ventre 

raiado; bonito; listado; 

gayado; sarrajao. serra 

Palamida lacherda 

Sklplack Luna; oceanic bonito: 

lesser tunny; bonito; 

aatermelon; katunkel 

Skipjack; atun de alfura; bonito 

de veintre rayado; bonito; 

palomida; bonitol; bonito de 

alrura; lampo; Ilampua; 

bonito del wi; boniia 

Balaya; scorai 

Bonite 

Okeanskti bonito; katwo; 

polosatyi tunets 

Striped bellied tunny; bonito 

Skipjack; arctic bonito; oceanic 

skipjack; striped tuna; 

watermelon; rtctor fish; 

striped bonito; oceanic 

bonito; rkippy; ocean bonito; 

mushmouth; aku (Hauaii); 

aku kina, (Hawaii) 

Tun] prugavac 

Body color in life is steel blue, tinged with lustrous violet 

along the dorsal surface and decreasing in intensity on the 

sides to the level of the pectoral fin base; half of the body, 

including the abdomen, is whitish to pale yellow; evanescent 

vertical light bars are seen on the sides of the body immediately after capture; and a light grayish tinge on the underside 

of the mandible merges posteriorly with the whitish color on 

the lower half of the body. Four to six black longitudinal 

stripes are conspicuous below the lateral line on each side of 

the body. Matsumoto et al. (1969) described t\+o specimens, 

one taken in Hawaiian waters, the other off Honduras, which 

lacked these stripes. They, however, represent extremely rare 

caw. 

The counts of body parts for specimens from all oceans 

are given in Table 2. There seems to be no difference in the 

counts on fish from the Pacific and Indian Oceans and in at 

least tw'o characters, first dorsal spines and gill rakers, on 

fish from all oceans. 

Anatomical descriptions of the species may be found in 

Kishinouye (1923) and Godsil and Byers (1944). The vertebral 

column (Fig. 3) differs sufficiently from that of other fishes 

to be worthy of comment. Godsil and Byers (1944) wrote 

"The spinal column is extremely complex in this species. In 

the posterior half of the precaudal, and the anterior part of the 

caudal region, haemapophyses arise at both ends of each 

vertebra, with the anterior haemapophyses of one vertebra 

articulating with the posterior haemapophyses of the preceding vertebra. These haemapophyses project far below the 

spinal column and alternate with the longer haemal spines 

described below. An osseous bridge roughly parallel with the 

spinal column unites the anterior and posterior haemapophyses 

of each vertebra. Viewing the spinal column laterally, the 

bridge and the circular opening it forms may be easily seen. 

A branch arises from each bridge and extends downward to 

unite in the median line with its fellow from the opposite side. 

This forms the haemal arch from the tip of which a single 

haemal spine continues ventralward." 

4 
Figure 2.-Thr \kipjack luna. harrirwnnrt< pela,ni\ (1.innseusI 

Table 2.-Counts of body part, for the skipjack tuna. ti~rrwuonir~ pelmrir. from Ihe Parific. Indian. 

and Allanlic Oceans. 

PaciSic Ocean 

Japan. Habail. 

E. Paclflc 

Characterr (Waldron 1963) 

lit dona1 $pine 14-17 

2d dorial ra)\ 13-16 

Dorm1 finlet\ 7-10 

Anal ray5 13-16 

Aria1 fink15 7-x 

Pcctoral ra)\ 24-32 

lndiaii Occan 

E. ASrica. Sri 

I ankn. Laccadlie AiianiiL Ocean 

Ii , Indone5ia. 

Au\rialia (Jones Cap? Veri 

and Sda5 1963) (Portel 1963) All ocean\ 

I 2.10il I ) 

14-17 15-16) 

13-16 - 13-16 (14-12) 

7-9 

13-17 - 17-17 (14.10 

7~8 - 7-8 17) 

~ 24-32 (26-30) 

14-17 14-16 

~ 7-10 (8) 

26-30 

GI raker* 18-22 t 18-21 + ~ 16-22 +( I R~20) 

I.ouer 35-43 33-42 - 33-43 (3R-41) 

Total 53-63 SO-63 5 1-62 50-63 (56-60) 

Upper 

Figure 3.-Axial skeleton of an adult skipjack luna (from (;odd and 19441 

5 
Such complex ba\ket\\ork also occur) in the Eurhynnus 

(Godsil 1954), hut \\hereas the haemal arch in k. pelamrs 

occurs adjacent to the \ertebral column, that in titfhynnur 

ii positioned a\\aq from the vertebral column (Fig. 4) at the 

lips of the haemapophysec. Vie\\iiip the vertebral column 

laterally, the inferior foramina in Eurh~nnirt are elongate 

and about twice as long as those in K. pelamis. 

The abwnce of con\i\tenl differences in both external and 

internal morphologic;ii characters of skipjack tuna from 

widely separated region> in the Pacific (Godsil and Byers 

1944) indicates that there is but one species of skipjack tuna 

and that it is not possible to define subpopulations on the 

basis of these characters. 

1.32 Cytomorphology 

No information is available on chromosome number of 

skipjack tuna. 

I .33 Protein specificit) 

The inability of standard morphometric analyses to resol\e 

the problem of subpopulation or stock separation in skipjack 

tuna has led to the utilization of population genetic methodologies. Initial attempts involved the stud, of blood types 

through antigen-antibody reactions, and this was follohed by 

some work on polymorphisms of the protein hemoglobin 

using starch-gel electrophoresis. As in other applications of 

population genetics, most work nom involves the study of 

various serum and tissue enzymes, again using starch-gel 

electrophoresis. Sera are more commonly used because collecting the sample does not decrease the commercial value of 

the catch. Early isoenzyme studies involved samples collected 

from the central Pacific, then rather quickly from the eastern 

Pacific, Atlantic, western Pacific, and finally the Indian 

Ocean. 

(I) Blood groups 

Cushing (1956). who pioneered the application of blood 

typing for the study of tuna subpopulations, found two 

phenotypes on the basis of the agglutination of skipjack tuna 

erythrocytes in normal bovine serum. Subsequently, Sprague 

and Hollosay (1962) named this cystcm rhe C blood group 

apparently after Cushing. In the 5ame paper, they published 

information on another blood group, B, and on blood factors 

A, D, and F \\hicli ucre 1101 \honn to belong to any inheritance sysiem. Fujino (1967, 1969) found that the frequency of 

the K,-positive phenotype of the B group was related to the 

size of skipjack tuna. Several papers were published after this 

purporting to habe found or not to have found evidence of 

subpopulations from various areas in the Pacific Ocean 

(Sprague and Nakashima 1962; Sprague et al. 1963; Sprague 

1963; Fujino 1969, 1970b, 1972). Neither the C nor B groups 

were shown to be amenable to Hardy-Weinberg analysis, and 

neither of these blood groups proved useful for population 

genetics studies. 

Fujino and Kazama (1968) found another blood group, Y, 

which again did not prove useful in separating subpopulations, 

except when used together with other gene systems (Fujino 

1969, 1970a). The Y group is made up of 15 phenotypes, 

coded by 6 codominant alleles; however, only 2 alleles, Y' 

and Y"', and the 3 resultant phenotypes are abundant enough 

to be useful in population studies. The system was shown to 

be amenable to Hardy-Weinberg analysis and independent of 

sex, sire (age), and the C and B blood groups. Samples from 

various areas in the eastern, central, and western Pacific 

Ocean and the Atlantic Ocean failed to show heterogeneity 

of genotypic frequencies; however, the lower frequency of 

the Y' allele found in the Atlantic Ocean indicates greater 

opportunity for divergence and, therefore, utility in subpopulation differentiation. 

Fujino (1970a, 1971) described some preliminary results 

for an H blood group consisting of two phenotypes. While 

there \vas some evidence for the existence of subpopulations 

within the east-central Pacific Ocean (by testing phenotypes), 

the system was not shown to be amenable to Hardy-Weinberg 

analysis. 

(2) Hemoglobins 

Little work has been performed using henioglobins because 

their instability requires preservation techniques that are hard 

to apply at sea and because they are not highly polymorphic. 

'. . 

6 
All samples examined by Sharp (1969, 1973) from off Baja 

California and Hawaii showed two phenotypic bands; therefore, the author made no inferences regarding subpopulation 

differentiation. 

(3) Serum isoenzymes 

While it is true that serum enzymes have provided the richest material for differentiating subpopulations of skipjack 

tuna, only esterase and transferrin, and recently guamine 

deaminase, have actually been found to be sufficiently polymorphic. In the plid-1960’s while testing a series of tissues 

sampled from skipjack tuna collected off Hawaii, Sprague 

(1970) found six electrophoretic zones of esterases, one of 

which he interpreted as serum esterase. He hypothesized that 

the four observed phenotypes are coded by three alleles. 

Using blood serum, Fujino (1967) and Fujino and Kang 

(1968a) found three clear and three faint bands of serum 

esterase activity resulting in nine observed phenotypes and 

hypothesized, on the basis of the Hardy-Weinberg law, that 

they are coded by six codominant alleles. We follow the conventional allele numbering system based on relative mobility, 

hence the most abundant allele in the eastern and central 

Pacific is Est 2, rather than number 1 as in Fujino’s papers 

(the reason for this difference is historical in that the now Est 

1 is rare and was found after much work was already published). Because the faint bands are so rare, the system is 

usually treated as a six phenotype, three allele system for 

subpopulation study. The transferrin gene system was first 

described for skipjack tuna by Barrett and Tsuyuki (1967) 

from samples collected in the eastern tropical Pacific. They 

observed six phenotypes and hypothesized that they are 

coded by three codominant alleles. Researchers at the Australian National University (South Pacific Commission (SPC) 

1981) have reportedly tested 42 gene loci (proteins) and found 

only 3 that were highly polymorphic. Glucose-phosphate 

isomerase, 6-phosphogluconate dehydrogenase (also reported 

by Sharp and Kane’), and apparently adenosine deaminase 

were variable but at low levels. Research on the esterase, 

transferrin, and guamine deaminase gene systems will be discussed in some detail below. 

After the observed serum esterases were hypothesized to 

compose a single gene locus consisting of six codominant 

alleles, the locus was shown to be independent of sex, size, 

blood groups B, C, and Y, and serum transferrin (Fujino 

1967, 1970a; Fujino and Kang 1968a. b). While these basic 

and essentially genetic factors were being established, the 

search for subpopulations, which was the real purpose of the 

studies, proceeded. Such population genetics studies involve 

testing individual sample lots for genetic equilibrium and 

testing for homogeneity both within and between sample lots 

from arbitrary areas. These tests are quite technical since 

they involve inheritance properties of the multinomial HardyWeinberg law, chi-square tests of homogeneity or, more recently, the G-test of Sokal and Rohlf (1969), and considerations of the effect of sample size on these tests. A great deal 

three different approaches being followed by various scientists 

and organizations. 

The first approach was advanced by Kazuo Fujino who 

was associated with the Honolulu Laboratory, National 

Marine Fisheries Service, and is now with the Kitasato University in Japan. Fujino (1970a) showed that the HardyWeinberg method is not very sensitive to deviations from expected proportions (genetic nonequilibrium) by employing a 

simple theoretical test. Taking the calculated allelic frequencies 

for the eastern-central and western subpopulations (to be 

described later) and constructing a one-to-one mixture of 

these subpopulations, lack of fit to Hardy-Weinberg proportions could not be shown with a sample size of 100 individuals 

(nor, we might add, would have been found if he had used a 

sample of 200 individuals). Later Fujino (1971) presented 

graphic evidence showing that a mixture of subpopulations 

could always be detected for a sample of 100 individuals 

from a one-to-one mixture of subpopulations having a difference > 0.443 in the frequency of the allele being tested, 

whereas a mixture could never be detected if the difference 

were < 0.328. For differences falling between 0.328 and 

0.443, ability to detect a mixture depended on the absolute 

allelic frequencies. While an examination of the effect of 

sample size shows that it is, of course, easier to determine a 

mixture with larger sample sizes, Fujino’s data indicate that 

the differences in the allelic frequencies between the hypothesized eastern-central and western subpopulations preclude the 

ability to consistently detect mixtures with a sample size of 

100 (or as we said, even 200). From this result, Fujino seems 

to have concluded that a reasonable strategy would be to 

collect small samples (generally < 100 individuals) and to 

sample more schools. He has then proceeded to search for 

subpopulations by grouping sample lots (schools) from similar 

geographical areas, by testing for intra- and inter-area 

homogeneity, and by setting up statistical confidence regions 

for classifying individual school samples into separate subpopulations, without considering whether the individual 

schools represented mixtures of genetic units. 

Fujino (1967) and Fujino and Kang (1968a) presented results from their initial examination of material from the 

Atlantic, Hawaiian Islands, the northeastern tropical Pacific, 

Palau Islands, and Japan. They found no evidence of heterogeneity of samples within these areas, nor between Hawaii 

and the eastern tropical Pacific or between Palau and Japan. 

However, their tests did show that samples from Hawaii were 

different from the combined samples from Palau and Japan; 

and Atlantic material was different from that collected in the 

eastern Pacific. Fujino (1969) used both the esterase system 

and the Y blood group in attempting to separate subpopulations. Using rejection ellipsoids for Y’ and Est 2, he found 

that the eastern-central Pacific samples overlapped with those 

from the western Pacific, and these, in turn, overlapped with 

samples from the Atlantic. Thus, while the Atlantic material 

was shown to be different from the eastern-central and western Pacific material, the employment of these two gene systems did not contribute to differentiating between easternof controversy has surrounded the employment of these tests, 

i.e., the confidence to be placed on the results, and has led to 

central and western Pacific material. He also stated that there 

was no evidence of heterogeneity (for any of the five gene 

systems, including esterase and blood group Y) between the 

7 
and restated that he had found no evidence of heterogeneity 

of samples within these areas, even with the inclusion of new 

material such as those from Ecuador. Also for the first time, 

Fujino described the boundary between these IWO subpopulations, and he did this by assigning schools to either of the 

subpopulations based on rejection limits for Est 2. While the 

western subpopulation was shown to be present the entire 

year in inshore waters off the east coast of the main Japanese 

Islands and around the Ryukyu Islands, the Bonin-northern 

Mariana Islands chain, and Palau Islands, he stated that the 

boundary between the subpopulations shifts easterly in the 

summer and'westerly in the fall and winter in the waters between the Bonin-northern Mariana chain and the international dateline. Then in the same year, Fujino (1970b) presented 

additional information on these subpopulations, namely, that 

there was no evidence for seasonal variation in the esterase 

gene frequency in either Hawaii or Palau, that new material 

from the Line Islands was not different from the historical 

Hawaiian material, that there was no evidence for heterogeneity within the western Pacific subpopulation based on 

samples from Japan, northern Mariana Islands, Ryukyu 

Islands, and Palau Islands, and further that the relatively low 

numbers of skipjack tuna larvae found in the area between 

long. 170"E and the international dateline in the Northern 

Hemisphere was supportive of the two subpopulation hypothesis. On the basis of new samples from schools in the 

western Pacific. which were assigned to either the easterncentral or western subpopulation based on rejection limits 

for Est 2, Fujino (1972) adjusted the boundary, indicating 

that the range of the western subpopulation extends out to 

long. 165"E (between the eastern Caroline Islands and the 

Marshall Islands/Gilbert Islands) without marked seasonal 

variation and hypothesized that the Iru-Bonin-northern 

Mariana Islands complex acts as a barrier to the easterncentral subpopulation. Using the estimated growth rare of 

skipjack tuna (based on Rothschild 1967), Fujino was able to 

separate the western subpopulation into A and B components 

associated with an apparent hatching time in the Northern 

Hemisphere summer and winter, respectively. Figure 5 depicts 

the hypothesized range of the western subpopulation, the 

overlap of northern sunimer and winter spawning areas, and 

the migration routes of the A and B units. Subsequently, 

Fujino (1976) rL 7orted some additional results from material 

collected from itie southwestern Pacific. He classified all four 

schools sampled in New Zealand and one from Tasmania as 

belonging to the eastern-central subpopulation. From Australia, two schools were classified as belonging to the western 

subpopulation and one to the eastern-central. Likewise for 

the New Hebrides (7 schools) and southern Solomons (12 

schools), both subpopulations were represented. All schools 

from New Caledonia (2) and Papua New Guinea (13) w'ere 

classified as being western subpopulation. Finally, Fujino 

(1980) presented nonoverlapplng Est 2 frequency ranges, 

namely 0.394-0.570 for the eastern-central and 0.578-0.758 for 

the western subpopulation. Using these values to classify 

schools, Fujino found that 40-5070 of the schools off eastern 

Japan in May-June belonged to the western subpopulation 

and averaged 65% for the entire year. Fujino also indicated 

that he could subdivide the western subpopulation using a 

new esterase gene system (EstR) from red blood cells. Taking 

samples classified as western subpopulation and testing for 

this new esterase, Fujino found that the Satsunan-Tosakigure S.--Uistribution and migration route of the western Pacific \kipjack luna 

subpopulation proposed b) kujino (1972). Eastern limits of distrihution during 

northern winter and summer are indicated b? a did line and a broken line. 

reapecli\el!. Intensive %pawning ground5 are shewn b? horizontal and bertiral 

hatching and migration route5 are indicated b? curred lines with arrows. 

Kishu group had an average ESIR 1 gene frequency of 0.754 

and the Bonin-Izu group had an average gene frequency of 

0.083. The relationship of these new genetic subpopulations 

to the previously described A and B units was not stated. 

Three schools assigned to the eastern-central subpopulation 

had an average EstR 1 value of about 0.38, thus falling between the two hypothesized western subpopulations. 

The second approach of determining subpopulations was 

advanced by Gary D. Sharp who was associated with the 

Inter-American Tropical Tuna Commission (IATTC) and is 

now with FAO. Sharp (1978) pointed out that if replicate 

samples were collected from homogeneous units, one should 

expect a normal distribution of estimates about whatever 

parameter was being estimated. He stated, however, that thir 

has not resulted for gene frequencies in the extensive, largesized samples of both skipjack tuna and yellowfin tuna, 

Thunnus albaeares, and argued that this was due to the genetic nonhomogeneity of the schools. As evidence that schools 

are genetic composites, he referred to his studies on shifts in 

gene frequency and the clumping of rare alleles in individual 

schools (Sharp'; Sharp and Kane footnote 3). While Sharp 

(footnote 4) was able to show significant differences between 

schools based on morphometric discriminant analysis, these 

differences seemed unrelated to genetic differences. Based on 

these considerations, Sharp recommended that sample sizes 

'Sharp. G. D. Studics OS Paclllc Ocean sklplack tuna (/(aisur,onuspelornrs) 

genetics through 1978. Manum in prcn. C N.. FAO. Fnh Den.. VLa della Terrne 

di Caracaila. 00100 Romc. Italy 

8 
should be on the order of 200 individuals per school. In his 

study of subpopulations, Sharp placed greater emphasis on 

the lack of homogeneity of individual schools, compared the 

distribution of gene frequency estimates among geographical 

areas, and employed the usual statistical tests for both intraand inter-area homogeneity. 

While he indicated that his large-sample data from the 

eastern tropical Pacific, Papua New Guinea, and New Zealand fit Hardy-Weinberg expectations in all but two cases (in 

the former site), Sharp (footnote 4) and Sharp and Kane 

(footnote 3) showed that nearly all sampled schools exhibited 

statistically improbable clumping of rare alleles or a shift in 

the estimated gene frequency midwjay in the length range of 

the samples, or both. These results indicate not only that the 

Hardy-Weinberg test is not sensitive to mixtures but that 

schools tend to be both mixtures of siblings as well as groups 

with differing gene frequencies. These results cast doubt on 

the utility of computing average gene frequencies for use in 

classifying schools to hypothesized subpopulations. Also in 

testing for homogeneity within geographical areas (eastern 

tropical Pacific, New Zealand, Papua New Guinea, Hawaii) 

using his own large samples and the larger sized samples of 

Fujino's, Sharp (1978, footnote 4) found significant heterogeneity in the esterase system (and in transferrin also, but to 

a lesser extent). Since tests among areas also show significant 

heterogeneity, Sharp's results are generally supportive of Fujino's results in that there is evidence for subpopulation differentiation between the eastern-central and western Pacific 

and within the western Pacific; however, they also indicate 

that subpopulation differentiation within the eastern-central 

subpopulation is likely. Sharp's approach then deviates from 

Fujino's by comparing the similarities of gene-frequency distributions among the various sample sites rather than assigning schools to hypothesized subpopulations based on rejection 

limits. Thus, Sharp (footnote 4) observed (Fig. 6) that the 

esterase gene-frequency distribution for Hawaiian samples 

seems to have components similar to those found in samples 

off eastern Japan, and other components similar to those in 

the northern areas of the eastern tropical Pacific and off 

Ecuador. Also, the typical gene frequency in the eastern 

tropical Pacific corresponds to the most common gene freFREQUENCY Cf THE ESTERISE 2 4iLELE Cf L4RGE SKIPJICK S4MPLES 

tigure L.-Frequenc~ dislribulien of Est 1 skipjack luna allele far large sample\ 

from +arbus (ource? (from Sharp text footnote 4). 

quency in Hawaii. For both Palau and Papua New Guinea, 

there seem to be three groups showing much overlap between 

the areas. For New Zealand, Sharp says there is one group 

similar to the eastern Pacific and one unique group, but all 

of these distributions fall into the upper end of the Hawaiian 

distribution. The distribution for the northern area of the 

eastern tropical Pacific is similar to that for Hawaii and contains some components similar to those from Ecuador. The 

latter was said to have a unique component (with low gene 

frequency), but this again falls within the range of the Hawaiian samples. Based on all of these findings, Sharp (footnote 

4) hypothesized the existence of six subpopulations in the 

Pacific (Fig. 7). Considerable overlapping of the subpopulation ranges occurs in the central and western equatorial areas. 

The third approach was used by the SPC in association 

with their Skipjack Survey and Assessment Programme. 

Blood samples and various biological information have been 

collected fairly uniformly across the South Pacific and into 

the Northern Hemisphere in the western Pacific. With their 

~. . . 

Figure 7.-Rangc\ of 4% \kipjar!. luna whpopulalicin\ (from Sharp 

le11 foolnote 4). milh c\ample\ of the mo\emrnl of 1aBEcd fkh. 

...... 

9 

^--_---I. . .- 
interest in determining uhat effect, if any, population structuring would have on yield and fishery management in the 

SPC area, they have emphasized the analysis of longitudinal 

gene-frequency \ ariations and of differences among \<estern, 

central, and eastern segments (areas A, B, and C) of the SPC. 

Having the benefit of much of the research described 

above, the SPC collected relatively large samples (100 individualslschool), integrated their genetic research nith tagging 

and sampling for size composition. and examined the quantitative aspects of the population genetic methods employed. 

Thus, they performed certain basic statistical tests of their 

data before analyzing for subpopulation5 (South Pacific 

Commission 1980, 1981). nhich provided them with the following rewlts. With a sample sire of 100, gene frequency of 

the samples was estimated with a precision of 0.07 at the 

95% confidence level. If t\<o genetic populations differ in 

their gene frequencies by 0.1, they could be statistically separated with a sample size of 25 at a pouer of 90°!o (this seems 

in direct conflict with the results in Fujino 1971). And if over 

six sample lots (600 individuals) \.cere combined within an 

arbitrary area, significant heterogeneity mas usually found. 

Regarding population genetic results, they found that 

samples from areas A (west of long. 170"E) and B (betaeen 

long. 170" and l65"E) showed a higher degree of heterogeneity than area C (east of long. 165 'E): however, thig was 

based on 76, 32, and 11 sample lots, respectively. ?rlost interestingly, they found a longitudinal cline of Est 2 (Fig. 8) 

from about long. 140"E (gene frequenc) of 0.71) to 130"M' 

(gene frequency of 0.37). As nould be expected from the 

results of the tests for heterogeneity for areas A, B, and C, 

rhe variance about the cline (regression line) decreased from 

west to east. To the east of long. 130"W, the cliiie appeared 

to wave or to level off. To the \<est, Lobis' felr that the cline 

continued into the Pacific and Indian Ocean ivaters of Indonesia, whereas the South Pacific Cornmission (1981) suggested 

Figure 8.-Relalion of esterme erne frequent! and longitude. Represcinn line 

and 9500 prediction limih indicated. Regre55ion c\clude5 \ample5 from ea51 of 

Ih? SPC area (east of dulled line). Y is numher of rampler: R i\ correlation 

coefficienl (from South Pacific Commi5\ion 19x1). 

that the cline may level off. More samples will be needed 

from the Indian Ocean to resolve this issue. 

While tagged fish were continuing to be recaptured (and 

indeed long-term recaptures of many fish tagged in French 

Polynesia were just being reported), the South Pacific Commission (1980, 1981) attempted to integrate the tagging and 

genetic experiments. On the basis of the calculated gene frequency of the tagged school, the expected longitudinal range 

of recapture was estimated from the regression line of gene 

frequency on longitude. Of the 106 rag recoveries at large for 

at least 90 d, only 1, released in Kiribati, was recovered in an 

unexpected area (South Pacific Commission 1980). Then, regressing the net west-east movement of all tagged fish from 

each school for which gene frequency was estimated (29 

schools) against the difference between calculated gene frequency of the school and the gene frequency estimated from 

the cline, they failed to find a significant relationship (South 

Pacific Commission 1981). A positive relationship would 

have indicated that fish tended to migrate to areas having 

schools with average gene frequencies more similar to that of 

schools in which they were at the time of tagging. While this 

second test may seem to be more critical, it may be biased 

because it includes short-term recaptures which tend to disperse very little and because fish tagged in the western perimeter of the Pacific basin may be restricted in their migration 

to the west. 

Returning to serum transferrin, Fujino and Kang (196%) 

stated that the system was independent of sex and, later, of 

the blood groups C. B, and Y (Fujino 1970a). They found no 

evidence for heterogeneity either within or among the centraleastern Pacific, Atlantic (also shown by Fujino 1969). and 

western Pacific areas. Ho\<ever, when they grouped the data 

by areas, the pooled data showed an excess of heterozygotes 

compared with Hardy-Weinberg expectations. They presented data purportedly showing that the frequency of certain 

alleles varied with size and hypothesized that differential 

mortality with size could maintain the balanced polymorphism. Subsequently, however, Sharp and Kane (footnote 3), 

using larger sample-size lots collected in the eastern tropical 

Pacific and off New Zealand, found that mo\t samples fitted 

Hardy-Weinberg proportions. Reexamining Fujino's data, 

they showed that only one size group out of five deviated 

from Hardy-Weinberg proportions. Lewis (footnote 5) reported that samples collected from Pacific and Indian Ocean 

waters of Indonesia had transferrin frequencies falling within 

the normal range seen in all other historical samples. Presenting results from samples collected all across the South 

Pacific, as well as from certain sites in the western North 

Pacific, the South Pacific Commission (1980) found no variation of transferrin gene frequency with longitude. By plotting the variation of the frequency of the major transferrin 

allele against those for the major esterase allele, the variations were shown to be independent and the variation for 

transferrin was the smaller of the two by far. The gist of all 

of these studies is that transferrin has not proven useful in 

differentiating subpopulations of skipjack tuna. 

(4) Tissue isoenzymes 

Sprague (1970) found as many as 12 esterase activity bands 

in samples of dark and light skeletal muscle, heart muscle, 

kidney, liver, spleen, gonad, brain, and whole blood collected 

10 
from skipjack tuna in Hawaiian waters. These 12 bands appear to be arranged in 6 regions, but only 2 of the regions 

were hypothesized to compose a genetic system. The fourth 

region contained serum esterases as described in the previous 

section. Sprague contended that the esterases in region 5 were 

also usable for subpopulation studies, but insufficient data 

were available at the time to do so. He hypothesized that the 

three observed phenotypes in this region are coded by two 

alleles. McCabe and Dean (1970) also found innumerable 

bands of esterase activity in samples of liver and skeletal 

muscle taken from skipjack tuna in the Atlantic Ocean. They 

indicated that the esterases in only two activity regions, Est 

Ib and Est 111, were stable and, therefore, usable for subpopulation studies carried out in the field. Region Est 111 

seemed to be a dimer system made up of four phenotypes 

coded by four alleles. Since one of the alleles had a frequency 

of 0.964, this esterase system is, for practical purposes, 

monomorphic and not likely of much use in studying subpopulations in the Atlantic Ocean; however, the situation in 

the Indian and Pacific Oceans could prove to be different. In 

region Est Ib, they observed 15 phenotypes in 300 samples 

and hypothesized that the phenotypes were coded by 7 alleles. 

No specific data were given, nor was it shown that the system 

was amenable to Hardy-Weinberg analysis. Nonetheless, Est 

Ib may have potential for use in determining subspecific 

differentiation, since no single allele had a frequency > 0.4. 

Unfortunately, the phenotypes must be determined by a twostage electrophoresis process, making the process more complicated and expensive than is the case with serum esterases. 

McCabe et a]. (1970) tested for dehydrogenase, acid phosphatase, and alkaline phosphatase activity in skeletal muscle 

sampled from skipjack tuna and reported finding polymorphisms of 6-phosphogluconate dehydrogenase (6-PGD) and 

alpha-glycerophosphate dehydrogenase (a-GPD) polymorphism in liver samples. The 6-PGD system was found to be a 

dimer with four phenotypes coded by four alleles, and the aGPD system was found to consist of three phenotypes coded 

by three alleles. Since the frequency of PGD and a-GPD 

were both over 0.9, both systems are essentially monomorphic 

in the Atlantic and would probably not be useful for studying population differentiation in that ocean; however, the 

systems could prove to be useful in the Pacific and Indian 

Oceans. 

Lewis (footnote 5) reported preliminary results from his 

investigation of 21 enzymes, encoded by 25 presumed gene 

loci, in liver tissues collected from skipjack tuna in north 

Queensland, Australia, and Padang, Indonesia. Only the 

adenosine deaminases ADA1 and ADA2 and guanine 

deaminase (GDA) were found to be sufficiently polymorphic 

(five, four, and three alleles, respectively) to encourage further research, whereas glucose phosphate isomerase (GPI), 

mannose phosphate isomerase (MPI), 6-PGD, isocitrate dehydrogenase (ICD), a-GPD, sorbitol dehydrogenase (SORD), 

glutamate oxaloacetate transaminase (GOT1 and GOT2), 

phosphoglucomutase (PGMI and PGMZ), peptida5e 1 and 2, 

malic enzyme (ME1 and ME2). fumarate hydratase (FH), 

malic dehydrogenase (MDH), lactic dehydrogenase (LDH), 

pyruvate kinase (PK), glutamate pyl-uvate IranTaminase 

(GPT), 6-phosphofructokinase (PFK), phosphoglycerate kinase (PGK), and superoxide dismutase (SOD) were found IO 

be absolutely or essentially monomorphic. No large differences were evident between the Australian and Indonesian 

material for ADA or GDA, but the samples from Indonesia 

seemed to be homogeneous and had a lower gene frequency 

than the samples to the east. The South Pacific Commishn 

(1981) found a decreasing cline in gene frequency for the 

GDA, allele from west to east (Fig. 9). 

.48 

.35 

. 30 

.25 

.20 

. 15 

. IB 

.05 

.00 t 

Figure P.-Relalion of guanine deaminaw gene frequenc) and lungitude. Regression line and Y5ob prediction limit\ indicated. Regression ewludes 3amples from 

ea$t of lhe SPC area (east uf dolled line). > is numher of samples: H is correlation 

coefficient (from buulh Pacific Commission 1981). 

2 DISTRIBUTION 

2.1 Total area 

Prior to 1959, the occurrence and distribution of the skipjack tuna had been determined essentially from records of 

captures in coastal waters and in those offshore areas subjected to commercial pole-and-line and purse seine fishing. 

Because pole-and-line fisheries were constrained by distance 

to areas within easy access of bait supplies and purse seine 

fisheries were confined to thotc areas with a relatively shallow mixed layer, a characteristic of the purse seine fishery in 

the eastern sides of both the Atlantic and Pacific Oceans 

(Brock 1959), there were large gaps in our knowledge about 

the distribution of skipjack tuna in midoceanic areas of the 

Pacific Ocean. Even less Bas known about the occurrence 

and distribution of skipjack tuna in the Indian and Atlantic 

Oceans, where fishing for this species on a commercial scale 

had not developed as in the Pacific Ocean. 

Since 1959 various authors (Brock 1959; Kasahara 1968; 

Miyake 1968; Marcille and Suzuki 1974; blatsumoro 1974, 

1975) have utilized the Japanese tuna longline data to determine the distribution of skipjack tuna in midoceanic areas. 

Although the catches made by the longline gear were mall, 

they constituted proof of the presence of skipjack tuna and 

the information thus obtained has expanded our knowledge 

about the distribution of the species. 

The distribution of \kipjack tuna (Fig. IO) was derived 

from atlases of the Japanese tuna longline carch data fol 

various years from 1964 to 1979 ([Japan.] Fi\heries Agency 

1967a, b. 1968, 1969, 1971, 1972, 1973, 1979a, 1980a. 1981), 

the Japanese southern water fishery for various year\ from 
~ 

ARE& OF OCCURRENCE 

Figure 10.-Skipjack tuna distribution and Fisheries in the major oceans. The mean 1S'C isotherms represent maximum late summer poleward displacement 

(data from various sources). 

1964 to 1979 (Tanaka undated a, b, c, 1978, 1979, 1980; 

Tohoku Regional Fisheries Research Laboratory (Tohoku 

RFRL) undated a, b, c, d) the Japanese skipjack tuna bait 

boat fishery catch data for 1977 and 1978 ([Japan.] Fisheries 

Agency 1979b, 1980b). and other sources (Wade 1950; 

Robins 1952; Brock 1959; Roux 1961; Jones and Silas 1963; 

Postel 1963; Sakagawa 1974; Calkins 1975; Sakagawa and 

Murphy 1976; Santos 1978; Kikawa and Higashi 1979; Habib 

et al. 1980a, b, c; Blackburn and Serventy 1981). The species 

occurs continuously from east to west across all oceans, and 

over a wide latitudinal range from about lat. 45"N to south 

of 45"s in the western Pacific and from about lat. 30"N to 

30"s in the eastern Pacific. In the Atlantic Ocean, skipjack 

tuna have been caught from about lat. 45"N to 40"s in the 

western side and from lat. 35"N to south of 40"s in the 

eastern side. Rare captures off Vancouver Island (Clemens 

and Wilby 1946) in the Pacific Ocean and off the British 

Isles, Scandinavia, and Denmark (Postel 1963) in the Atlantic 

Ocean extend the limits of occurrence northward to lat. 

49"N in the eastern Pacific and beyond lat. 55"N in the 

eastern Atlantic. Skipjack tuna have been taken as far south 

as lat. 40" to 45"s in the western Indian Ocean and south of 

Australia, and, although not indicated in Figure 10, in the 

eastern part of the Persian Gulf on rare occasions. The 

species occurs also in the Mediterranean Sea, from Gibraltar 

to Italy, but not in large concentrations as in the Atlantic 

Ocean (Belloc 1955). 

2.2 Differential distribution 

2.21 Spawn, larvae, and juveniles 

Larvae of skipjack tuna have been found over a wide area 

in all the major oceans, including the Gulf of Mexico and 

throughout the Indo-Pacific Archipelago (Fig. 11). The known 

distribution of skipjack tuna larvae in the western and cenFigure 11.-Distribution OF skipjack tuna larvae. 1952-15 (data From various sources). 

12 
tral Pacific extends from near lat. 35"N off Japan to as far 

south as lat. 375, off the southeastern part of Australia 

(Matsumoto 1958; Nakamura and Matsumoto 1967; Ueyanagi 1969, 1970; Mori 1970; Seckel 1972; Chen and Tan 

1973; Nishikawa et al. 1978), and in the eastern Pacific from 

lat. 15'N to 5"s (Klawe 1963; Love 1970, 1971, 1972, 1974). 

The distribution in the Pacific is broad latitudipally in the 

west but narrow in the east due to the constriction of warm 

water favorable for spawning by cold currents flowing toward 

the Equator in both hemispheres. Both northern and southern boundaries shown may hell represent the limits of distribution of skipjack tuna larvae in that ocean. 

In the Indian Ocean, the distribution of larvae extends 

southward to the southern tip of Africa, to about lat. 36"S, 

and unlike that in the Pacific Ocean, larvae have been taken 

in the eastern part of the ocean from as far south as lat. 

30"s (Ueyanagi 1969). The northern boundary is limited to 

lat. 11" to 15'N by the Asian Continent (Jones 1959; Gorbunova 1963, 1965a; Jones and Kumaran 1963; Ueyanagi 

1 969). 

The distribution in the Atlantic Ocean differs somewhat 

from that in the Pacific Ocean. Although the distribution is 

wider in the western side, the southern boundary does not 

reach much beyond lat. 25"s and the northern boundary 

does not extend beyond lat. 30"N (Klawe 1960; Suarez,Caabro 

and Duarte-Bello 1961; Gorbunova and Salabarria 1967; 

Richards 1969; Zhudova 1969; Richards and Simmons 1971; 

Ueyanagi 1971; Juarez 1974; Nishikawa et al. 1978). The 

large dip in the northern boundary in the region of the Sargasso Sea may be due to infrequent sampling. The distribution of larvae in the eastern sector of the ocean differs from 

that in the Pacific Ocean. Whereas the distribution narrows 

and the concentration of larvae diminishes greatly in the 

eastern Pacific (Klawe 1963), the distribution is broader and 

the abundance of larvae is much greater (Richards and Simmons 1971) in the eastern Atlantic Ocean. 

Within the vast range over which the larvae are distributed, 

certain areas of high abundance can be recognized. Kawasaki 

(1965a) suggested that the center of abundance of skipjack 

tuna larvae in the Pacific Ocean was in the area bounded by 

lat. 5"N and 4"s and long. 160"E and 140"W. He later (1972) 

revised his western boundary to long. 120"E. on the basis of 

Ueyanagi's (1969) data, which rhowed a peak in larval abundance at long. 120"-150"E along a band of water from lat. 

10"N to the Equator. Matsumoto (1975), after adjusting the 

catches of larvae made with different sized nets and different 

towing methods in a band of water from lat. 10"s to 20"N 

(Table 3; Fig. 12), reported that the center of larval <kipjack 

tuna abundance was between long. 160"E and 140%' (similar 

to Kawasaki's first estimate), that the abundance was moderate between long. 100" and 140"W and between long. 120" 

and 160"E, and low in the eastern Pacific east of long. 100"W. 

Other areas north of lat. 20"N uith relatively high larval 

abundance include the waters around the Hawaiian Islands, 

where the catches in night surface tous with a 1 m net during 

the spawning season averaged 4.1 larvaelto- (data from 

Seckel 1972) and waters fo the south of Japan between lat. 

25" and 30"N, hhere the catches in subsurface tows (20-30 m) 

with a 2 m net averaged nearly 4.0 larvaeltow (Ueyanagi 

1969, fig. 22, Area A). 

In the Atlantic Ocean, larvae are apparently more abundant in the eastern half of the ocean betw'een lat. IO"N and 

I 71 

I 

I 

87 

a l6 -6 

c 2 51 59 

1 8. 4K 

L L 

5 E4 

VI : 3' 35 

21 n 

A2 2 

I I 

JO E 140' 160' 180' 160' 140' 120- 100' W 

LONGITUDE 

Figure 12.--Calch rates of skipjack tuna larvae and adults b) loniitude in 

the Pacific Ocean between 1st. 10% and 20% (from Matwmoro IY75). 

Table 3.-Calch rates of larval skipjack tuna in night surface lows from the Pacific Ocean (from Matsumoto 1975). 

13 
10"s (Richards 1969). There are no other reports from the 

Atlantic or the Indian Oceans pro~iding rneasiircs ut' ahundance. 

Juvenile skipjack tuna are rarely \een at sca and are e\- 

tremely difficult to capture. They have been collected in 

various types of tra\\l and plankton nets, by dip netting at 

night under light, and from stomach contents regurgitated by 

seabirdr, but the greatest number ha\e come from stomachs 

of tunas and billfishes. 

The relatively small number of recorded capturer attest to 

the difficulty in capturing the juveniles. Higgins (1967) compiled all records of 512 juveniles, 1.2 to 30 cm in length, 

taken from the Pacific for the period 1916-66. Yothida (1971) 

obtained 1,742 juveniles, 1.6 to 40 cm in length, from the 

stomach\ of billfishes from Hawaii,, and central South 

Pacific naters in the period 1962-66. Higgins (1970) examined 

578 "juveniles," 7 mm to 4.7 cni in length. taken in miduater trauls in Hawaiian naien. Roughly 40mo of the latter 

catch consisted of skipjack tuna lar\ae < 1.2 cm in length. 

Simmons (1969) compiled records of I63 ju\enile\. 13 to 15 

cm in length, taken by variow method\, including samples 

from stomach content5 regurgitated by seabird\. from the 

Atlantic Ocean. Mori (1972) examined 5,851 juveniles and 

young. 3 to 35 cm in length including some adults Io 70 cm, 

from stomachs of tuna5 and billfi\hcs taken from the Pacific. 

Indian, and Atlantic Oceani during the period 1949-69. Of 

this number, 4.104 were juveniles < 35 cni in length: 3.778 

fiom the Pacific Ocean, 202 from the Indian Occan, and 124 

from the Atlantic Ocean. Thus, the total iuvenile ship.iack 

tuna collected from 1916 through 1969 is < 7,000. of nhich 

roughly 300 Mere from the Atlantic Ocean, 2W from the 

Indian Ocean, arid the remainder from the Pacific Ocean. 

Despite the small numbers of juvenib coilccted from the Indian and Atlantic Oceans, the dispersion of the capture sites 

presents a fair coLeraye of thc two ocean\. 

The above data 5ho\\ that jmenile skipjack tuna are uidel) 

distributed in the tropical and subtropical waters of all oceans 

(Fig. 13). In the Indian Ocean the distribution extends from 

lat. 15"N to 35"s. In the Pacific Ocean the distribution extend5 generally from lat. 35"N to 35"s in the nest and narro\\s to lat. I0"N and 5"s in the east. In the Atlantic Ocean 

the distribution estends from lat. 40"N in the west to lat. 

30"s in the east. In all oceans the pattern of distribution resembles that of the larvae. From information acquired recently about the extremely rapid growth of skipjack tuna and 

other tunas reared from artificially fertilized eggs through the 

juvenile stages (Harada et al. 1971; Mori et al. 1971; Harada, 

Murata. and Furutani 1973; Harada, Murata, and bliyashita 

1973; Ueyanagi et al. 1973), it would be expected that juveniles of skipjack tuna occur in all localities where larvae have 

been found. 

In certain localities, i.e., North and South Atlantic, central 

Indian, and northwest and southeast Pacific, the distribution 

of juveniles is shown to extend beyond the range indicated 

for the larbae. This could be due to juveniles leaving the immediate spawning area as they grew larger and became more 

mobile. Mori (1972) suggested that this could occur when the 

juveniles attain the length of 30 cm. 

2.22 Adults 

Excluding those areas from \%hich only rare captures have 

been reported, the distribution of skipjack tuna shown in 

Figure 10 (medium shaded areas) shows the maximum northnard and southnard limits. Both boundaries vary seasonally 

and annually as the skipjack tuna respond to seasonal changes 

in the environment. The boundaries generally shift poleward 

in the summer and fall as seasonal warming of the water 

occurs in higher latitudes and recede toward the Equator in 

winter and spring as the uater cools. However, the northern 

and southern boundaries do not appear to shift in unison 

with each other and the north-south shifting of the boundaries does not occur simultaneously across the ocean, due to 

the varyin8 effects of warm and cold currents in different 

parts of the oceani. 

Matsumoto (1975) has shown that in the Pacific Ocean the 

northern boundary moves poleward in the third and fourth 

qnarters and recedes toward the Equator in the first and 

second quarters (Fig. 14). The shift occurs at about the same 

time in both the western and eastern halves of the ocean. In 

, ._ 

,i 

, k 

c 

', 

e I 

Figure 13.--l.iicaIiona of knmn cspIure\ of juvenile \kipjarb tuna in the major owan\ from 1916 through 1969 (data from various sources). 

14 
Figure 14.-Oullines of the northern and suulhern houndaria of skipjack luna caught in the Japanese luna lonpline fisher). 1964-67. h) )earl) 

quarters (shaded area). The broken lines denule the maximum range for all four quarlers (from Matsumolo 1915). 

15 

~ .... ._.._ I 
16 
the Southern Hemisphere. ho\\e\er. the western half of the 

boundary moies pole\\ard in the second and third quarters 

and toward the Equator in the fourth and first quarters, 

\+hereas in the eastern halt. the shifting ol tlie boundary 

occurs about 3 nio later. Although similar studies ha\e not 

been made in other ocean\, ue believe that the boundaries of 

skipjack tuna distribution in those oceans also undergo similar changes. 

\+?thin this \tide distribur.L>iial range. adult shipjack tuna 

occur in varying den\itic\ it1 Jifferenr parr\ of the oceans, 

with area\ of moderare IO Iii~li densitic\ quite often bein3 defined by the existence 01 a fishery (darkly shaded areas, Fig. 

10). In the Indian Ocean, skipjack tima fidierit.\ occur in the 

Gulf of Aden and the Licinity of Laccadlie I5laiids. Jlaldi\e 

I4ands, and Sri Lanka. and \poradically along the coastlines of countrie5 bordering tlie ocean frorn Australia across 

the Indian subcontinent to South Africa. In the \restern 

Pacific Ocean skipjack tuna fisheries occur in the iicinity of 

Jepm, Ryukyu I<l-nd\, Tainan, Philippines. Bonin Islands, 

Mariana Islands, Ciroline Islands, Palau Islands, I'apua New 

Guinea, Solomon Islands, Ne\+ Caledonia, Au\tralia, and 

Ne\\ Zealand. 111 the north along lat. 40" to 45"N. the fishery extends eastnard to long. 176"W. In tlie Equatorial Countercurrent and North Equatorial Current the fi\hery extends 

eastward to long. 154%'. In the central Pacific Ocean, fisheries occur around the Ha\\aiian Island5 and French Polynesia. In the eastern Pacific Ocean, fisheries occur off Baja 

California, the Revillagigedo Islands, Clipperton Island, 

Central America, Cocos Island, the Galapago\ Island\, and 

northern South America. The fisheries extend \\est\\ard 

along the equatorial currents to long. 140"W. In the Atlantic 

Ocean, fisheries occur in the Gulf of bleuico, the Caribbean 

Sea, around Cuba, and off Africa bet\\een lat. 2O"N and 

15"s. 

2.3 Determinants of distribution changes 

2.31 Larvae and juvenile\ 

Surface current5 influence the distribution of skipjack tuna 

larvae indirectly by modifying the distribution of rea-surface 

temperatures (Brock 1959; Blackburn 1965). In botli tlie 

Atlantic and Pacific Oceanr the warm North and South 

Equatorial Currents flon westuard and are deflected poleward as they reach the coritinents. Cold currents at high latitudes flow eastnard and are detlected tonard the Equator 

upon reaching the v.estern shores of the c9ntinents (Fig. 15). 

This results in the expansion of the narm nater area falorable for spawning in the and the restriction ol similar 

area in the east, as s1iotr.n by the 25 "C isotherm in Figure 11. 

A similar situation esisti in the Indian Ocean, except that it 

applies only to the Southern Hemisphere. 

Currents can directly influence the distribution of skipjack 

tuna larvae by transporting the egg5 during the incubation 

period and early growth stages of the larvae. Honecer, such 

influence is localized and not easily di5cernible because it is 

overshadoned by the estensiveness of the spanning area and 

the lengh of the spawning sea5on. Even around islands, 

where the currents are modified to form wakes and eddies 

(Barkley 1972) in which zooplankton and fith larvae can be 

entrapped, skipjack tuna larvae were found to be randomly 

distributed (Nakamura and Matsumoto 1967; Ponles 1977). 

Skipjack tuna, as well as other tunas, require warm water 

for spawning and larval survival; consequently, the limits of 

larval dimibution are governed largely by water temperature. 

The range of distribution can be described roughly bq the location of the mean 25°C surface isotherm (Fig. ll). Others 

(Uepanagi 1969; Mor1 1972) have used the 24°C isotherm to 

describe the distributional range. This does not imply that 

kigure tS.--Orean current r>\lern\ (from Hrrich 1959). 

17 
sbip.iack tuna larvae are not found in water temperatures 

below 25" or 24"C, for they have been take at surface temperatures as low as 22. I "C (Matsumoto 1974). 

Both the optimum and minimum temperatures for \kipjack tuna larvae differ by areas (Table 4). In the werrern 

Pacific Ocean between lat. 30"N and the Equator, all larvae 

taken in surface night tows from 10 cruises of the RV Sho.vo 

Marir (data from Ueyanagi 1969) were from waters 25" to 

29°C. The most produit!ve tows, in occurrence and number 

of larvae, were made in 28°C water. The lowest and highest 

temperature5 in ahich \kipjack tuna larvae were caught were 

23.5" and 30"C, respectiLely. These larvae were taken in subsurface tows at 20-30 m (not included in table). In the vicinity 

of the Hawaiian Islands between lat. 10" and 26"N (Table 3), 

nearly all larvae caught in surface night to\\\ from nine 

cruise? of the RV Torcnsend Crot?7we// in 1964 and 1965 (unpubl. data, Honolulu Laboratory), \\ere from waters 23 o26"C, and the most productive tows, in occurrence and number of larvae, were in 25 and 24°C \\ater. respcctivel!. The 

coldest water in which skipjack tuna lar\ae were caught \\at 

22.1 "C. Strasburg (1960), using opening and cloting plankton nets towed obliquely at 0-60, 70.130, and 140-200 m in 

the equatorial central Pacific, reported the capture of a skipjack tuna larba (Fig. 16) in v.ater \\ell below 6jcF (lR.5"C) 

and felt that 60°F (15.6"C) \\as the minimum temperature in 

Nhich tuna larvae could occur. 

Studies of tuna larvae taken in plankton nets b) othert 

(Matsumoto 1958; Strasburg 1960: Klawe 1963: Ueyanagi 

1969: Richards and Simmons 1971) ha\e 5hown that skipjack 

tuna larvae generally are limited to the upper 50 ni of nater, 

that they undergo vertical diel migration, and that the vertical migration is most pronounced in the upper 30 m. All of 

thete 5tudiec show that surface to~t at night caught considerably more larvae than day tnw. Although Strasburg's 

result? (Fig. 16) suggest that larvae may be present below the 

thermocline, Klawe (1963) reported that none were taken 

belov. the mixed layer in the eastern Pacific Ocean. 

Ju\enile skipjack tuna (up to about 47 mm) also are found 

near the surface. Sampling for juvenile tunas with a small 

mesh midwater trawl, Higgins (1970) found that shalloa to\\s 

(maximum depth of 20 m) at night tended to catch the most 

juveniles and deep tows (100 m) at night, the fewest. Catches 

during the morning were not significantly different from 

those at night, hut shallow morning tows tended to be less 

Figure 16.--lcolherm and thermocline deplhs at clnring net plankton ttatiiin~ in 

thecentral equatorial Pacific Ocean (lal. ll"N-S?j: long. 140%'). IlliRh C1. .hhh 

cruise 33. Circled number5 denote \kipjack luna lanae. dot\ represenl cloring ne1 

 to^* )ielding other tuna larvae (modified from Strashurg 19601. 

producti\e than shallow night tws, and deep morning tows 

tended to be more productive than deep night tows. Daytime 

catches were about the same in shallo\+ and deep to~t. The 

differences in the catch rates for juvenile skipjack tuna betueen shallow and deep tow5 at different times of the day 

were astumed to be the result of diel vertical migration. 

Higgins (1970) found also that juvenile skipjack tuna were 

significantly larger in deep tows than in shallow tows, and 

night and morning tows at the deeper depth also tended to 

catch larger juveniles than day tows. He tentatively concluded 

that the smaller juveniles live primarily in the upper isothermal layer, whereas the larger juveniles tend to occur in or 

migrate to deeper water. 

2.32 Adults 

(I) Surface temperature 

The temperature range within which skipjack tuna appear 

to be restricted has been reported on a world scale by Laevastu 

and Rosa (1963) as 17" to 28°C and 19" to 23°C for the 

major fisheries. Specific regions of the world oceans not only 

show some variations, but extend the range further. Uda 

Tahle 4.-Shipjack tuna lanap taken in wrlare tow\ during IO cruise\ of the .Shii.vo lloru in the western 

liorth Pacific Ocean (Ile?anagi 1969) and in 9 rrui\e\ of the 7uwnwid (rovinell in the central hrth 

Pacific Ocean (unpuhl. data. hMFS Honolulu Laboralor)). 

Ue\tern Paulic (131 30 K-0") Central Pacific (lat. IO"-26"N)' 

Surface humhsr Number Perisni Lar\ar Numher Number Percent Larvae 

temp ('C) iou\ lanap occurrmcc perioa inn\ lanae nceurrence Der IOU 

18 
(1957) gake the range off Japan as 17.5" to 30°C for all occurrences: Broadhead and Barrett (1964) and Williams (1970) 

reported a range of 17" to 30°C in the eastern Pacific Ocean; 

Jones and Silas (1963) reported regular catches ol skipjack 

tuna off southern India from 27" to 30°C {{ater; Robinc 

(1952) reported skipjack tuna taken in temperature5 of 14.7" 

to 20.8"C in Tasmanian \\aten: and Eggleston and Paul 

(1978) gave a range of 16" to 22°C off Neu' Zealand. Thus, 

the overall temperature range of skipjack tuna occurrence 

could be taken as 14.7" to 30°C. Figure IO shon5 that the 

15°C isotherm adequately demarks the range of skipjack 

tuna occurrence. (.Note: The 15°C isotherms shown \\ere for 

the warmest month of the year in both hemispheres and, 

hence, represent the ma,ximum average poleward displacement .) 

As in the distribution of the larxae, temperature plays an 

important role in the distribution of the adults. Its influence 

is usually not obvious in the tropics, %here the temperature IS 

relatively uniform throughout the year, but IS most noticeable at the higher latitudes. This is readily seen in the persistence throughout the year of high catch rate cells of skipjack 

tuna taken on the tuna longline in the equatorial Pacific, as 

compared with the latitudinal shifting of such cells near the 

northern and southern boundaries 01 distributlon in response 

to seasonal changes in temperature (Matsumoto 1975), and in 

the marked seasonality seen in the curface fisheries near the 

northern and southern limits of distribution in the northeastern and southeastern Pacific, Hamaii, northwestern Pacific, 

Papua New Guinea, and Neu Zealand (Kauasaki 1965b; 

Joseph and Calkins 1969; Uchida 1975; Habib 1976. 1978; 

Forsbergh 1980; Habib et al. 1980a, b, c; Wankowski 1980). 

That skipjack tuna seek preferred temperatures is indicated 

by catch patterns in the various fisheries. These temperatures 

vary by regions: 20" to 24°C in the northwestern Pacific 

fishery (Uda 1957). 20" to 30°C in the easlern Pacific fishery 

(Williams 1970). 28" to 29°C off southern India (Jones and 

Silas 1963). 16" to 18°C off Tasmania (Robins 1952), and 

19" to 21°C off New Zealand (Eggleston and Paul 1978). 

Sharp (1978) listed the optimum temperature range of commercial activities and rchooling behabior of skipjack tuna lor 

five hypothesized subpopulations in the Pacific Ocean a\ tolow: northeastern Pacific, 20"-26 "C; southeastern Pacific, 

20"-28"C; Ne\\ 7ealand, l7"-23 "C; Papua Ne\\ GuineaSolomon Islands, 28 "-30°C; and northbbestern Pacific, 20'- 

28°C. Others who have inrestigated the relationship of skipjack tuna and mater temperature include Kuroda (1955). 

Katrataki (1958, 1965a). Schaefer (1961b), Radoiich (1963), 

Kasahara and Tanaka (1968). Uchida (1970), Seckel (1972), 

and Yasui and Inoue (1977). 

(2) Salinity 

According to Blackburn (1965), "Salinity measurements 

can be important in characterizing and detecting oceanic 

features with which tuna are arsociated but salinityperse has 

no known direct effect on tuna distribution." Barkley (1969), 

however, showed that the habitat of adult skipjack tuna in 

the Pacific Ocean appears to coincide geographically with 

areas where a shallo\\. salinity maximum is present seasonally 

or throughout the year (Fig. 17). and suggested that the normal habitat of skipjack tuna is the lomer salinity water which 

overrides the shallow salinity maximum. 

Seckel (1972) observed that skipjack tuna landings in the 

Hanaiian skipjack tuna fishery were high in those years when 

init.ial warming of the water occurred before the end of February and the mean monthly salinity during spring was between 34.6 and 34.8"/00, the latter value indicating the northern boundary of the California Current Extension v;ater. 

Yasui and Inoue (1977) also noted good to excellent fishing 

off southern Japan in those years when the mean surface 

salinity was 34.81"/~ and the mean surface temperature was 

Zf.2"C. In poor years the mean salinity was 34.87"/,,,, and 

the mean temperature was 19.8"C. In both instances, good 

fishing was related to favorable salinities, as well as favorable 

water ternperat ures. 

Figure 17.-Di~trihution 01' ,kipjack tuna in the Pacific Ocean (after Holhwhild and l'rhidn 1968). 1 he contour\ mark 

the boundaries of the area where a shallow ruhrurfnrr salinit) maximum ib present either wd\Orla~~> or permanmll? (from 

Barklet 1969). 

19 
Ihiig~:! et al. (1978), in comparing chart\ oi shipjack tuna 

catclie\ b) the Japanese in tlie nestern Pacific for the period 

1974-76 and turf'xe dinit) chart\ lor 1957-75. noted that 

the catchec of tkipjack tuna were greater 111 naters \rith dinities 01 35"/,!,, or less in the area east of Papua Ne\\ Guinea. 

In thib instance the 3SL'/,,,! iwhaline in the region uas I'ound 

tn be associated \\ith the pre\ence of a co~verpeiice. 

(3) Surface currents 

'The circulation features of the major current\ in both the 

Pacific and Atlantic Ocean\, described pre\iou\l)- in conncction uith distribution of laivae, ah affect the di5tribution 

of adults. The flow toward thc Eqiiator of cool \rater on the 

eastern side of the oceans tends to re\trict tlie distribution of 

fish in that region, and the pole\\ard tlo\b at the \restern side 

of the oceans perniitr a broader di5tribution there. 

Surface currents affect the distribution ol' \kipjach tuna in 

other nays. In currents llouing meridionally, tuna are con- 

>idered to he distributed along the axit of the current and in 

greater abundance than in adjacent natev (Blachburn 1965). 

Such a relation \\a> noted by Kairasaki (1958) and Uda 

(1962a) in the Kuroshio (n'arm current fi-om the south) off 

Japan. Uda obserked that inore skipiach tuna became a~ailable to the fisher) when the \\arm Kuroshio water extended 

over a broad area; and conversely, that a strong Oyarhio 

(cold current from the north) hindered the niovement of fish 

and catches were Ion. 

Other effects of currents on the movement of fish have 

been indicated in the models of migration of skipjach tuna 

proposed by Seckel (1972), Williams (1972), Matsumoto 

(1975), and others. These migration models are described in 

Section 3.51. 

(4) Hydrographic features 

Fish tend to aggregate in areas where harm and cold water 

intrusions are well developed, in eddies, in areas of turbulent 

mixing, in oceanic fronts, comergences, upuelling, and other 

hydrographic features (Uda 1961, 1962b). Kawasaki (1958) 

noted tha! skipjack tuna fishing grounds developed off Japan 

when the warm Kuroshio uater spread thinly over colder 

uater and when tlie Kuroshio meandered in a complicated 

pattern. Uda (1962a) reported that good catches were also 

made when both the Kuroshio and the cold Oyashio currents 

were strong, under which conditions the skipjack tuna were 

concentrated along the boundary betueen the Harm and cold 

water (polar front). Uda (1962b) further reported that in the 

eddies associated with the polar Iront, skipjack tuna were 

found in the warmer anticyclonic eddies along the front. 

Donguy et al. (1978) showed that \kipjack tuna catches by 

the Japanese in the western Pacific, east of Papua New 

Guinea, were concentrated along and on the low salinity side 

of convergences (Fig. 18). Forsbergh (1969) reported that a 

marked salinity front was found at all seasons in the Panama 

Bight and that the abundance of skipjack (and yellowfin) 

tuna in the northern part of the Bight appeared to he related 

to the seasonal cycle of up\\elling. 

(5) Vertical distribution 

Based on studies of tuna 1o:igline catches, Yabe et al. 

(1963) indicated that tunas are distributed by depth layers 

Mith the tkipjack tuna occupying the shallo\~est layer, follo\red in order b) the bluefin tuna, Thitnnits rhvnnus; yelloufin tuna: bigeye tuna, T. ohecirs; and the albacore, T. alalfmea. Skipjack tuna \\ere assumed to occupy the shallowest 

depth layer largely on the hasis that the) are taken in commercial quantities almost entirely by fishing gear that require 

the fish to be at or very near the surface, and that the skipjack tuna are taken only incidentally on the deep-fishing 

long line. 

Rerultc from the use of echo sounders and underwater ohwr\ations from small submarines hale sho\rn that ihe rkipjack tuna not only occur o\er a much greater depth range 

than previoutly realized, but also in depths occupied by other 

tunas. Kimura et al. (1952) and Yamanaka et al. (1966) observed fish schools at depths of 140 and 120 m, respectively. 

uhich they identified as skipjack tuna \\hen the schools subsequently rose to the surface. Strasburg et al. (1968) encountered skipjack tuna schools at depths of 98 to 152 m in 

Hanailan waters during their numernus dives in a submarine. 

Ti-acking skipjack tuna tagged aith ultrasonic transmitters in 

Hawaiian maters (Dizon et al. 1978) indicated that this species 

spends its time between the surface and 263 rn during the 

day, but remain5 near the surface (within 75 m) at night. 

Thete depths are well Hithin the depth range in which the 

yello\\fin tuna are usually caught by longline. 

3 BIONOMICS AND LIFE HISTORY 

3.1 Reproduction 

3. I1 Sexuality 

The skipjack tuna is normally heterosexual; however, a 

few instances of hermaphroditism have been recorded (Nakamura 1935; Raju 1960; Uchida 1961; Thomas and Raju 1964). 

In the two specimens described by Uchida from Hawaiian 

waters, the left and right gonads of one fish (Fig. 19) and the 

left gonad of the second fish were divided into three segments: 

an anterior ovarian section, a middle testicular section, and a 

posterior ovarian section; whereas the right gonad of the 

second fish was divided into two segments: an anterior twothirds testicular section and a posterior one-third ovarian 

section. In both fish the anterior ovarian sections contained 

ripe resorbing ova but not in the posterior ovarian sections, a 

condition that could result from extrusion of ova from the 

latter during spawning but not from the former, due to the 

lack of an adequate oviduct. The testicular portions contained 

no running milt, but were described as well developed. Based 

on the developmental stages of the ovarian and testicular 

sections, Uchida believed that both fish were functional 

males and females. 

3.12 Maturity 

The minimum size of skipjack tuna at first spawning has 

not been defined, but a reasonable estimate can be made 

from the various published reports of fish with mature, ripe 

and spent ovaries, and of fish with ova diameters approaching 1.0 mm. 

In the Pacific Ocean, Marr (1948) recorded skipjack tuna 

as small as 40 cm fork length (FL) uith spent ovaries from 

20 
Figure 18.--l)istribution of skipjack luna catches and surfare salinilj in the mestern Pacific: a) JanuarJ-March IV74 h) JuI)-Srplember 1974. c) Octohrr-lkrcmhrr 

1974. and dl January-March 1975. No calrh denoted h) 0. catches h) +,and dark squares reprewnl > 7uO I (from Ihngu? el al. 1978). 

Figwe 19.-Ovulerlr~ of a hermaphrodilir \kipjack luna (from Urhida 19611. 

the Marshall Islands; Wade (1950) recorded fish in the 40.0 

to 40.9 cm size class with ripe and spent ovaries from Philippine waters, and reported the capture of a female in the 

34.0 to 34.9 cm sire class having ripe ovaries; and Brock 

(1954) noted that the smallest fish that posessed maturing 

ova during the spawning season in Hawaiian Haters \\ere 

around 40 to 45 cm. Raju (1964~) estimated the 5i7e of sexual 

maturity of skipjack tuna in the Indian Ocean around hlinicoy Island as about 40 to 45 cm. He deribed his estimate 

from his observations that I) the smallest skipjack tuna that 

possessed ovaries with maturing ova during the spawning season \+as around 40 to 45 cm and 2) remnant5 of mature o\a 

were present in fish abo\e 40 cm but none in fich belo\\ thi\ 

sire. Stequert (1976) reported first spawning of Yhipjach tiina 

at 41 to 43 cm off the northwest coast of Madagascar. Simmon5 (1969) reported minimum size of females at maturity in 

the Atlantic Ocean as 41 cm, as determined from ltrh \kith 

spawned or spent ovaries. 

From the above, it appeari that [he minimum si7e of fcmale ikipjack tuna at maturity is 40 sm and that initial 

spawning can occur in fish between 40 and 45 cni or larger. 

The unusuallv small female (34.0-34.9 cni stir cia\\) nith ripe 

ovaries reported by \\adc. (1950) could be a rarity or a caw 

of erroneous Ftaeing of the gonads (we Section 3.15 tnr d~\- 

cussion of maturity stage5 of gonad\). 

21 
3.13 and 3.14 Mating and fertilization 

Iversen et al. (1970) observed what they beliebed to be 

courtship of skipjack luna from an underwater sled towed 

from the RV Charles H. Gilbert. While observing a pair of 

skipjack tuna approximately 50 cm long, swimming close together back and forth in front of the sled, they noticed that 

the fish's bodies would tilt about 30" from the vertical in 

opposite directions with the ventral portions of their bodies 

almost touching each other. At another time they sa\& two 

fish swimming in tandem, one ahead of the other. The latter 

fish, tituated slightly below the lead firh, uould approach it 

closely from the rear, its snout coming within a few centimeters of the caudal fin of the lead fish. At the closest point 

of approach, the trailing fish uould display dark vertical 

bars on its side, similar to that displayed during feeding. A 

second pair of skipjack tuna shimming in tandem were observed from the deck of the vessel. On two occasions the lead 

fish was seen to wobble from side to side and the trailing fith 

would exhibit vertical bars along its flanks. These fish showed 

no interest in the baitfish throw'n into the water, so that the 

appearance of the bars were not likely connected with feeding (see Section 3.41 on feeding behavior). 

Observations of similar behavior, believed to be mating or 

courting acts, have been observed by longtime skipjack tuna 

fishermen (e.g., R. M. Oka6) in the Hawaiian Islands. Schools 

of fish displaying such behavior were reported to be reddishor purplish-hued and as being disinterested in feeding. The 

senior author has observed similar schools on several occasions and when the visibility, sea, and light conditions were 

right, whitish trails of spawn could be seen at the point of 

contact when two fish came together. The contacts seemed 

to occur randomly throughout the rapidly moving schools. 

These observations suggest that mating is most likely promiscuous, and other observations such as the collection of 

eggs in various stages of embryonic development in plankton 

nets suggest that fertilization occurs externally. 

3.15 Gonads 

In the early years, sexual maturity of the skipjack tuna was 

determined by the general appearance of the gonad (Matsui 

1942; Marr 1948; Yao 1955; Williams 1964). The characters 

most used were size of gonad and degree of softness or turgidity. Others (Buzag 1956; Raju 1964~; Kawasaki 1965a) felt 

that these general characters alone were inadequate even for 

grots clastification. Some improvements were made by including the description of ova in the various stages of development (BuTag 1956; Schaefer and Orange 1956; Raju 

1964~; Yoshida 1966; Simmons 1969; Batts 1972c), but the 

classification still remained subjective and was prone to variations caused by the observer's interpretation of the various 

st ages. 

Buiiag (1956) and Raju (1964~) classified sexual maturity 

on the basis of ovum diameter measurements. Yoshida (1966) 

and Batts (1972~) provided ovum diameter measurements 

along Mith descriptions of ova, ovaries, or both. Yet the 

classification by these authors differed noticeably, since a 

standard scale defining the Larious stages of development 

had not been established for tunas. BuFiag (1956) attempted 

to classify the stages according to the International Maturity 

Scale,' but no one else has followed his example. A summary 

of the maturity stages of female skipjack tuna based on ova 

diameter and descriptions of ova and ovaries from the preceding studies is presented in Table 5. 

To aboid measuring ova, some authors have used gonad 

indices (G.1.) to determine maturity (Yabe 1954; Schaefer 

and Orange 1965; Orange 1961; Raju 1964~; Yoshida 1966; 

Baits 1972~). Schaefer and Orange (1956) obtained a linear 

relationship between ovum diameter and (3.1. (Fig. 20) of 

fish taken in the eastern tropical Pacific. They thus concluded 

that maturity of female skipjack tuna could be determined 

on the basis of G.I. values. Their data, however, consisted 

mainly of G.I. below 50 from samples that included only feu 

fish larger than 60 cm. Raju (1964~) also obtained similar 

results (Fig. 21) from fish taken off Minicoy Island in the 

Indian Ocean; however, his sample included larger fish a.id 

the largest G.I. was about 66. 

Yoshida (1966), whose study was based on fish from the 

Marquesas and Tuamotu Islands in the central Pacific, reported G.I. values ranging from 20 to over 90 for ovaries in 

the developing stage (ovum diameters 0.5 to 0.6 mm) and 

from 30 to nearly 100 for ovaries in the advanced stage (ovum 

diameters 0.6 to 0.7 mm). He noted that there was a large 

overlap in the G.I. values between these two stages and that 

the linear relationship reported by Schaefer and Orange 

'Maturity wale adopted by the lniernational Council tor the Exploration ot 

theSea.a5clted b) Clark(1934). 

Table 5.-!wmmar? 01 malurit? stages nf female rhipjack luna. 

I Immature 0 IC O\a transparent; uith nucleu\ Slender. elonpate. \ex 

II tarl, inaiurine 0 35 OLaopaque: )OIL forming Enlarged: ova !not \Ivble 

111 1 ate niatorinp 0 60 Obaopaque: )clk eranular Fnlarged. turgid: oba i:\ible 

IV hlatittc 0 X5 O\atran\parent: uithoil Enlarged. hesin lo\ing turgidit); 

V Ripe I.(N) O\a \\ith )ello\r CUI droplet Grratl) enlarged. not turgid: ma 

determinahle 

cIu5icr or oil droplet ma \~sihle, cavly di4odeed 

eaol\ dlclodged. some free In 

lumen: extruded hy preswre 

VI Spent 0.55 (ha remnant, I.Onini Flaccid, utth remnant5 of ripe 

ox a 

22 
50 I I 1 I I I I I 

Figure 2O.-Relationship belween gonad index (ovar, weight relative to fish length') and mean diameter of most advanced 

group of ova of skipjack tuna from the Marquesas and Tuamotu Islands (from Yorhida 1%). 

1 

0 70 

40 ~ 

, 0 IO 

'0 lo 20 M 40 M 60 70 0 

GONAD INMX 

Figure 21.-Relalionship between gonad index (ovar) weigh1 relalive to fish 

length') and modal length of largest group of ova of \kipjack tuna from Minico) 

(from Raju 1964~). 

(1956) was not applicable to (3.1. values above 36 (Fig. 20). 

We should also add that the linear relationship shown by 

Raju (1964~) in Figure 21 does not seem valid either. Because 

of the wide range of (3.1. values for ovaries in the same developmental stage, the gonad index does not seem to be a 

sound measure of maturity for skipjack tuna. 

3.16 Spawning 

(1) Fecundity 

Relatively little has been published concerning the fecundity 

of skipjack tuna. The fecundity range has been reported as 

100,OOO to 2 million ova for fish 43.0 to 87.0 cm long from 

the Pacific Ocean, 87,600 to 1,977,000 ova for fish 41.3 to 

70.3 cm long from the Indian Ocean, and 141,000 to 1,331,OOO 

ova for fish 46.5 to 80.9 cm long from the Atlantic Ocean 

(Table 6). Plots of number of ova in the most advanced 

mode against fish size by Joseph (1963), Raju (1964a), SimTable 6.--Obserred fecundilr e,timatea for skipjack luna. 

Source of 

data 

Fi\h length 

Numher (range) Estirnaied ova 

Localit\ oi fi,h (mm) (ranee) ~~ ~ 

Pacific Ocean 

Yabe (19%) 

Rothschild (1963) 

Joseph (1963) 

Yoshida (1966) 

Raju (1964a) 

Stequert (1976) 

Atlantic Ocean 

Simmons (1969) 

Batts (1972~) 

Indian Ocean 

Ryukyu Islands S 

Hawaiian Islands 3 

Eastern Pacific 42 

Marquesas Islands 4 

Minicoy Island 63 

Madagascar 64 

Caribbean Sea 13 

North Carolina, 31 

U.S.A 

468-6 10 

W-870 

614-715 

430-750 

418-703 

44-565 

46s-809 

498.704 

I13.364-854,897 

280,000-I ,900,wo 

210,000-1.4w.wo 

100.ooo-2,ooo.ooo 

ISI.Y00-1,977.900 

87,600-824.000 

262.(xx)-1,331.00 

I4 I .ooO- I ,200,000 

23 
mons (1969), and Batts (1972~) shorn considerable variation 

among fish of the same size. All four authors fitted regrecsion lines to the relationship between fecundity and length 

(Table 7; Fig. 22), but due either to inadequate sample size 

or to samples of limited size ranges, the regression lines \dried 

noticeably. Raju (1964a) fitted regression lines also to the relationship between fecundity and the square of the fish length, 

and both he and Simmons (1969) fitted regression lines to 

the relationship between fecundity and the cube of the length. 

The latter regressions had the higher coefficients of correlation, although all four regressions were statistically significant 

(P = 0.01). Joseph (1963) and Raju (1964a) fitted regression 

lines to the 

I 

relationship between fecundity and fish weight 

I 

I 

I JOSEPH 1963 I 

I 0 RAJU 19640 I 

I 

I* 1 ROTHSCHILD 1963 

0 SIMMONS 1969 I I 

V BATTS 1972c I 

I A YOSHIDA 1966 I 

on 

1800 

4 1600 

a 

LL 

1400 

n 

4 

2 1200 P - 

t 

: 1000 - 

n 

3 ; 800 

600 

400 

200 

Figure ZZ.-Hegreaaion~ of fecundil? on length and iihrerred fecundit) erlimalea 

of skipjack luna aheve 690 mm. I) Jo5eph (IY63); 2) Haju (1964a); 3) fecundit). 

length', Haju (1964a); 4) Simmons 119691; and 5) Baltr (1972~). 

also. Both reported smaller standard error of estimates than 

those obtained from any of the fecundity-length rslationship5, 

Nhereas the latter obtained a coefficient of correlation only 

slightly higher than that of the fecundity-length cubed relationship. The closeness of the coefficients of correlation is 

understandable, since weight is nearly proportional to the 

cube of the length for skipjack tuna. Consequently, tither 

the length cubed or weight may be used to describe the relationship between fecundity and fish size. 

Using Raju's (1964a) fecunditb-length cubed relationship, 

the calculated fecundity estimate of skipjack tuna at minimum spawning size (40.0 cm) is 110,OOO ova. That Raju's 

fecundity-length cubed relationship holds reasonably well for 

fish larger than about 69.0 mm is noted in Figure 22, which 

shows the extension of the curve and also the observed fecundity of fish above this size by various authors. 

In spite of the acceptable fecundity-length cubed relationship noted above, it would be most difficult to estimate the 

total annual egg production of the skipjack. tuna because of 

I) the extremely large variation in number of ova among 

fish of the same size, 2) the uncertainty in the number of 

times a fish spawns during the year (Section 3.16), 3) the 

possibility of reduced fecundity at successive spawning in 

the same year (Joseph 1963), and 4) incomplete knowledge 

concerning the size composition and total abundance of the 

spawning stocks. 

(2) Frequency 

The possibility of skipjack tuna spawning more than once 

during a season has been discussed by several authors. Brock 

(1954) was of the opinion that the multimodal distribution of 

ovum diameters during the spawning season and its absence 

at other times indicated that individual skipjack tuna taken 

in Hawaiian waters spawn several times. As supporting evidence, he cited the absence of an overall trend in ovum 

diameters during the spawning season (Fig. 23) and the absence of spawned-out fish until after the end of the spawning 

season. Bukg (1956) determined multiple spawning by following the movement of ovum diameter modes through the 

various stages of development as indicated by ovum diameter frequency plots (Fig. 24). Raju (1964~) determined multiple 

spawning from the Indian Ocean by satisfying the four lines 

of evidence used by Clark (1934): I) The presence of multiple 

modes in diameter frequency polygons, 2) obtaining a high 

correlation in the progression of successive modes, 3) the 

Table 7.--Comparison of fecundil~-lenplh and fecundi1)-weight relationships of skipjack luna from variou? 

sources. 

/, 

6.326 

XR.613 

2 713 

0.00254 

0.OOXXl342 

3.23X 

I .x54 

67 01 

- 

213 

28.6 

27 d 

24 7 

22.5 
c STAGE I 

STAGE II 

STAGE 111 Y 1 

Figurr 23.-A1emye modal diamelrr of the Isryrst poup of o\a in Aipjack luna 

war) \ample\ rdlected from1949 to I951 in Hawaiian water\ (from Rrorh 19541. 

presence of ova remnants in maturing ovaries, and 4) the 

decrease in the numerical ratio of ova bet\\een maturing 

and mature group. 

From the results of these studies, there seems to be little 

doubt that skipjack tuna spawns more than once during a 

season. How many times it spawns is not yet known. but 

utilizing the reports by Brock (1954) and Bu%ag (1956), the 

number of spawnings can be estimated, at leayt for fish in 

Hawaiian waters. 

From Brock's 1949 and 1951 data in Figure 23, \\e note 

that in Hawaiian waters, fish \vith immature o\a (ova diameter 0.1 nim) occurred in February, and in 2 mo the o\a developed into the mature stage (ova diameter 0.7 nim). If 

development continues at this same pace, the ova should he 

in the ripe stage (Yabe 1954; Yoshida 1966; Batts 1972~) nith 

diameters from 0.8 to 1.2 mm in late April or early May. 

This is supported by the first captures of larval skipjack tuna 

in Hawaiian waters in late April and early May (Seckel 19723. 

BuEag (1956), as well 2s others, had observed that after the 

first hatch of ripe ova has been spawned, the ovaries revert 

to the condition similar to that of ovaries in maturity \tape 

111 (Fig. 24), i.e., ova in the most advanced group are not 

much larger than about 0.58 mm (18 micrometer units). OVP 

of this size would require about 6-7 wk to reach the ripe 

stage (based on Brock'c data), evclusibe of a probable resting 

period after the initial spawning. Hence, the earliest that a 

skipjack tuna could spawn a \econd time after having 

spawned once in early May would be in late July or early 

August. Since the spawning season in Hawaiian waters terminates in September (Brock 1954), it \\auld be unlikely for 

this specie\ to spawn a third time in the area. Thls does not 

mean that a third or even a fourth spa\\ning could not occur. 

In fish with ovaries de\eloping more than two groups of ova. 

migration to warmer waters in the fall and winter could allou 

it to spawn a third or even a fourth time during a year. 

Another bit of information may be gleaned from Figure 

23. If the first spawning occurs as dacribed, then the duration of ova development from the immature to the ripe or 

spawning stage can be estimated as being from 3 to 3.5 mo. 

(3) Spawning in captivity 

Current interest in mariculture has resulted in many atSTAGE V % STAGE V-A 

STAGE VI1 

L 

L STAGE VII-A 

6 6 10 12 I4 16 I6 20 22 24 26 28 30 32 5 

DIAMETER IN MICROMETER UNITS 

Figure 2J.-Ora dinmrtrr frequrnr) pc,l)gon\ of \hipjack tuna with the 

*laye* (ifma dr\rlopmrnt (trim HuZq IYShI. IEarh micromrtrr unit equals 

0.0325 mm.1 

tempts to rear tunas in captivity. Collection of ripe ova from 

females at sea has only resulted in sporadic success, probably 

due to the paucity of running ripe females in the commercial 

catches and because attempts to hold adults as brood stock 

has proven inefficient and costly. 

In June and July of 1980. female skipjack tuna (1.4-2.2 kg) 

confined at the Kewalo Research Facility of the U.S. National 

Marine Fisheries Senice in Honolulu ovulated ripe ova \sithin 8 h after capture (Kaya et al. 1982). The rapid debclopment of the o\a, followed by o\ulation and spawning, apparently was induced by stresses experienced by the fish during 

capture and confinement. The ripe ova measured approximately 1.0 mm as compared uith unobulated ova, 0.59-0.74 

mm. obtained from fish taken from the 5ame school and iced 

immediately upon capture. Artificial fertilization of the ripe 

ova resulted in successful embryonic developement and subsequent hatching of the larvae. Males brought live to the 

holding tanks did not re\pond in the same manner. Their 

testes \bere identical to those of fish iced immediately upon 

capture. Although all testes nere nqature, they failed to yield 

milt on moderate stripping pre\wre. Small amounts of thick 

milt obtained from evcised testes, ho\\ever, were successfully 

used in fertilizing the ripe ova stripped from captive females. 

The rapid maturation and ovulation procesws brought on 

by capture and confinement stre\ses are currently being exploited to routinely produce larval skipjack tuna5 for debelopmental and gro\bth studies at the Kewalo Research 

Facility. 

25 
(4) Areas and seasons 

Gonadal stdie\ in the Pacific, Atlantic. and Indian Oceans 

indicate that skipjack tuna spann throughout the year in 

tropical water\ near the Equator and from spring to earl! fall 

in subtropical saters, with the spauning period becoming 

shorter as distance from the Equator increases. Because sampling for gonads requires the capture of adults, these vudies 

have generally been restricted to \pecific areas where fishing 

has been done on a comrnerical basis or in area\ subjected to 

heavy experimental fishing. Such areas include uaters south 

of Japan, Philippine\, hlarshall Islands, Ha\\aiian lilands, 

Ne\\ Caledonia, Marquesas-Tuamoto Archipelago, and off 

Central America in the Pacific Ocean (Kishinouye 1923; 

Marr 1948; U’ade 1950; Brock 1954; Yabe 1954; Yao 1955; 

Schaefer and Orange 1956; Orange 1961; Ymhida 1966; 

Legand 1971; Naganuma 1979); uaters off Ghana, Ivory 

Coast, Sierra Leone, Cape Verde Islands, Lesser Antilk\, 

Cuba, and North Carolina in the Atlantic Ocean (RiLero and 

Fernandy 1954; Frade and Postel 1955; Gorbunova and 

Salabarria 1967; Simmons 1969; Zhudova 1969; Battc 1972~); 

and uaters around Madag r, Minicoy Ihnd. and throughout the tropical lone in the Indian Ocean (Raju 1964~; Marcille and Suiuki 1974; Stequert 1976). 

Lana1 captures, also indicative of spawning acti\ity, ha\e 

occurred not only in the arcac mentioned abo\e and in similar 

months, hut also in intervening waters, e.%.. Pacific OceanMatsumoto (1958). Stra$burg (1960), K1av.e (1963), Nakamura and Matsumoto (lY67), Ueyanagi (1969. 1970); Atlantic 

Ocean-Richards (1969), Richards and Simmons (1971 ), 

Ueyanagi (1971); Indian Ocean-Jones (1959), Gorbunova 

(1963, 1965a), Jones and Kumaran (:96?), Ueyanagi (1969). 

3.17 Spawn 

Ripe ovarian eggs are spherical, smooth, transparent, and 

usually contain a \ingle yellou oil droplet (Brock 1954; Yabe 

1954; Yoshida 1966). Eggs are 0.80 to 1.17 mill iii diameter, 

with mean diameters ranging from 0.96 to 1. I35 mm. The oil 

droplet caries greatly in six: Yabe (1954) reported a range of 

0.22 to 0.27 nim, Brock (1954) gave a range of 0.22 to 0.45 

nim, and Yoshida (1966) reported an average of 0.14 mm. 

That eggs of this six range are fully ripe and capable of 

being fertilized has been shown by Ueyanagi et al. (1973, 

1974) and Kaya et al. (1982). 

Eggs of other species of sconibrids that liace been fertil:zed 

artificially (frigate tuna, Auxis /hazard; bullet tuna, A. roche;; 

and yellowfin tuna) and reared through the hatching stage 

are comparable in size and appearance with those of \kipjack 

tuna (Harada et al. 1971; Harada, Murata, and Furutani 

1973; Harada, Murata, and Miyashita 1973; Ceqanagi et al. 

1973). Consequently, tuna eggs collected at sea in plankton 

nets are extremely difficult to Identify to species. 

The spauned egg\ of tunas are buoyant in nature (Kikawa 

1953; Harada et al. 1971; Harada, Murata, and Furutani 

1973; Harada, Murata, and Miyashita 1973). During experiments in rearing art ially fertilized yellowfin tuna eggs, 

Kikaua (1953) observed that most of the eggs uhich \\ere 

presumably dead turned opaque and sank to the bottom, bur 

thocc still viable were transparent and buoyant. The opaque 

eggs nere found to be in arrested stages of cleavage, whereas 

the buoyant eggs contained developing embryos. 

3.2 Preadult phase 

3.21 Embryonic phase 

Development of the skipjack tuna embryo has been studied 

in detail by Ueyanagi et al. (1973, 1974) from artifically fertilized eggs. They noted that embryonic development proceeded 

rapidly after fertilization (Fig. 25) and the larvae hatched in 

about I d. The incubational period noted by the former authors 

was 26 to 31 h in 23” to 25°C water. 21 to 32 h in 24.2” to 

27°C water, and 21 to 22 h in 26” to 29°C water; whereas, 

the latter reported 22 to 27 h in 27°C aaier. Hence, there is 

some indication that incubational period is related inversely 

to water temperature 

3.22 Larval phase 

Total length (TL) is used in discussing tuna larvae. It represents the distance from the snout to the end of the longest 

caudal ray before the caudal fin has forked. On larger larvae 

total length equals fork length. 

The newly hatched larva (Fig. 25) is about 2.6 mm TL 

(Ueyanagi et al. 1974). comparable with the size estimated by 

Matsumoto (1958). According to Ueyanagi et al., the larvae 

absorbed their yolk sac within 2 d after hatching, but due to 

unsuccessful initial feeding, they died within 5 d. 

The development of the larvae is shown in Figures 25 and 

26. Ueyanagi et al. (1974) described the earliest stages of the 

larvae thus: “Yellou pigment spots on the finfold were concpicuous in the prelarval stage. Small melanophores appeared 

on the dorsal edge of the trunk in the early prelarval stage 

and moved toward the ventral edge [where they] tended to 

concerge toward the caudal peduncle.” The convergence of 

these melanophores at the caudal peduncle leads to rhe eventual formation of a single, distinct black spot (Fig. 26a-d), 

one of the principal identifying characters of the skipjack 

tuna larva. Other characters describing the Iarbae have been 

reported in Wade (19511, Yabe (1955), lshiyama and Okada 

(1957), and Matsumoto (1958). These include a disproportionately large head which is bent slightly downward in relation to the body axis, the appearance of 2 or 3 melanophores 

over the forebrain area when the larvae are about 7 mm long 

(the number of melanophores increases to about 12 in larvae 

about 14.5 mm in length), heavy pigmentation over the midbrain area throughout all siiec, and the appearance of the 

first dorsal fin spines in larvae about 7 mm long (the number 

of spines increases to about 13 in larvae 1 I mm TL). Pigmentation on the first dorsal fin is limited to scattered melanophores near the outer edge of the fin (Fig. 26d). The full 

complement of 16 spines and 15 rays in the first and second 

dorsal fins, respectively, and 15 rays in the anal fin are developed by the time the larvae are about 12 mm long. At this 

stage the young are considered as juveniles. 

The juvenile stage is characterized by a more fusiform 

body and by a general increase in pigmentation over all parts 

of the body. Juveniles up to 47 mm (Fig. 27d) are easily 

identified by the concave outline of the first dorsal fin and its 

characteristic pigmentation, the moderately long snout, and 

26 
ye1 low 

Figure 25.--lkvelopment of egg, and prelarlal stage of skipjack tuna. a) I2 h after fertilitation. 1.01 mm in diameter: b) IS h. 

1.00 mm; c) 21 h, 0.97 mm; d) 13 h, 0.99 mm; e) just after hatching. IL 2.65 mm; f) 4 h. after hatching. TL 2.96 mm; g) 10 h. 

TL3.30mm; h) 19h,TL3.36mm;i)33h,TL3.57mm; j)43 h.TL3.70mm; k)84h.Tl.3.SSmm(fromUe)anagietal. 1974). 

27 
Figure ZS.--(b,iiinued. 

the number of vertebrae (20 precaudal and 21 caudal). The 

first dorsal fin has scattered melanophores on the first three 

or four interspinal membranes and along the distal edge of 

the remaining membranes. The only other genus in which 

the juveniles are known to have similar-shaped first dorsal 

fins is Eurhynnus (Schaefer and Marr 1948; Wade 1950; 

Mead 1951). In Eurhynnus, however, the first dorsal fin is 

either completely or almost completely pigmented. Also, unlihe Eurhynnus, juveniles of skipjack tuna do not develop 

dark vertical bands over the dorsal half of the body. 

According to Godsil and Byers (1944) and GodGI (1954), 

adult skipjack tuna lack a swim bladder. Richards and Doke 

(1971), however, reported that all tuna larvae, including larvae of the skipjack tuna, develop shim bladders and gas 

glands at the smallest sizes studied (2.6 mm standard length), 

but that the swim bladder degenerates in skipjack tuna before the fish reaches a size of 20 mm. According to these 

author$, degeneration of the swim bladder begin5 when the 

larvae reach a Size of 9.0 mm and the shim bladder is nearly 

absent in specimens 24.0 mm long. Only the gas gland remains in iuveniles above this size. 

3.23 Adolescent phase 

exponential curve to a set of size data and report the estimate 

of Lm thus obtained as the maximum size of a fish. It should 

be remembered that Lm is a parameter mathematically characterizing the asymptotic phenomenon we call maximum size 

of a fish. Our estimate of it is a statistic whose accuracy is 

dependent upon the representativeness of the sample used in 

the calculation, and whose variance is, among other things, 

dependent upon the sample size. Thus if we need an estimate 

of maximum size for other model building, e.g., estimating 

sustainable yield, then a statistic is what is needed. If we 

want to describe the maximum size of a fish, then it may suffice to report the largest fish recorded to date. In the following paragraphs we will follow the latter procedure, while later 

in Section 3.43, we hill present estimates of Lm. 

Miyake (1968) listed a skipjack tuna in the 106.5 to 108.4 

cm size class taken on tuna longline. Although he made no 

reference to this fish, it could well be the largest skipjack tuna 

ever recorded. A fish of this size class, using the length-weight 

relation developed for fish in the central North Pacific 

(Nakamura and Uchiyama 1966), would weigh between 32.5 

and 34.5 kg (71.6 and 76.0 Ib). On the basis of growth parameters computed from data on modal progression and otolith 

readings (Table 8). such a fish would have a minimum age of 

8-12 yr. 

Juveniles below the size of first spawning (ca. 40 cm) have 

not been described and there is no information in the literature describing the initial appearance of the longitudinal 

black stripes along the sides of the body below the lateral line. 

3.32 Hardiness 

As already discussed under Section 2.32, skipjack tuna can 

withstand a wide range of temperatures. At sea they have 

been found in water with temperatures as high as 30°C and 

as low as 14.7"C, which approaches the lower lethal ambient 

temperature of 13°C. as proposed by Stevens and Fry (1971). 

However in the short term and locally, skipjack tuna inhabit 

epipelagic waters characterized by narrow variations in oceanographic features. (Additional discussion in Section 3.6.) 

3.3 Adult phase 

3.31 Maximum size and longevity 

It is comnion practice to fit a von Bertalanffy or negative 

28 
Figure 26.-Skipjack tuna larvae. a) 3.1 mm TI ; h) 6.1 mm TI : c) 8.75 mm TI ; d) I0.Y mm 1 I (from Matwmoto 19581. 
\' _, . x 

.. 

.. 

. .. 

Figure 27.-Juvenile *hipjach luna. a) 14.5 mm TI : h) 21.0 mm n.: cl 27.0 mm TI.; d) 47.0 rnm TI. (from SEharfer and Marr 1948: Wade IY50; 

Marwrnolo 1958. 1961). 

30 
Table 8.--Sizr (fork length in centimeters) at axe and growth parameters of skipjack tuna from the Pacific. Indian. and Atlantic Oceans. 

Age in year5 Growth parameters 

Source Area 012 3 4 5 6 7 io K L,(cm) hlethou 

Pacific Ocean 

Aikawa (1937) East ofJapan <26 26 34 43 54 ~ - ~ - - - Vertehrae 

Aikawa and Kalo (1938) Palau <27 21 37 46 55 M 12 80 - - - Vertehrae 

Chi and Yang Taiwan <21 27 47 62 73 81 - - -0 016 0.302 103 6 Vertcbrac (1973) 

Yokota et ai. (I 961) SouthofJapan- - 37 52 64 73 85 - - - - IG.0 hlodal analpis 

Joceph and Calk,ns(lY69) East ofJapan 0.3 25 45 62 75 - - - - 0 19 141.8 hlodal anal!\i\: La\+a\aki'\ 

Kawasaki (196%) South ot Japan - 15 45 63 73 71 - - - - - hlodal anal)\!\ 

Chi and Yang(1973J Taibran <36 36 60 75 - - - - -0.016 0.432 103.8 Modal analyw 

Yorhida (1971) Central Pacific - 35 - - 

Uchiydmaand Struh5aker (1981) Hawan - 43 68 7Y x3 - - - - 

data 

Jo\eph andCalkinr(196Y) Eastern Paritic 0.3 30 50 62 70 - - - - 0.44 85.1 hlodal anal)\ii: SLhacter'5 

(19hla)data 

Joseph and Calkins(196Y) Eactcrn Pacific 0.3 37 60 76 X7 - - - - 0.41 107.5 hlodal anal>ai\ 

Joseph and Calkin\ (1969) Hauaii 0.3 44 65 74 7X - - - - 0.77 82.3 Tagging. corrected data from 

Joseph and Callinr (1969) Hawaii 0.3 40 63 75 82 - - - - 0.59 W.6 lagging. uncorrectcd data 

Skillman' Hawaii - 43 61 74 83 - - - - 0 39 101.1 Tagging; malesonly 

Skillman Hawaii - 43 61 73 80 - - - - 0.41 92 4 Tagging; male\ and female, 

Josephand Calkins(lY69) tactern Pacific 0.3 31 51 M 72 - - - - 0.431 88. I Tagging; areraged data 

Josephand Calkin,(l969) Eastern Pacific 0.3 41 59 67 70 - - - - 0.829 72.9 Tagging; non-a>craged data 

Uchiyamaand Struhsaker (1981) Hawall - 44 68 83 91 - - - -0.02 0.55 102.0 Orolitti 

Indian Ocean 

Shabotiniets (1968) Indian Ocean - - - 40-45 40-60 - - - - - - First dorsal \pine 

Atlantic Ocean 

Battc Western North - 41 49 57 64 - - - -4 329 0.195 79.6 Fir31 dorral\pinc (1972a) 

Sulu Sea 

(1963)data 

and Hawaii 

- _-- - 

Rothschild (lY67) 

from Rothschdd (1967) 

Atlantic 

'See text footnote io. 

Under abnormal condition>, such as when fish are caught 

and handled for experimental purposes, skipjack tuna are 

not quite as hardy as other tunas and not nearly as hardy as 

some other marine or freshwater species commonly used in 

laboratory experiments. Skipjack tuna can be used as an experimental animal but laboratory procedures must be designed 

with care. Mark-recapture experiments have been conducted 

successfully, but the animals must be handled quickly and 

with care. 

3.33 Competitors 

Fish that have been observed or taken with feeding aggregations of skipjack tuna (Gudger 1941; Waldron 1963) have 

been considered as competitors of skipjack tuna. These fish 

include yellowfin tuna; albacore; kawakawa, Eurhynnus 

affinis; frigate tuna; dolphin, Coryphaena hippurus; rainbow 

runner, Elagaiis bipinnulata; and whale shark, Rhineodon 

iypus. 

In addition, seabirds may be considered as competitors, 

for both they and skipjack tuna feed on the same aggregations of forage fishes at the sea surface. 

3.34 Predators 

Predators of either juvenile or adult skipjack tuna are 

listed in Table 9. 

3.35 Parasites, diseases, injuries, and abnormalities 

Parasites found in skipjack tuna have been listed by Waldron (1963), Silas (1967), and Silas and Ummerkutty (1967). 

The list by Waldron includes 26 trematodes, 4 cestodes, 2 

nematodes, 1 or more acanthocephalids, and IO copepods: 

that by Silas includes 33 trematodes and 1 cestode: and that 

by Silas and Ummerkutty includes 10 copepods. The last two 

studies also provide information such as host, locality, location of infestation, and pertinent remarks on nomenclature 

or occurrence. Chen and Yang (1973) listed one species each 

of trematode, cestode, and nematode, and two species of 

acanthocephalids in skipjack tuna from waters off Taiwan. 

We have not seen the report by Love and Moser (1977). however, Forsbergh (1980) stated that these authors listed 36 

species of trematodes, 1 specie of cestode, 12 species of 

copepods, and 6 species of nematodes and acanthocephalids. 

Cressy and Cressy (1980) presented a comprehensive account 

of copepods parasitic on tunas and tunalike fishes on a worldwide basis. Because of the unreliability of most past literature, they attempted to clarify the taxonomic status of both 

parasites and hosts. They resolved problems of synonymy of 

several species of copepods and listed five species parasitic on 

skipjack tuna which they had collected during the course of 

their study. 

The list of parasites (Table IO) was compiled from the 

papers examined above. 

31 
3.4 Nutrition arid grouth 

3.41 Feeding 

U’aldron and King (1963), E. Nakainura (1965), and Dragovich (1971) determined that feedins peaks in [lie early morning from about 0800 to 1200, drops to a low betueen 1300 

and 1600, and peaks again in the late afternoon, roughly 

from about 1600 to cunset (Fig. 28). E. Nakamura (1965) associaed this feeding pattern with the availability of food. He 

stated, “Zooplankter5 move do\bn\vard during the early daypattern. From studies of stomach content\. Yuen (1959), light hours, presumably to reach a preferred level of illumi- 

(1) Tiine 

Feeding b\ skipjack tuna follous a more or le,s regular 

32 
i (A] STOMACH CONTENTS OF HAWAIIAN SKIPJACK TUNA 

I n 

0 

;; (8) STOMACH CONTENTS OF HAWAIIAN 

II SKIPJACK TUNA I 1 1500 

0600 0800 1000 1200 1400 1600 1800 

0459 08159 Id59 1259 1459 1659 1059 

TIME OF DAY 

Figure 28.-Diurnal variation in volumes of stomach contents of Hawaiian 

skipjack tuna (panels A and B) and diurnal variation in vnlumes (panel C) 

and number of forage fish (panel d) in stomach5 of Marquesan skipjack 

tuna (data from Waldrun and King 1963 (A); Yuen 1959 (6); and E. Nakamura 1965 (C and D)). 

nation. Since they serve as food for the forage organisms, 

some of the latter, also seeking lower levels of illumination, 

will tend to move downward and thus minimize their availability to the surface-dwelling skipjack. The minima occur 

around noon, the period when sunlight penetration is greatest, when zooplankters are deepest, and when forage fish are 

least available to skipjack. During late daylight hours the 

zooplanters and forage fish begin their upward movement. 

Skipjack begin feeding heavily in the late afternoon hours 

before dark as food becomes more available.” He suggested 

also that the diurnal variation in the volumes of stomach 

contents may also reflect the effects of satiation. He reasoned 

that “Skipjack, starting the day with their stomachs empty, 

feed actively during the early morning hours, and food consumption reaches a peak sometime before noon. A period of 

satiety occurs midday while dige5tive processes reduce the 

stomach contents. As the ctomach empties, skipjack forage 

again, and the volume reaches a second peak prior to darkness.” That this may not always be the case is suggested in 

the hourly catches of skipjack tuna in the Hawaiian fishery 

r In W 

P 50 

G 

W 

25 

0 0700 0% II ~ 

I100 

TIME OF DAY 

Figure 29.-Skipjack tuna catches bi time of day for wveral Hawaiian hait 

boats during Ihe 1956 and 1957 fishinz 5easnns (data from Yuen 1959). 

(Fig. 29). If the fish were satiated near midday, then chumming with bait, which would involve more feeding, should 

not result in large catches; yet, the figure shows that the 

highest average catches occurred at about that time. Hence, 

E. Nakamura’s observation on the association of feeding 

with the availablility of food is preferable to the latter hypothesis based on volume of stomach content. 

Some seasonal variations in feeding are indicated in studies 

of stomach content volumes but for the most part, the variations reflect the seasonal changes in abundance of forage 

organisms. Waldron and King (1963) observed a noticeable 

decrease in the average volumes of food in stomachs collected 

in Hawaiian waters during midsummer as compared with 

stomachs collected in late spring and early fall. The trend of 

the average volume followed closely that of the forage fish 

component. E. Nakamura (1965) observed that though the 

occurrence of forage did not vary from season to season in 

skipjack tuna stomachs from the central equatorial Pacific, 

the proportions of the three major food categories did $0. 

Crustaceans were low in fall and highest in summer, and 

molluscs were lowest in spring and highest in fall. In waters 

around the Laccadive Islands, Raju (l964b) found that the 

total average forage volume showed a peak in early spring 

and a lesser peak in the fall. The trend closely followed that 

of crustaceans, which dominated the food category in the 

areas sampled. 

(2) Food selectivity 

It is reasonable to expect that food requirements and the 

capacity to satisfy the requirements, such as larger mouth 

and stomach, increase as the sire of fish increases. All studie\ 

of this relationship (Yuen 1959; Alcerson 1963; Waldron and 

King 1963; Raju 1964b; E. Nakamura 1965) agree that the 

average volume of food in skipjack tuna stomachs increa\e\ 

with sire of fish. Differences in rhe diet have also been found 

among the barious tizes of shipjack tuna. Generally, the 

smaller skipjack tuna rely mainly on crustaceans for food; 

33 
the larger skipjack tuna. on juve~lile and small fishes. Yuen 

(1959) obsened that the percent either by iolunie or occurrence of fish in stomach\ of skipjack tuna collected near 

Haizaii increased with an increase in rile. wtierear the percent of tiioIIuscs and crustaceans decrea\ed. Similar obsenatiniis were noted generally for $hipjack tuna in tlie h’larqiiesa\ 

Islands (E. Nahaniura 1965). eastern Pacific Ocean (AIberson 1963), cetitial Indian Ocean (Raju 1964h). and nestern 

Atlantic Ocean. off North Carolina, U.S.A. (Bat[\ 1972b). 

hlagnuson and Heit7 (1971), in their study of the gill raker 

apparatus among sconibridc and dolphins (Coryphaenidae), 

found that gill raher gap differed niarhedly among species 

and lengths of fish. A 50 cm \Lipjack tuna. a 30 cm ycllo\\- 

fin tuna, and a 10 cni striped bonito, Sordu orrenra/r.r, all had 

an e\timated gill raker gap of I mni: and con\ersely, the gaps 

of a 50 cm fi\h of each species \\ere estimated to be ca. 1.0. 

1.7, and 4.5 mm, respectively (Fig. .30). Comparing mean gill 

raker gap with the \tomacli contents of fishes based on the 

literature, they \honed that the percent bolumes of crustaceans 

were inversely related to mean gill raker gaps and concluded 

that gill raker gap \\as related functionally \+itti the quantity 

of smaller organisms in the stomachs. The presence of 

euphausids in stoniachs of skipjack tuna and their absence in 

yelloufin tuna were cited as evidence for selection of organisms a$ determined by the magnitude of the gill raker gaps. 

50 100 I50 

FORK LENGTH (cml 

(3) Distance from land 

In examining feeding with respect to distance from land, 

arbitrarily zoned at 16, 40, 80, 161, and 322 km (10, 25, 50, 

100, 200, and over 200 mi), Waldron and King (1963) found 

the average volume of stomach contents increased with distance from shore up to 50 mi and varied irregularly at 

greater distances. The trend was due largely to \ariation in 

the volume of fishes, wliicli were the dominant food element. 

Among the families of fish consumed, Scombridae, Mullidae, 

and Caraiisidne ucrc ~niportant nithin I6 hm: Scombridae 

was dominant betneen 40 and 80 km, followed by Carangidae, 

Gempylidae, and Holoccntridae; and Scombridae, Gempylidae, and Euocoetidae \\ere important in offshore areas betiveen 161 and 322 kin. E. Nakamura(lY65) found no relarion- 

,hip between stomach content \olunies and distance from land 

of skipjack tuna taken around the Marquesas Islands. He 

noted, ho\ve\er, that food items categorized as reef-originating forms decreaxd \ignificantly from inshore to off5hore 

along four 362 km (225 nii) \ur\ey tracks. The percentages of 

reef-originating forms in the inner, middle, and outer 121 km 

(57 mi) sectors \\ere 61.5, 39.9, and 8.8, rcspecti\elq. 

(4) Feeding hehabior 

Only a fen studies have heen made of the feeding he1iaLior 

of skipjack tuna both in the v,ild and in captivity, but thew 

have provided a wealth ol information. Nakaniura (I962), 

from his study of skipjack tuna in captivity, found that I) 

when fed hourly until satiated, skipjack tuna exhibited an 

initial high feeding followed by feeding of small ainounts 

throughout the day; 2) skipjack tuna had difficulty in seizing 

the food at night; 3) new fish introduced into tanhs learned 

to feed faster when evperienced fish Here already in the tanks; 

4) food particles \\ere alnays taken at the surface or middepth, never off the bottom. and rwimming \vas often accelerated prior to tahing the food; 5) they rejected \\hole shrimp 

or squid attcr ingesting a feu pieces, but when the rejected 

pieces were cut up into smaller pieces. the food was again 

accepted; and 6) capti\e skipjach tuna were never seen to 

prey upon small fish which \\ere present in the pool. 

Magnuwn (1969) studied other a\pects of feeding heha\ lor 

of skipjack tuna in capti\ity. In niie experiment, skipjach 

tuna were fed at intenals of I5 min throughout the da?. A 

period of intense feeding occurred berveen 0630 and 0830 

(Fig. 31). During this period the fish did not fill their stomachs to capacity the first or elen the second or third time 

they \\ere offered food, but filled their stomachs slo~l! o\eithe whole 2-11 period. The ma\iinurn capacity of the stomach 

\vas about 7u0 of the body ueight. but during the \\hole day 

the fish ate an equivalent of lSro of their body \\eight, and 

the stomachs were conipletel) empty by dawn. In another experiment, skipjack tuna that had been deprived of food for 

24 h \rere fed at a uniform ratc for 42 min from 0900 to 

0945. The stomach contents approached a niauimum after 

about 30 min (Fig. 32.1). The proportion of food panicles 

attacked decreased a\ the stomach\ filled (Fig. 32-11), but 

very little change \+as noted until the contents exceeded 50mo 

of the stomach’\ capacity. Then the likelihood that a skipjack tuna \\odd attack a food particle decreased rapidly. indicating that satiation had occurred. The fi\h responded to 

food particles (i.e.. food particles re\ponded to but not attacked) even \\hen tlie stomach was full (Fig. 32-111). 

Passage of food through the alimentary canal was estimated from 54 fish, 39 to 50 cm FL, that had been denied 

food during the previous 24 h. The\e fish were fed a\ many 

thawed smelt as they would eat and their stoniaclis and 111- 

testines were examined at intervals over a 24-h period. The 

skipjack tuna ate the equivalent of about 8.6Oh of their body 

34 
TIME OF MY I HOURS I 

Figure 3l.-Diel changes in the feeding of skipjack tuna in capti\it). 1. Quantity 

of food eaten. 11. Cumulative weight of food eaten. 111. Estimated weight of 

material in the slomach prior lo each meal. Food was offered 81 15-min intervals 

from WOO to 1845 (from Magnuson 1969). 

weight; about 10% of the initial quantity eaten had passed 

from the stomach each hour during the first 8 h; and the 

stomachs were eTsentially empty within 12 h after a meal. 

Magnuson (1969) noted also that the contents of the intestine 

increased rapidly during the first 3 h after the meal and the 

intestine was essentially empty about 14 h after the meal. 

Most waste material was evacuated from the intestine while 

the intestine was being filled from the emptying stomach. 

Defecation soon after eating was not surprising because of 

the short intestine in skipjack tuna. 

Another aspect of feeding behavior is the appearance of 

banded color patterns on the skipjack tuna. Strasburg and 

Marr (1961) reported the appearance of such coloration on 

skipjack tuna weighing from 1 to over 14 kg, but the bands 

or bars were not observed on all members of a school. As 

they explain it, the bands are formed when the dark horizontal stripes are interrupted by light vertical bars. The transitory 

barred or banded appearance is likely under various conditions and is produced presumably by the contraction of 

melanophores. The vertical bars were noticed from a ship's 

subsurface viewing chamber during the chumming of fish 

schools. Nakamura (1962) confirmed the relation of the appearance of the bars to feeding among skipjack tuna held in 

captivity. In his feeding experiments he noted that, in addition to excited swimming behavior during feeding, the skipTlME (MINUTES) 

____- 0 25 5@ 75 I03 

FULLNESS OF STOMACH I% CAPACITY1 

Figure 32.-Changes in feeding behavior as skipjack tuna eats a meal. 1. &timated weight of material in stomach. ll. Proportion of food panicles attacked. 

111. Proportion of particles eliciting a response. Means and ranges shown in 

each plot. Proportion of stomach filled, shown along the time axis, was estimated from panel I (from Magnuson 1969). 

jack tuna exhibited such bars or bands, which disappeared 

when the fish were no longer interested in food. 

Similar markings have been reported for Pacific bonito, 

Sardu chiliensis (Magnuson and Prescott 1966), kawakawa 

(Nakamura and Magnuson 1965), and dolphin (Strasburg 

and Marr 1961). 

3.42 Food 

Various authors have examined the food of skipjack tuna 

(Kishinouye 1923; Suyehiro 1938; Welsh 1949; Ronquillo 

1954; Hotta and Ogawa 1955; Postel 1955; Tester and Nakamura 1957; Yuen 1959; Alverson 1963; Waldron and King 

1963; Raju 1964b; Thomas 1964; E. Nakamura 1965; Sund 

and Richards 1967; Dragovich 1971; Batts 1972b; Roberts 

1972). In addition, there are other reports with brief mention 

of the food of skipjack tuna. The most comprehensive studies 

to date are those by Hotta and Ogawa (1955), Alverson 

(1963). Waldron and King (1963). and E. Nakamura (1965) 

in the Pacific Ocean; Dragovich (1971) and Batts (1972b) in 

the Atlantic Ocean; and Raju (1964b) in the Indian Ocean. 

The major food items fall into three groups: fishes, crustaceans, and molluscs. Although fishes comprise the most 

important source of food for skipjack tuna in most of the 

35 
ocean\, variatinn\ in the order of importance (it thc major 

proirp\ 01 tood orpanisins are tound iii different areas and 

\ca\on\ (Table 1 I). In the \re\terii and central Pacific and 

\\e\tern Atlantic Ocean$. t'islw (49-950s of total \olurne) 

c~nipri\cd tlie mo\t importair group, follo\red h) niollu\c\ 

(4-5OO'u). and ci titracean\ (I -23"in) (Wcl\h 1949: Konqtiillo 

1954; Maldrou wd Kiug 1963: I)rago\ich 1971: Halts 1972b). 

M'aldron arid King (1963) I-cported diftercnce\ in the I'ood 

compo\ition of \kipiach tuna by i\land area\ in the central 

Pacific: "(I) Skipjack from the Hawaiian area depended on 

firlie\ to a grcater e\tciit than thobe froni tlic Plioeni\ or 

1 iiie I\lands; (2) \hipjach tioni the I'hoeni\ I\land area utili& ~iioIIu\c\. cru~t;iccaiis. and fi\hcr cquall! a\ source\ of 

food; and (3) \hipjach from the 1 inc I\land\ xrea depended 

on fishes to a le\\er degree rhau those troiri the other t\\o 

area\ arid to a greater cxteiit on \quid\ arid cru\~act'ans." 111 

the ea\terri side\ of [lie Pacitic and Atlantic Ocean\. cru\titceaiis \cere the iiiaiii wurcc ot food. The! compri\ed 50-6200 

of the tood. fish comprl\ed 33-37"o. and nlollu\c\, 4-13°'c~ 

(Schaeter 1960: Al\ei-\on 1963: Drago\ich 1971). 111 the Indian Ocean around [lie Laccadi\e I\lands. crii\tacean\ \\ere 

the mo\t important food group. They made up 5gVn 01 the 

total \olurne, mollusc\ were 22"'u, arid h\li \\ere IOO'[I (Raju 

1964b). At nearby Minicoy I\land fi\he\ tornied 4X"o of the 

food, crustaceans 47Oi1, and n~i\cellane~~~us item\, rnoctl) 

nioIIuscs, 5O;n (Thomas 1964). 

The food of skipjach ttiiia in all thc major oceans includt.s 

a \vide \arict) of organi\nis. representing I I in\ertebrate 

orders and 80 or iiiore lisli tamilies (?able 12). Pa\t \ludic\ 

sliou that of the iri\errebratcs, t\\o group\ of arthiopods 

(decapods and \tomatopods) and t\\o groups ot cephalopod\ 

(octopods and squids) were found in \tomachi from nearl) all 

localities. T~K fith families represented in ~tomach content5 

vary considerably. with \ample\ taheri in the Pacific Ocean 

ha\ing the greatest nurnber by tar. Hotta and 0ga.a (1955) 

li5tt.d 48 families, U'aldroii and King (1963) listed 42, and 

both Konquillo (1954) and t. Nahamtira (1965) li\ted 33. In 

the Atlantic Ocean. DragoLich (1970) listed 28 fi\h families in 

the western and 21 in the eastern \ectors. Raju (1964b) listed 

only 10 fish families in the stomachs of skipjack tuna collected from inshore \balers (about 8 hm) around Laccadive 

Islands and Thoma\ (1964) listed I I fish families from Alinicoy Island in the Indian Ocean. Of the li,h lamilicc represented in the diet of the skipjach tuna. only tiw, Carangidae 

and Rali\tidac, \\ere common to all area\ o1'all oceans. Others 

that were coninion to most of the areas \\ere Euocoetidae, 

Slngnathidae, Holoccntridae, Bramidae, Chaetodontidae, 

Gempylidae, Sconibridae (iiicluding tlie tuna\), Ostraciidae, 

and Tetraodontidae. 

Tlic presence of \combrid\ in the \to~iiaclis of skipjack 

tuna de.;erve\ special consideration, \ince this group of fish 

occasionally constilute a major portion of the \kipjack tuiia 

diet. Data from Waldron and Kiiig (1963) indicate that \coinbrids occurred in 10.6"h of 707 \hipjach tima stomaclls examined and that this group comprised 25.O"i, of the total 

\olunie, the highe\t of all fish familie\ arid larger thaii the 

entire mullusc group. E. Nahamara (19653 reported the occurrence of juvenilc tuna in 24.8 to 4J.li1'u of 603 >tomactis 

e\arnined from fi\c cruise\ around tlie 3larquesas and Tuanioiu Islands. This group ranhed the highest anicing all fish 

familie5 on four crui\e\ and fifth on one cruise. Dragovich 

(1971) also indicated high (25.OU'o) occurrence of tcombrids 

in 369 shipjach tuna stomach\ Iron1 the Caribbean Sea and 

adjacent uaters. Thi\ group rariked second only to the large 

unidentified fish group. 

The wide variety of food organism\ in the stomach content\ and the \ariation\ in the importance of the major food 

groups have led to the conclu\ion that skipjack tuna are opportunistic teeders and will prey upon any forage organisms 

that are aiailable to rhein. Cannibali\m among the skipjack 

tuna lend5 additional support to this concIu5ion. In the central Pacific Ocean, E. Nakamura (1965) reported juvenile 

skipjack tuna in l2.4U:o of the skip.iach tuna stomachs containing food. He alto indicated that 7.80'0 ot 707 stomachs 

collected by U'aldron arid King (1963) contained juvenile 

skipjack tuna. Conand and Argue (1980) reported that 3.3OIo 

of 5,956 \tomachs of \kipjack tuna from the western Pacific 

Ocean (lat. 25"N-42"S, long. 134"E-I40'h') contained juvenile skipjach tuna. In the Atlantic Ocean, Suarer Caabro arid 

Duarte-Bello (1961) and Drago\icIi (1971) al\o repoi-ted the 

occurrence of ju~enile skipjach in \tomachr of skipjack tiiiia. 

3.43 Groirth rate 

Various methods have been used to determine the grow111 

of skipjack tuna and other tunas, including analyses of growth 

36 
37 

I. . . .. 
Table U.-< bnrinrred. 

Arlantic Indian 

Pacific Ocean Ocean Ocean 

38 
marks on hard parts such as vertebrae (Aikawa 1937; Aikawa 

and Kato 1938; Yokota et ai. 1961; Chi and Yang 1973). first 

dorsal spine (Shabotiniets 1968; Batts 1972a). and otoliths 

(Wild and Foreman 1980; Uchiyama and Struhsaker 1981); 

temporal progression of length-frequency modes (Brock 

1954; Kawasaki 1955a, b, 1963, 1965a; Schaefer 1961a; 

Yokota et al. 1961; Joseph and Calkins 1969; Yoshida 1971; 

Chi and Yang 1973); and data from tagged fish (Schaefer et 

al. 1961; Rothschild 1967; Joseph and Calkins 1969; Josse et 

al. 1979). Reviews of some of these studies were given by 

Shomura (1966a). Rothschild (1967), Joseph and Calkins 

(1969), Chi and Yang (1973). and Josse et al. (1979). A summary of the results of these studies is presented in Table 8 

and representative growth curves are shown in Figures 33-35. 

Josse et al. (1979) reviewed the methods and results of 

most of the studies made to date and have concluded that 

counting seasonal growth marks on vertebrae, scales, and 

dorsal spines and following modal progressions of length 

frequencies were least reliable, that counting daily increments 

on otoliths was more reliable, and that measuring growth 

between tagging and recapture was the most reliable. Their 

low regard for counting growth marks and for following 

modal progressions stemmed from the failure of investigators 

to show irrefutably the periodicities in the appearance of 

growth rings and from the results of their ohn analysis of 

length-frequency data. They found that clear progressions of 

modes for more than a few months, other than in exceptional 

instances, were difficult to demonstrate and that the apparent 

modal progression could give in a single region, in different 

years, growths which were rapid, slow, nil, and even negative. They pointed out, first, the inadequacy of determining 

growth from otolith increments to resolve the problem of 

AGE IN MONTHS 

Figure 33.-Growth estimates of weslern Pacific skipjack luna. I) Aikawa 

and Kato (1938). vertebral rings: 2) Kawasaki (1963), modal progression 

(Joseph and Calkins 1969; 3) Yokola el al. (1961). modal progression: 4) 

Chi and Yang (1973). bertebrae; and 5) Chi and Yang (1973). modal 

progression. 

age, should the skipjack tuna pass through phases of no 

growth during the breeding period, and second, the uncertainty in reading daily marks close to the nucleus (Lewis"). 

'I.euic, A. D. 1976. Tile relevance 01 data iolircted 111 P,ipun Ned, Giiinea to 

skipjack population \iudte\ in the uc\tern Pacihi. I)ocunicnt I066/76. S p IW 

partrnent of Primary Industry. I'apua Neu Guinea. blinj\ir! 01 Apricuitiirr. 

Fore5ts. and Fishcrier. Fisheries Ui\i\ion. Su\a F ~JI 

Figure 34.-Gro*lh eslimales of cenlrdl Pacific skipjack luna. I) Lchijama and 

5lruhsaker (1981). otolith incremenls: 2) Skillman, lap. relurns. males onl? (see 

text foolnote IO): 3) Rothschild (1967). tag returns, uncorrected data; 4) Skillman. 

tag relurnc. males and females (s?e lex1 footnote IO); 5) Brock (1954). modal 

progression; and 6) Rolhschild (1%7), lag relums. correcled data (from I:chi)ama 

and Slruhsaker 1981). 

E 

70 

60 - 

- 50 

0 

W 

40 

30 

20 

10 

r 

"0 4 8 12 16 20 24 28 32 36 40 44 48 

I 

AGE IN MONTHS 

Figure 35.--Crowlh eslimales of easlern Pacifir skipjack luna. I) Joseph 

and Calkins (1969), tag relurns. averaged dala: 2) Joseph and Calkin, 

(1969). tag returns. nonaveraged dala; 3) Schaefer (1961a). modal progrersion; and 4) Joseph and Calkins (1%9). modal progression (from Joseph 

and Calkins 1969). 

39 
While Josse et al. (1979) considered tagging data to be the 

most reliable, the pos5ibility of an interruption in gronth 

caused b! tagging itself has yet to be determined. \+'e cite as 

an example the nark by \Vild and Foreman (1980). \rho inve\tigated the relation of otolith increment, and time for 

tagged and recaptured skipjack and )ello\\fin tuna\ inarhcd 

cline. The) observed that tlie number of increrestimated time by appro\irnatel> 24oh during 

gro\\th from 42 to 64 cni o\er a period of up to 249 d. They 

further reported that the yo\\th ratc' of \hipjack tuna, estimated froni the Icngth at recapture and linear change in 

otolith dimension of tagged fish injected \\it11 tctrac\clinc 

wa\ I. 15 cm/ino. The gronth rate \\as comparable \\it11 that 

obtained fiom earlier ragging studies by Iowph and Calkins 

(1969) in the ca\tern Pacific, but much le\\ tliiui that estimated from the latter's rt'\ults bawd on iiiodal progres\ion. 

The agrecment of the gronth rate\ I'rom otolith iiicr~iiieiitr 

after tetracqcline treatment and froin ~ncrcniental change in 

si7c indicate\ that both methods are cqtially good at niea\uring gro\rili rate of tagged fish but proiide no inforinatioii on 

the effect of tagsing itself. 

The matter of reliability in the length of fish at relcase 

sliould be considered also in tagging data. 111 iiio\t pa\[ Fludies 

the aicragc length of fish from the same <ctiool \\as assigned 

to the tagged fish or tagged fith \%ere incastired io the nearesi 

5 cni. Onlq reccntl) in Papua Ne\\ Guinea (.lo\\e et al. 1979) 

Pacif~c (IA ITC") ha\e tagged fish been 

elq at time of release, i.e., to tlic nearesi 

centimeter. Until similar data are obtained from other areas 

and the adLerse effects of handling fish during tagging ha\e 

been clarified, the age and growth ol skipjack tuna nitist be 

con5idered only at appro~imations. 

Gro\%th estimates of skipjack tuna in tlie \+estern North 

Pacific have been determined only from ana1)sis of Lertcbrae 

and modal progreuion. Aikawa (1937) and Aikaira and Kato 

(1938) made no serious attempl to \alidate growth rings as 

annual markh. Their lengths at age appear to be greatl) underestimated. Chi and Yang (1973). u\ing both methods, observed that two rings were formed annually on Lertebrae of 

skipjack tuna taken from waters around 'Taiwan and that 

growth estimates from modal progress~on were higher than 

those obtained from growth marks on \ertebrae (Table 8; 

Fig. 33). The growth of skipjack tuna, based on modal progression, \%as 24 cm from age 12 to 24 mo and 15 cm from 

age 24 to 36 mo. Growth estimates from modal analysis by 

Yokota et al. (1961) were lower than those obtained by Chi 

and Yang (1973) and comparable with the latter's estimates 

from vertebral analysis. 

Growth e\timates of skipjack tuna in the central and eastern Pacific have been obtained from modal progression, otolith increments, and tagging (Figs. 34, 35). Growth estimated 

from modal progresssion in both areas \vas comparable with 

that obtained by Chi and Yang (1973) in the western North 

Pacific and generally higher than that estimated from tagging. 

Growth between ages 12-24 and 24-36 rno were 25 and 1 I cm, 

respectively, in the central Pacific (Uchiyama and Struhsaker 

1981) and 23 and 16 cm, respectibely, in the eastern Pacific 

(Joseph and Calkins 1969). 

Uchiyarna and Struhsaker (1981) obtained growth estimates 

from otolith increments of skipjack tuna in Hawaiian waters. 

Their estimate5 varied some\\ ha1 from those obtained from 

modal progressions iii tlie same area and in the eastern and 

\\estern Pacific (Joseph and Calkins 1969; Chi and Yang 

1973). Although gro\rth \\as similar for fish increasing in age 

from 12 to 24 nio (24 cni), 11 \bas much higher for fish increating in age from 24 to 36 mo (25 cm). 

Gronth estimates derived from tagging she\+ little or no 

difference for fish in the central and eastern Pacific (Table 8). 

Gro\\th of fish increasing in age from 12 to 24 mo and 24 to 

36 mo \\ere 18-23 and 9-13 cm, respectively, in Hawaiian 

\\aters (Joseph and Catkin, 1969, Skillman'") and 18-20 and 

8-13 cni, respecti\el\. in the eastern Pacific (Joseph and 

Calhtn5 1969). 

Tagging studies in Papua Ne\+ Guinea by Kearney (1978) 

ha\e indicated that 5kipjack tuna in that rcgion grew at an 

alerage anntial rate of 7 cm. The estimate \vas derived from 

fish betueen 2 and 6 hg (ca. 38.0-64.4 cm) uhich uere at 

liberty for tip to 789 d. Kearriey stated "The available tagging 

information . . . lea\es little doubt that the growth rate and 

ina~iniuni sire of \kipjack in the we\rern subpopulation are 

much loner than that for the niore easterlq \ubpopulations." 

Subsequent aping srudic, from otolith\ (Uchi>ama and Struhsaker 1981; Fig. 36) and tagging by JO~X ei al. (1979) and the 

I4TTC (footnote 9) have confirmed this. Josse et al. (1979) 

anal\zed published and unpuhlislied tagging data from the 

eastern Pacific, Ha\+aii. and Papua New Guinea and determined that the Io\rest gronth rates were observed in skipjack 

tuna from Papua Ne\\ Guinea and the highest from Ha\baii. 

The IATTC (footnote 9) compared ne\\ tagging data (fish 

length ai release measured to nearest centimeter) from the 

eastern Pacific \+ith similar data from Papua New Guinea 

40 
and found that skipjack tuna in the eastern Pacific grm at a 

significantly faster rate than those in the Papua New Guinea 

region. 

Studies of skipjack tuna age and growth in the Indian and 

Atlantic Oceans have been minimal. Shabotiniets (1968) 

determined the size at age of skipjack tuna in the Indian 

Ocean from growth marks in the first spine of the first dorsal 

fin (Table 8). Since he did not validate these marks as annuli, 

the size at age he obtained thus appears questionable. Batts 

(1972a) also made a study of the growth and age of skipjack 

tuna from growth marks in the first dorsal spine of fish 

from the western Atlantic Ocean. He, too, failed to show 

that the growth marks were annuli. His sires at ages 1 through 

4 essentially represent a straight line and differ greatly from 

that obtained by others using data on modal progression or 

tag returns (Table 8). 

3.5 Behavior 

See Section 3.41(4) for feeding behabior; Section 3.13 for 

reproductive behavior. 

3.51 Migrations and local movement5 

Migrations of skipjack tuna have been determined for certain areas and hypothesized for others through various 

methods, such as following the movement through a fishery of 

fish groups identified by size or age, from tagging, population genetic studier, and from tuna longline catch data. The 

most direct evidence has been from tagging data, but there is 

one weakness in this method-it only gives the net movement 

from point of release to point of recapture and does not 

show the actual path taken by the fish in the interim. 

In the central-eastern Pacific, Rothschild (1965), on the 

basis of available evidence on lar\al distribution. gonad 

indices, size distributions, tag recoveries, catch predictions, 

and immunogenetic studies, hypothesired that a laree portion 

of the skipjack tuna taken in the eastern Pacific Ocean originate in the equatorial PaciTic; that a large component of prerecruits (< 35 cm) moves eactward and is \plit into a northern 

group that enters the Baja California fishery area and a 

southern group that enter5 the Central and South American 

fishery areas; and that the skipjack tuna remain in the eastern Pacific for several month\ before they move offshore 

into equatorial waters to spawn (Fig. 37). 

A tagging study by Fink and Bayliff (1970) showed detailed 

movements of skipjack tuna in the eastern Pacific fi\heries 

(Fie. 38). They reported that there appear to he Inn maiii 

CENTRAL AND 

Figure 37.-H)pothe\ired origin and migration of skipjack tuna between the 

central equatorial Pacific and the northern (Mexican) and wuthern (Crntrdl and 

South Ameriral fisheries of the eastern Pacific Ocean (from Rothachild 196s). 

Figure %.--Inshore migration of skipjack tuna of the A) northern fisher) group nnd 8) southern fisher! group based on tagging data; numbem refer 

to months (from Fink and Ba)liff 1970). 

41 
groups of skipjack tuna in the eastern Pacific; that recruits to 

the northern fishery first enter the fishery in the Revillagigedo 

Islands in April, migrate north along the Baja California 

coast during spring and summer, return south in the fall, and 

then migrate to the central Pacific; and that recruits to the 

southern fishery enter the fishing area in or near the Panama 

Bight and migrate both northwest to Central America and 

south to the Gulf of Guayaquil. Although a few fish tagged 

off Ecuador in the Gulf of Guayaquil were recovered off 

Baja California, they reported little interchange of skipjack 

tuna between the northern and southern fisheries. 

Williams (1972) has extended Rothschild's hypotheses by 

proposing three migration models for the recruitment of 

skipjack tuna into the eastern Pacific fisheries: An active 

migration model (Fig. 39A), where the fish migrate eastward 

actively along zonal productivity bands at the northern and 

southern edges of the North Equatorial Countercurrent 

(NECC); a passive migration model (Fig. 39B), where the 

fish are passively carried eastward in the equatorial countercurrents; and a gyral migration model (Fig. 39C), which involves both active and passive migration. In the last model 

Williams proposed that the fish of the northern group move 

counterclockwise around a zonally narrow' equatorial gyre 

consisting of the NECC and the North Equatorial Current, 

with the western limit some distance west of long. 130"W; 

and that fish of the southern group move eastward in the 

NECC and return to the central Pacific in the South Equatorial Current. 

Recoveries in Hawaii of skipjack tuna tagged in the eastern 

Pacific showed the capability of the skipjack tuna to negotiate long migrations. Seckel (1972) developed a numerical 

drift simulation model based on geostrophic and wind-driven 

currents as a potential mode of skipjack tuna migration from 

the eastern Pacific northern fishery into Hawaiian waters. He 

determined that fish could drift from an area between lat. 

10' and 20"N at long. 120"W to an area between lat. 19" 

and 22"N at long. 155"W in 21 to 23 mo (Fig. 40). Although 

the time intervals are within the range of values for skipjack 

tuna tagged in the eastern Pacific and recaptured in Hawaii, 

they are, by far, longer than most (Table 13). 

In the northwestern Pacific for fish caught in the Japanese 

coastal fisheries, lmamura (1949) noted that skipjack tuna 

migrated annually from the southern seas northward in the 

warm Kuroshio in the spring and summer and returned south 

as the cold Oyashio became stronger in the fall. Kawasaki 

(1955a, b) proposed that there are two groups of skipjack 

tuna in the southwestern Japanese waters which migrate 

north from the Ryukyu Islands to southern Japan and back, 

Figure 39A.-Active migration model: A) routes of )oung skipjack tuna into the southern fisher) and B) routes of young skipjack tuna into the 

northern fisher). NEC = Norlh Fquatorial Current: \ECC = Norlh Equatorial Countercurrent: SEC = South F4uatorial Current; SECC = South 

Equatorial Countercurrent. (From Williams 1972.) 

, , , , -r-r---- r -7 

, /.h' 'j i 

t ". 

Figure 39B.-Passive migration model: A) routes of )oung skipjack tuna into the southern fisher) and B) route3 of >omg %kipjack tuna into the 

nonhern fisher). See Figure 39A for abbreviations. (From Williams 1972.) 

42 
, .. A . ,"..I.* \.,". 

I.. 

I 

- ,...~, . . * -* ". .. , .I 

>- 

%_.Id. I. 

.I/. .e. .I , . . .. , .. ~ 

-. 

L__ .~ 

."/. ,. .O, ... .. _d 

I.. 

...1". ". , 

I. 

Figurr U).--lacation\ of drifting ohjrctr in June of Ihr third modd year that wrrc introduced in each of the pre\iour 30 mo a1 long. 120-\\. I>d*hrd 

linr, indirsle mort wrrlrrl) location rrachrd h? object\ introduced wrr? quarlrr (frnm Srckrl 1972). 

43 
tuna in the Pacific. a \\c\tern and an ea\tern central subpopulation, arid t\\o groups of skipjack tuna \\ithin the \\estern Pacific uubpopulation. an A group, deriicd from spawning in the northern winter, and a B group, derited from 

\pa\\ninS in the northern \unimer. The migrator! paths of 

Ihese group\ (Fig. 5) are clearly restricted to the \\ectern 

Pacific west of long. 170"E and the migratory paths in Japaiicse coa\tal waters agree w~th the path\ proposed by Ka\\a- 

,ahi (1955a. b). 

In the western equatorial Pacific intenri\e tagging of skipjach tuna has been carried out in recent years by [lie Tohoku 

KIKL (Japan), the Department of Agriculture, Stock and 

I.isheries (Papua Ne\\ Guinea), and the Far Seas Fisheries 

Research Laboratory (Japan). Preliminary analkses of tag iecoveries have revealed considerable pole\rat d niigration of 

shipjack tuna wtrhin the boundaries of long. 130' and 160'E 

\\ith none being captured farther east than long. 173"E 

(Kearney 1975). F;or the Paptta Ne\\ Guinea area, Lewi\ 

(19x0) reported that the c\\ential feartire\ of migration \\ere 

the interchange of shipjack tuna betueen the Bismarck and 

Solomon Sea\ during Nobember-April and [lie northerly 

nio\ement from the Biwiarck Sea in the Iuly-October period. 

Long-term reco\erie\ (fish 60-390 d at large) in other routhern and \\e\tern Pacific area\ indicate straight-line migrations 

in ewes\ ot 1,000 mi (South Pacific Conimt\\ion 1981). Fish 

ragged off Ne\\ South \'ale\, Australia, \\ere reco\eretl in 

the Solomon, Samoa. and Society Islands, Kiribati, arid 

northern Ne\\ Zealand; fish tagged off northern Ne\\ Zealand \\ere recovered in West Samoa, Losalt), and Society 

Island\; fish tagged off Tu\alu \\ere recovered in the north- 

\\?stern Haaaiian I\land\ and near Palmyra Island; fish tagged north of Ne\\ Ireland were recovered in Kiribati and the 

long I I? 45 '\\ 

long. I5Y-l:'t 

limp Ill 01 w 

I dl II 5"\, 

la1 21 lh'h, 

1 a. 25 45 I\. 

LJI 21 I6 N. 

tat 2PI17'h. 

la1 Zl-lh". 

la ?I"lh N. 

lonp. 112'47 '\\ 

long III'IW \\ 

long. I I I 'I6 '\% 

lonp. I I I '04 'M 

Ion$. I I I =w 'H 

Marshall Islands; and fish tagged off Volcano Island were recovered in Kiribati (South Pacific Commission 1981, Fig. 6). 

R'hile straight lines connecting tag release ard recovery 

points do not necessarily reprewnt the actual paths of the 

fish, the net movement <,f fish from the south equatorial 

Pacific to the North Pacific and vice versa is significant. 

hligrations of hhipjack tuna ha\e been examined on a 

wider scope in the Pacific. Knsahara (1968) cited Naganurna," 

who postulated teberal migratory routes of skipjack tuna 

(Fig. 41), based on the catch as well as monthly length frekigore Il.-Origin and migration route, of \kipjab tuna po\lulatrd from luna 

longline data (from Kmaharn 1Y6X). 
quencies of skipjack tuna iahen in the titila lonpline fi\her\. 

Matsunloto (1974, 1975). on the basis of tuna longline catch 

data for four siiccessive years (1964-67), proposed that there 

are a number of semi-independent stocks of skipjack tuna in 

the Pacific Ocean and that the\e Ftockc mipate in roughl! 

circular paths (Fig. 42). The migrator! pathr \\ere siio\\n to 

follow a clockwise direction in the central and nertern North 

Pacific, and counterclockwise in the eactern North Pacific 

and in the Southern Hemisphere, corresponding \\it11 the 

flow of surface currents of the major nater masses. By following the movement of high catch rate cells by quarter\ mer 

several successive years, Matsumoto (1975, Fig. 12) has \ho\\n 

the probable routes by uhich fish can rnobe from one area to 

the next and from the eastern to the v.e<tern Pacific in the 

Northern Hemisphere and vice versa in the Southern Hemisphere within a period of 2 or more years. 

Long distance migations recorded for skipjack tuna in the 

Pacific Ocean are shown in Figure 43. The migrations are 

Figure 42.--Mo~emenl of the \ahus geographic \lock* of shipjack tuna in the 

Pncific Ocean. The numeral) along lhe migrator? roule, represenl quarlm and 

Iocitlions of high catch-per-effort cell\ of skipjack luna taken h) Ihe Japanese 

longline fishery. 1964-67. Slock de4gnalions are shown in parenlhece\ (from 

Malsumoto 1974). 

16W, 4 

I ~- 

. -DAY TRACKING 

, t_ NIGHT TRACKING 

I HOUR INTERVALS 

\ho\vn a\ \traipht-line nio\enicnt\ bct\\cen tapping and ICCIII>- 

tiire sites, and do not necessaril! reflect the actual pit[lis 

taken by the fish in mi gratin^ from one point to another, nor 

the time required to negtiate the distance\. Neiertheless. 

mo5t of the midoceanic mo\ernents reflect the routes prw 

pohed b) Ivlatcunioto (1975). 

Local mo\emcnrs of skipjach tuna Iiri\t. been \ttidietl b! 

tracking fish tapped nith ultrasonic transmitters. A small 

skipjack tuna (44 cni) trached for X d in Ha\\aiinn \\aters h! 

Yuen (1970), made niphtl) journey\ of 25-106 kni a\\ay Iron1 

a bank and, nith one exception. returned to the bnnh each 

morning (Fig. 44A, B. C). On the one exceptton. the ti\h \\a\ 

9 km short of the bank; ho\\ecer. it remained there all day 

and returned to the banh on the follo\\ing niornins (Fis. 

44D, E). The mo\ement of the tracked fish \\a\ typical of 

fish which frequented the banks for feeding. Becau\e of the 

repeated returns to the same spot and their consistent arri\dl 

times, Yuen suggested that skipjack tuna can nailgate and 

that they are able to sense the passage of time. 

In another study, one small (ca. 45 cm) and three large (ca. 

70 cm) skipjack tuna \\ere tagged with ultrasonic transmitters 

d: 

160.1. B 

16C01. ID 

. .6J 21'. KbULA 

BANK 4 ' 'S 

I 
3.5 2 sell 00 I I11 g 

46 
Uda (1933) stated that skipjack tuna congregate around 

whale sharks (Rhinodontidae) in fear of spearfish. Tominaga 

(1957) also reported that skipjack tuna associate with basking 

sharks (Cetorhinidae) for protection from marlin. The latter's observations of this relationship indicate that the shark 

is the likely leader and the skipjack tuna the follo\\ers, and 

that the attachment of the skipjack tuna to the shark is quite 

strong. The skipjack tuna "swim in front, behind, to the 

sides, above, and below the shark. When attacked by a spearfish, the skipjack close in toward any part of the shark'r 

body. Upon chumming such a school, the skipjack will come 

toward the bait, but if the shark moves auay from the ship, 

the skipjack will proceed after the shark immediately." 

Association of skipjack tuna with whales may also be for 

protection, but is not as close as that with sharks (Tominaga 

1957). According to Tominaga, the association usually occurs 

with one or two whales, seldom with a herd. When attacked 

by marlin, the skipjack tuna merely comes in the vicinity of 

the whale. In this association, the skipjack tuna always precede the whale and may abandon the whale while the school 

is being fished. 

Skipjack tuna are found associated also with anchored 

floating objects. Fishing for tunas around such objects (bamboo rafts) has been done for many years in the Philippines, 

where tuna were caught by hook and line (Murdy 1980). The 

introduction of purse seiners in the early 1960's (Aprieto 

1981) and the rapid development of techniques for purse 

seining around the rafts have resulted in a highly successful 

commercial fishery for tunas (Matsumoto"). Recent experiments with fish aggregating devices constructed of two 55-gal 

steel drums and moored in depths of 400-2,200 m in waters 

around the Hawaiian Islands have been successful in attracting skipjack tuna and other tuna schools in sufficient quantities for commercial pole-and-line fishing (Matsumoto et al. 

1981). Similar experiments are being conducted by several 

island states in the S 11th Pacific and in the Indian Ocean. 

(3) Distribution of types of schools 

The distribution of the various types of schools differ by 

area and time. In the northwestern Pacific Ocean, off northeastern Japan, Uda (1933) reported that skipjack tuna schools 

associated with birds, whales, or logs generally appeared in 

greatest number in the main Kuroshio system, while schools 

associated with sharks appeared for the most part in warm 

water cells where the Kuroshio and Oyashio impinged upon 

each other. He also noted that schools associated with whales 

were seldom found in the southern portion of the Kuroshio. 

Kimura (1954), in a more extensive study off northeastern 

Japan, obtained additional details: I) Schools unaccompanied 

by birds were more often found at the boundaries of the 

Kuroshio and Oyashio, 2) schools associated ~ith sharks also 

occurred in warmer waters slightly south of the two current 

boundaries, and 3) schools associated with birds were seldom 

seen in offshore waters (ca. 800 km). He further determined 

the percent occurrence of the various types of schools and 

plotted the average occurrence of the three mmt common 

types of schools in the area (Fig. 45). Schools associated \\ith 

- 

tipurr45.1Ratiir of delerlicin of sch~wIs with bird\ (Hb), school, wilhout birds (Rg). and schools wiih sharks (Es) lo total schools in each IO-d 

period (from Kimura 1954). 

birds were abundant in hlay and June but \\ere rclatively 

scarce during the peak of the seawn, July to September. 

Schools associated with sharks first appeared in early June, 

were predominant in July, and dimppeared in October. 

Schools unaccompanied by birds \\ere abundant in August 

and September and predominant in October. 

In more southern areas west of the Bonin Islands (lat. 19"- 

24"N, long. 136"-147"E), schools associated with whale 

sharks have been reported in October and schools associated 

with logs in February (Tohoku Regional Fisheries Re\earch 

Laboratory undated a). Further south, in the area ]at. 2 "-SON, 

long. 134"-150"E, schools associared uith logs and bird 

flocks regularly occurred from December through February. 

Lastly, in waters south of the Equator, around Papua Ne% 

Guinea and the Solomon Islands, schools were generally associated with bird flocks, with few instances of association 

with sharks or logs (Tohoku Regional Fisheries Research 

Laboratory undated b, d). Living Marine Resources (1977), 

however, reported that numerous logs of all sizes from tuigs 

to entire trees were sighted and fished by three U.S. purse 

seiners in August-October 1976 in the area northeast of 

Papua ,New Guinea (lat. 3ON-45, long. 138°-154"E) in the 

South Equatorial Current. More specifically, T. K. Kazama'l 

reported few log sighting in the area lat. 8"-3"N, long. 

147"E, but that sighting, increased conyiderably to the south 

(lat. 2"-4"S, long. 138"-147"E), numbering several hundred 

per day with the greatest single concentration of logs extending over an area approximately 15.5 m (SO ft) Nide and 1.8 

km (I mi) long. 

A vastly different situation prevails in the central Pacific 

Ocean, where almost all of the fish schools sighted are accompanied by birds. Murphy and lkehara (1955) obserbed 

that about 85Vo of all fish school> \ighted in water5 around 

and between the Ha\\aiian, he, arid Phoenix I4and5 were 

accompanied by birds. Like\\i\e, for tho scouting cruises 

around the Hawaiian Island\ during the \pring of 1953, 

Royce and Otsu (IY55) reported that every one of 253 fish 

schools sighted \\as accompanied by bird$. Undoubtedly, this 

dependence on bird flocks for finding tuna \chciol\ in Ihe 

central Pacific Ocean ari\e\ from ilie proalrncc of bird\ in 

the area and partly from llie proailin$ \ea \1atc, \\hicIi I\ 

47 
BIRD FLOCKS SKlPJ4CK SCHOOLS TOTIL FISH SCHOOLS 

terlderic! to aggregate h! \pccie\ 111 adtl~tion. Broadhead 

and Orange (1960) ob\er\ecl tliiit the frequency distribution\ 

of the lenyrli\ of \hipiach tiiiia 111 purr arid mixed specie5 

school\ \\ere \itiiiI:tr, \\heres\ tho\c of !ello\\ fir1 tuna differed 

noticeablq (Fig. 37). 

Kairasaki (1963) compared length-frequency distributions 

of skipjack tuna taken from \ariciui type, of schools in Japane\e \raters and noted that the \ariatioris in length within 

each school type increased in tlie following order: pure school 

(~irhout birds), bird-associated school, whale-ascociared 

schoot, shark- or drift\rood-associared \chool. Although 

Ka\\asaki's classification of pure- and bird-associated schools 

is confu\iiig, a pattern is discernible here, i.e., the less mobile 

the object of aswciation, the greater the size range of fish 

associated \%it11 it. This observation is, thus, consistent with 

Brock's (1954) hlpothesia, since schooling under slow moving 

objects does not require uniform maximum suimming ability. 

Catches of tunas from under stationary floaing objects 

further support BI-ock's hypothesi\. h,latsunioto (footnote 12) 

obsened that piit re heine catches brneath anchored bamboo 

raft\ in tlie Philippine\ included tuna\ (skipjack, yelloibfin, 

and bige)c tiilia>, kanakawa, and frigate and bullet tunas) 

ranging vidcly in 5izes from 23 to 63 an. One or t\ro large 

yeIlo\\fiii tuna or- bigeye tuna, 30 to 70 kg or more. were also 

tahcn occa~ionallv. 

H4WAll4N ISL4NDS ~ ~~ 

I i 

I Eigure 46.--hrdrilnPI %ariation in rdle of bird flock. *kipjack luna whoolr. and PURE IKCJACKIISJII I.+ 1CWOOLI 

total fish whaol \ighting* for three major island group\ (from Waldron 1964). 

(4) Length distribution of \chool\ 

Brock (1954) noted a comparaticely mall range of lengths 

within iiidi\idual xhools in the Hai%aiian fisher), in contra51 

to rhe range of lengths in the landings a\ a \\hole. The nieaii 

range of lengths for single school sample\ \%a\ 11.3 cni, nith 

5 and 21 cm being the least and greatest, whereas the range 

for the wasonal landing \+as 47 cni. He thus concluded that 

the skipjack tuna school \+as highly \i/e-sclecii\e and h)potheaized that tlie segregation by \iLe \\a\ likel? due IO the 

niauiinuni \\\iniming speed attainahle by fi\h of the \ame ,ize. 

Bawd 011 obsercations from the eatern Pacific tiiiia fisher^, 

Schaefer (1948) suggested that the tendenc! of tuna\ IO aggregate by \ire might, in \ome ca\es, be \tronger than the 

1 I 1 I I i I 1 0 400 100 '00 700 *w s00 ,ow ,100 ItW 

LIYTH D# -TU. 

Figure 47.-Erequenc? dialributicins uf lengths of jellow fin luna and skipjack 

luna from school, of pure and mi\ed-,prcie\ compusilinn sampled from bait 

'escl cMches (from Broadhead and Orange 1960). 

(5) Sire of school 

Estimation of school size has been attempted in the eastern 

Pacific Ocean by Orange et al. (1957), based on the catchper-set by purse seine vessels, and by Broadhead and Orange 

(1960), based on the catch-per-school by bait boats. Estimates bated on purse seine catches were considered more re48 
liable than those based on bait boat catches because it is 

assumed a greater percentage of a school is taken by seining 

than by baitfishing. However, actual school sizes would still 

be greatly underestimated, since entire schools are not always 

caught by the purse seine (Orange et al. 1957). 

The frequency distribution of catch-pcr-succersful set by 

species composition categories plotted by Orange et al. (1957) 

showed that the distributions of pure skipjack tuna schools 

were skewed very strongly toward small catches (Fig. 48). 

with more than 50% of the sets falling in the two smallest 

size classes, i.e., 0.25-4.9 and 5.0-9.9 tons. (Note: Orange et 

al. used short tom.) The average catch-per-ret of pure skipjack tuna schools in the eastern Pacific for the 4-yr period 

studied was 13.5 tons, with a range of 1 to 130 tons (from 

their table 2). Along the northeastern coast of Japan, the 

average catches-per-successful set of one-boat and two-boat 

purse seine vessels were 6.3 and 8.6 tons, respectively (Inoue 

1959). Either the schools in the eastern Pacific were larger 

than those in waters off Japan, or the fishing environment 

and technique were inore conducive to greater catches from 

similar-sized schools in the eastern Pacific. It is also possible 

that seiners in the eastern Pacific are less apt to set on small 

schools than seiner5 off Japan. On the basis of Figure 48, 

one can say that the occurrence of small schools (<20 short 

tons) was the rule and that large schools over 50 diort tons 

occurred less frequently. 

1955 916 SETS PURE YELLOWFIN 

245 SETS PURE SKIPJACK 

129 SETS MIXED 

0" s 30b\ 20 

1954 463 SETS PURE YELLOWFIN 

299 SET9 PURE SKIPJACK 

172 SETS MIXED 

1953 605 SETS PURE YELLOWFIN 

142 SETS PURE SKIPJACK 

191 SETS MIXED 

1952 563 SETS PURE YELLOWFIN 

208 SETS PURE SKIPJACK 

216 SETS MIXED 

TONS OF CATCH PER SINGLE SET 

Figure 48.-Prrrrnfage frequenr? dictrihutinnp of ratch-per-srl in the eiehl principal fishing area* of Ihe ea\Iern Pacific 

tuna fkher?. fur each ?ear 1952 through 1955. hr ~prcie+rornpohilion calrporir\. Open circle5 repre\enl catrhr* of pure 

)ellow fin tuna. Solid circle\ reprebent calche, of pure Ekipjark luna. Trianglrr reprewnl ratrhr* of mixed aprrir, (from 

Orange et al. 1957). 

49 
The frequency distribution of catch-per-school from poleand-line fishing (Broadhead and Orange 1960). was similarly 

skewed strongly toward the small catches (Fig. 49). The majority of the catches (about 65%) from pure schools fell into 

the two smallest size categories, 0.5 and 1.5 tons (note: Broadhead and Orange used short tons), and the mean catch for all 

schools was 2.8 tons. The difference in the mean catches by 

bait boats (2.8 tons) as compared with that of purse seine 

vessels (13.5 tons) probably reflects lower efficiency of poleand-line fishing and the likelihood that the purse seine vessels 

may shun small schools more often than pole-and-line boats. 

Broadhead and Orange (1960) compared the sizes of schools 

fished in the eastern Pacific and Hawaiian fisheries by plotting the distribution of catches from the two areas. Data 

from the Hawaiian bait boat catches yielded a J-shaped 

curve similar to that of the eastern Pacific catches but the 

points fell well below that of the latter. The mean catch-perschool was about 1 ton compared with 2.8 tons from schools 

fished in the eastern Pacific Ocean. On the basis of this comparison, they stated, “It appears that the skipjack tuna ichools 

fished in the Eastern Tropical Pacific area are, on the average, 

Tomewhat larger than those fished in the vicinity of the 

Hawaiian Islands (assuming that the bait vessels capture the 

same percentage, on the average, of each school encountered 

in the tu’o regions).” Such a comparison, houever, is dubious 

since their assumption is incorrect. Both ves5el size and number of men fishing per boat differ greatly betueen the two 

fisheries, thus resulting in disproportionate catches. 

TONS PER SCHOOL 

Figure 49.-Perceniage frcquenc) distrihulions of catch-per-school (Ions) b) 

hail vessels, h) calegor) of school. 1956-58 (from Broadhead and Orange 1960). 

(6) Schooling at night 

Some evidence for skipjack tuna schooling at night is 

available in the literature. Nakamura (1962) observed that 

skipjack tuna in captivity swam slowly about the perimeter 

of the pool, often with their bodies normal to the light rays 

from the floodlight. Fish were observed once at night without the floodlight. On this occasion (moonlight was sufficient to allow the fish to be seen), the fish remained in a 

school and swam slowly around the pool. Other evidence of 

schooling at night comes from Yuen (1970). While tracking 

skipjack tuna fitted with ultrasonic tags, he observed that a 

school of skipjack tuna was often visible at the position of 

the tagged fish. On several occasions, both day and night, 

switching the sonar to the active mode disclosed many fish 

targets in the vicinity of the tagged fish. 

(7) Activity of schools 

That schooling occurs at depths in excess of 100 m was 

shown by Kimura et al. (1952) and lwasaki and Suzuki (1972). 

From vessels equipped with echo sounders, Kimura et al. 

(1952) located fish schools between depths of 20 and 150 m. 

They identified them as skipjack tuna by tracking them on 

the echo sounder and fishing them as they reached the surface. In some instances chumming with baitfish caused the 

schools to rise to the surface from depths as great as 100 m 

within 45 sec, a speed of 2.2 m/s (4.3 kn). 

Underwater observations of skipjack tuna schools by Strasburg and Yuen (1960), Strasburg (1961), and Yuen (1970) 

provide further insight into the behavior of skipjack tuna 

schools. Strasburg and Yuen (1960) observed that the activity 

of schools varied with fish size. First, small skipjack tuna 

(ca. 20 cm) schooled in large numbers (in the thousands), 

and in spite of their size, the schools seemed to respond instantaneously to stimuli. The fish were closely spaced and 

maneuvered with precision. Even during feeding, these schools 

never entirely lost their integrity. Second, in schools of medium-+ed fish (45-65 cm), feeding activity was frenzied and 

most semblance of schooling uas lost. The fish made rapid 

dashes to the surface at speeds of about 12.9 m/s (25 kn), 

either singly or in pods of half a dozen or so; hence, their 

snimming uas seldom horizontal but oscillated vertically in 

patterns resembling a series of sine curves. Third, schools of 

large skipjack tuna (70-80 cm) appeared lethargic in comparison with schools of smaller fish. There were no signs of 

schooling during feeding, and the individual fish never appeared to swim faster than about 5.1 m/s (10 kn). More 

often the fish cruised at 1.0 to 2.1 m/s (2 to 4 kn), swimming 

nearly horizontally, with surface dashes being relatively unhurried. 

Strasburg and Yuen (1960) also noted that I) water sprayed 

on the surface while chumming increased feeding frenzy, resulting in increased catches, 2) fast chumming (twice normal) resulted in decreased catches, 3) slow chumming or 

insufficient bait caused the fish to scatter and reform into 

schools away from the vessel, resulting in decreased catches, 

and 4) neither sound, blood, or skin extracts appeared to 

have much effect on actively feeding schools. 

One other school activity was observed by Strasburg (1961). 

During experimental fishing in Hawaiian waters, he noted 

that a school being fished would abruptly dive vertically and 

vanish from view. Examination of stomach contents of fish 

from such schools showed that they had consumed postlarval 

Synodus variegaius and juvenile Holocentrus lacteoguitatus, 

both reef-type fishes, and that the occurrence of these fishes 

in the stomachs was significantly related to diving frequency. 

Observations on the number of diving schools indicated that 

diving was the rule rather than the exception. The number of 

dives per school ranged from 0 to 8 (Fig. 50) with a mean of 

2.7. The schools were away from the surface as briefly as 3 s 

or as long as 28 min. In a later study (Strasburg et al. 1968) 

reef-type fishes, such as those found in the stomachs above, 

were observed from a small submarine at depths of 107 to 

50 
88 (c) Condition of gonads 

:1 

Brock (1954) commented on the rarity of skipjack tuna 

with fully ripe gonads and the lack of spawned-out ovaries 

within the spawning season in his extensive sampling of fish 

in the Hawaiian skipjack tuna fishery. He suggested that fish 

with ova larger than 0.7 mm in diameter become progressively 

less available to the fishery. This suggests that female skipjack tuna may abstain from feeding, immediately prior to 

and during the spawning period. Yuen (1959) found that 

maturation of the ovary did not appear to affect biting response, except in fish with ripe eggs. That skipjack tuna likely 

undergo a period of fasting is borne out by fishermen’s remarks (see Section 3.41(4)) concerning the futility of fishing 

such schools. 

(d) Distance from land 

NUMBER OF DIVES PER SCHOOL Suyehiro (1938) measured biting quality by catch rate 

(number of fish per man per 100 min), designating a catch of 

16 or less fish as poor biting and 17 or more fish as good 

biting. Applying this criteria to schools fished in Japanese 

waters by 17 vessels, he found that skipjack tuna schools 

around islands bit poorly, whereas schools in the open sea 

generally bit well. Yuen (1959) reported a similar relationship; however, this was true only for large (>60 cm) fish. 

Figure 50.-Frequency of diving in Hawaiian skipjack luna schools (from 

Sfrasburg 1961). 

192 m, suggesting that skipjack tuna schools may have descended to these depths to feed. 

(8) Biting response of schools (e) Time of day 

The factors influencing the biting response of skipjack 

tuna schools have been studied by a number of scientists. 

These factors include (a) state of hunger, (b) behavior of 

prey, (c) condition of gonads, (d) distance from land, (e) 

time of day, and (f) weather. 

(a) State of hunger, as determined by the 

amount of food in the stomachs 

Uda (1933), Suyehiro (1938), and Hotta et al. (1959) noted 

that skipjack tuna responded to bait well when their stomachs 

were empty, and poorly when full. Uda (1933) further observed that skipjack tuna with stomachs between the extremes 

of fullness and emptiness tended to respond more poorly 

than when their stomachs were emptier. This statement, however, is contrary to Yuen’s (1959) observation that skipjack 

tuna schools responded to bait longer when the major food 

items in the stomachs were in the earlier stages of digestion 

and that the response became greater as the stomachs emptied. 

Similarly Uda’s (1933) statement is contrary to Magnuson’s 

(1969) observation that attacks by captive skipjack tuna on 

food particles decreased rapidly only after the stomach contents exceeded 50% of the stomach’s capacity. 

(b) Behavior of prey 

Skipjack tuna feeding on fast-swimming fishes such as 

tunas, carangids, and gempylids, exhibit a more favorable 

biting behavior than skipjack tuna feeding on slower swimming chaetodontids, scorpaenids, molids, and acanthurids 

(Yuen 1959). 

Uda (1940) reported that there is a relation between time 

(hour of catch) and total catch, as well as between time and 

catch-per-school. He observed that catches peaked three 

times during the day, a primary peak between 0500 and 0900, 

a secondary peak between 1200 and 1300, and a tertiary peak 

between 1600 and 1800; and that about half of the day’s 

catch was made between 0500 and 0900, and about twothirds of the day’s catch prior to noon. He explained the pattern of catch thusly: “The fish school and feed most actively 

in the early morning, and once their stomachs are filled their 

appetites decline, but around noon, for reasons connected 

with the time required for digestion, their appetites again increase and they become slightly active.” He believed that the 

decrease and increase in appetite occurred again in the evening just before sunset. While his explanation above agrees 

with the results obtained by Magnuson (1969) from feeding 

experiments conducted on skipjack tuna in captivity (see Section 3.41(4)), similar peaks were not reflected in the catch 

rates shown in 2 of the 4 yr he examined. 

A similar study was done in the central Pacific by Uchida 

and Sumida (1971), but rather than using catch and catch 

rates as the variables, they used number of schools sighted 

and number of schools fished successfully. These variables 

plotted against time of day (Fig. 51) resulted in curves not 

unlike that obtained by Uda (1940); however, their interpretation of the curves differed. Fishing success (i.e., schools 

yielding catches) increased rapidly from daybreak to a peak 

at 0901-1OO0, dipped slightly at 1001-1100, and was followed 

by two peaks at 1201-1300 and 1401-1500. The authors attributed the dip at 1001-1100 to reduced scouting effort due 

to time taken for lunch. They suggested that the time taken 

for breakfast and dinner also may have partly affected the 

51 
15 I 

0 

10- 

SCHOOCS SIGHTED 

10 

- 

$5 

a 0 

LO 

I 

.. AVERAGE SIZE 

SCHOOLS WITH CATCHES z IO 

5 

'0501 0701 0901 1101 I301 1501 1701 

0600 0800 1000 1200 1400 1600 I800 

TIME OF SIGHTING OR FISHING 

Figure Sl.-Frcquencie\ of xhool \ighting\ and \choolc wilh calche\. b) 

lime of day far seven Hawaiian \kipjack tuna fi3hing \ei\el*. June-4ugu\t 

1961 (from t chide and bumida 19711. 

reduced sightings and catcher near wnrise and sunset. Additionally, unlike the .lapanere fishery, \\here most of the day's 

catches were made prior to noon, most of the day's catch in 

the Hawaiian fishery occurred after midday (Table 14). 

Tshle I4.-Perccnlagr of the dab', total cnlrh ahoard 

the \e\\cI\ in the fin1 lh) 0900). wcond (0901-12WI, 

third (I20I-lS~H)). and fourth (after 1501) quarter, of Ihe 

fkhing daj for seven Hawaiian \kipjach tuna fishing 

\es%h, June-Augusl 1967 (from I'rhida and bumida 

1971). 

Vec5ris 

A 

B 

birct Second Third Fourth 

3 3 31.5 42 4 22.8 

I5 5 15.3 35 4 33.8 

17 0 20.2 23.7 39.1 

9.2 24.9 35.2 30.7 

7.1 41.6 28.5 22.8 

7.2 40.9 33.3 IR.6 

16.4 10.3 21.0 52.3 

10.8 26 4 31.4 31 4 

10.8 37 2 68.6 100.6 

(f) Weather 

To what degree weather affects the biting behavior of skipjack tuna schools has not been clearly determined. Suyehiro 

(1938) reported that in the western Pacific Ocean biting 

seemed to be better in cloudy than in clear weather, and that 

biting appeared equally good in rain and fog as in clear 

weather. He noted, however, that the reliability of the latter 

was questionable due to few data. He also reported that wind 

velocity was not related to biting quality. In the central Pacific Ocean, Yuen (1959) observed that weather conditions 

were predominantly uniform and biting behavior did not 

change by much. On a few days darker than usual, the ueather 

affected fishing only in decreasing the chances of sighting 

schools. 

(g) Other aspects of biting behavior 

In the western Pacific Ocean, IJda (1940) indicated that in 

most of the schools fished, catches generally occurred between 10 and 40 min of fishing, that the catch rate (catchper-school) increased in direct proportion to the increase in 

fishing time up to 80 min, and that when fishing time exceeded 80 min, neither the total catch nor the catch rate 

showed any increase. Similarly, in the central Pacific Ocean, 

fishing duration of most schools was between 5 and 40 min 

(Yuen 1959; Uchida and Sumida 1971) and the catch-perschool increased proportionally with fishing duration (Fig. 52). 

Yuen (1959) also measured other factors that seemed to influence catch. Of these, I) fishing duration, 2) peak catch 

rate, 3) postpeak duration, 4) rate of increase of prepeak 

catch rates, and 5) mean number of hooks per minute were 

all found to be related significantly to total catch (Table 15). 

Of these, 2) and 3) were subsequently used as measures of 

skipjack tuna response, on the premise that peak catch rate 

would measure the degree of interest or intensity in feeding 

and postpeak duration would measure the duration of interest. 

NUMBER CAUGHT PER SCHOOL 

... .. 

4. 

WEIGHT CAUGHT PER SCHOOL 

.. .. 

0 

10 

K 

i+ z 

I 6 II 16 21 26 31 3: 4, 46 51 %>bo /I# I, ,I, 

5 10 15 20 25 30 35 40 45 50 55 60 

FISHING DURATON (MINUTES) 

Figure SZ.-RelaIion between fihhing duration and arerage balues of rdlch 

per school. amount of bait wed per rchool. numher of men fishing per 

school. and size of fish per school. June-August 1967 (from Uchida and 

Sumida 1971). 

52 
Yao (1962) examined the biting qualities as judged and recorded on fishing reports by Japanese fishermen. In these 

reports biting qualities were judged as good when: 

I) Fish were attracted in large numbers immediately upon 

chumming and there was a sudden initial increase in catch, 

2) the number of fish increased beyond the fishermen’s capacity to catch them, 

3) the duration of fishing above a minimum catch rate was 

long, 

4) catch per school was large, and 

5) maximum catch was attained quickly. 

Yao calculated the biting qualitia of schools, employing Yoshihara’s (1960) theory on the mechanisrn of skipjack tuna 

fishing, by the equation 

W=N (I - (I ~ Ar)e-Al) 

where W is the number of fish caught, I is fishing duration. 

N is number of fish in \cliool, and 1 is the biting coefficient 

expressed as the reciprocal of the length of time until the 

catch rate reached a nlaximum. Yao’s calculations indicate 

that I) through 4) tend to be judged higher as school \ize 

increases, even when the degree of biting was the same. lgnoring school size, he found no statistical differences in the 

mean biting coefficient among schools judged as good, average, and poor by the fishermen. 

Yao (1962) also observed that the mean biting coefficient, 

0.135, for small schools (c 1,OOO fish) was extremely high, as 

compared with the mean coefficient, 0.085, of large school, 

(>2,000 fish). He suggested that the high mean coefficient 

for small schools was due to selective fishing of only good 

biting schools, whereas fishing was done on both good and 

slow biting schools \\hen the schools were large. 

Summarizing the above, it Teems that fish respond well to 

chum when their stomachs are less than half filled and when 

they are feeding on fast-swjimming prey; that schools in offshore areas respond better than those inshore; that fish, 

particularly females, respond poorly during spawning; that 

the response reaches a peak in the early morning between 

0800 and 1000; that there may be more than one feeding 

peak during the day; that the response generally lasts from 5 

to 40 min, depending on school size and state of hunger; 

and that weather may not be a factor in influencing feeding 

behavior. 

3.6 Physiology 

3.61 Energ) transfer (metabolism) 

Rapid and efficient energy turnover, more than any other 

single biological factor, characterizes the skipjack tuna’s 

physiological ecology. Life in the warm and relatively unproductive tropical waters necessitates elaborate adaptations for 

locating and efficiently utilizing the patchily distributed food 

resources. Studies of these adaptations and estimates of energetic rate processes have provided information arid insights 

for modeling of important management parameters (Sharp 

and Francis 1976). The detailed understanding of the skipjack tuna’s energy budget has had its genesis in basic biological information obtained from experiments \bit11 captive 

specimens at the Kewalo Research Facility of the National 

Marine Fisheries Service in Honolulu. Indeed, this research 

facility is the source of much of what is known about the 

basic biology of tropical tunas. 

(I) Energy budget 

In all animals, energy transfer in and out of the animal is 

quantified in the energy budget or mass balance equation 

(Ricker 1968): 

C=R+U+f+G+B 

where C = rate of food consumption 

R = rate of metabolism 

U = rate of excretion 

F = rate of egestion 

G = rate of gamete production 

B = rate of growtli. 

For our purposes the units in the analysis of this rclationship will be given in caloric units (kcal). Skipjack tuna, for 

instance, contain an average of 1.46 kcal/g of live weight 

(Kitchell et al. 1977), and the mean caloric value of skipjack 

tuna forage is assumed to be 1.1 kcal/g wet weight (Kitchell et 

al. 1978). 

The rate of metabolism (R) is lumped by Kitchell et al. 

(1978) into two component\: I) The energy, or specific dynamic action (SDA), required to process the forage, and 2) 

the remainder of the metabolic work (M). Table 16 present5 

the energy budget values of a 1 kg skipjack tuna under six 

input-output levels (Kitchell et al. 1978). Values for SDA, U, 

and Fare assumed from work on other fish to be IS, 5. and 

15% of ingested calories. Thus, the energy available for M 

and storage as growth or gonadal development is 0.65 of the 

calories eaten. Input of calories in the table range from starvation, maintenance calories (no growth) of 5.9% of body 

weight per day, ingestion of necessary calories to produce 

“normal” growth (0.7% of body weight per day, Uchiyama 

and Struhsaker 1981) at three arbitrary multiples of activity, 

and maximum ingestion of 30% of body weight per day. The 

maximum ingestion rate was determined in laboratory studies. 

Growth under these input conditions varied from - 3.6% 

under starvation conditions to a maximum growth rate of 

6.9% of body weight per day when fish were feeding at maximum rations (30% of body weight per day while maintaining 

levels of activity nearly three times those at minimum sus53 
tainable cruising speeds). If activity levels include metabolic 

costs to five times that at minimum cruising speed, growth 

rates would be only 0.7%/d at maximum rations. Kitchell et 

al. (1978) reasoned that these levels of activity were unrealistically high and, therefore, the observed growth rates of 

0.7%/d indicated limited food availability rather than high 

levels of activity. 

In skipjack tuna, unlike most other animals, the routine 

metabolic rate per unit of weight does not decrease as size 

increases (Brill 1979; Gooding et al. 1981). Large fish have 

approximately the same metabolic rate per unit of mass as 

small fish. The reasons for this phenomenon are obscure. 

However, in skipjack tuna, as in most other animals, the 

relative capacity for feeding does decrease as size increases. 

In absolute terms, large fish can eat more than small fish but 

in relative units, the maximum ration per unit of w,eight decreases as size increases. Ability to acquire energy and the 

energy costs of metabolism thus converge as skipjack tuna 

size increases. This interesting relationship can be used to 

predict the limits of growth. It has been thown that to pursue 

and capture prey takes a metabolic rate approximately tmo 

times the rate required to just swjim at minimum speed (speed 

required to maintain hydrostatic equilibrium, Magnuson 

1973). Then the maximum ability to provide surplus energy 

for growth (0.65 of maximum ration) is equivalent to the 

minimum activity necessary for feeding by skipjack tuna 

weighing 25-30 kg. 

For yellowfin tuna, unlike skipjack tuna, the metabolic 

rate per unit weight declines as size increases. The maximum 

feeding rate also declines but at a slightly more rapid rate 

than metabolirm. As in skipjack tuna, both raies converge 

but at levels that predict maximum yellowfin tuna sizes of 

150.200 kg. The maximuiii predicted \izes of the tno turns 

can be compared xith maximum sizes in Section 3.31 for 

skipjack tuna (22-25 kg) and the largest known yellowfin 

tuna (180-200 kg). 

An economic analogy may help make these concepts more 

understandable. If: I) feeding rate is taken as income; 2) 

egestion, excretion, and SDA rates as taxes (proportional !o 

income); and 3) metabolic rate as fixed co~ts, then growth 

(somatic and/or gonadal) can be considered net profit. Small 

animals have the potential for large profits because their 

fixed costs are low and their income potential is high. As size 

iicreases, so too do fixed costs and taxes, but potential profit 

margin narrows due to a less rapid increase in income potential. Eventually they converge at a point that dictates the 

maximum size. In yellowfin tuna, in contrast to skipjack 

tuna, the fixed costs decline rlightly (an economy of scale) as 

size increases, allowing it to become larger before the convergence of costs and profits occur. Obviously, any differences between real income (foad availability) and maximum 

income potential will cause the zero-profit level to develop at 

a smaller size. 

(2) Respiration 

Intermediary metabolism is the process wjhereby energy is 

made available for biological pofier, the fixed costs referenced 

in the preceding section. Intermediary metabolism, sometimes called internal respiration, is measured in units of oxygen consumed, calories transferred, carbon dioxide liberated, 

or power in watts. All are stoichiometrically relatcd. External 

respiration is defined as the mechanism which delivers 0, 

and expels CO,. 

Since skipjack tuna cannot use buccal or opercular pumping to force water over their gills, they must continually swim 

or they \rrill suffocate. They are “ram ventilators.” A 44 cm 

skipjack tuna swimming at a basal speed of 66 cm/s, has a 

gill resistance equal to 7% of its total swimming resistance 

(Brown and Muir 1970). The cost of this extra resistance is 

approximately 1 to 3% of its total metabolitm when swimming at basal speeds. 

Within the gills, O2 is extracted from and CO, delivered to 

the Hater. Here, as in other structures dealing with energy 

transfer in the tuna, there exist modifications for rapid exchange. Within the secondary lamellae of the gills, blood 

flows obliquely to the long axis rather than with it as in other 

teleosts (Muir and Brown 1971). This oblique pathway allows 

a greatly shortened length of the absorbing vessel, reducing 

the total blood pressure drop across the skipjack tuna’s gill 

by 16 times. The short path length allows for small diameter 

absorbing vezsels as well as thinner, more efficient lamellae. 

The latter reduces resistance to water flou. Large gill areas 

result from the increase in total number of lamellae and their 

length. These adaptations result in a gill, which, while delicately constructed and fine grained, has an area of from 4 to 

30 times larger than other teleosts. 

Thus with this large gill surface area, the tuna gill’s effectiveness in removing oxygen from the water is higher than 

other teleosts’. Stevens (1972) has estimated that skipjack 

tuna can extract up to 90% of the oxygen in the water that 

enters the mouth. Other fish can extract from 10 to 30%. 

Four techniques have been employed to estimate oxygen 

uptake: 1) Whole animal, in which one or more tuna are con54 
fined within a sealed tank and the drop in dissolved oxygen 

is measured as the tunas respire; 2) starvation experiments, 

where energy consumption is measured by loss of weight and 

caloric density; 3) in vitro experiments that measure oxygen 

uptake of various tissue samples; and 4) restrained experiments, where oxygen consumption is measured from a perfused, paralyzed fish. 

Of these four techniques, the measurements of oxygen 

consumption from fish swimming in enclosed chambers is the 

most common. Gooding et al. (1981) found that skipjack 

tuna’s “standard” metabolic rate (SMR) was two to five 

times that of similar-sized, active non-tuna species. However, 

the true SMR for tunas is oxygen consumption rate at zero 

activity. This true rate must be extrapolated from rates collected from fish swimming at various voluntary speeds. The 

regression relating consumption rate to size and swim speed 

is as follows: 

log VO: = - 1.20 + 0.19 log W + 0.21 S 

where VO, = oxygen uptake (mg OJg per h) 

W = fish weight (g) 

S = swim speed (body lengths/s). 

Of note is the weight exponent of near 0.2 compared with 

-0.2 of typical fish. Because the swim speed of routinely 

active skipjack tuna is inversely related to weight, routine 

metabolic rate is virtually independent of fish weight. 

Lacking any caloric input, skipjack tuna lose mass and 

caloric density from their tissues from metabolic demand. 

The oxycalorific equivalent relates oxygen uptake values to 

those of calories transferred, 3.4 cal/mg 0,. A tuna swimming at its weight-dependent characteristic swimming speed 

(about 1.4 body lengthsls) has an energy demand (from 

Gooding et al. 1981) of log VOi = -0.54 + 0.08 log N,’. 

Thus, a 1,500 g skipjack tuna, swimming at its characteristic 

respirometer tank speed would consume 64 kcalld. In a fasting fish, the energy comes from mobilization of body tissues; 

Kitchell et al. (1978), citing other sources, estimated that 

20% overhead must be obtained from the tissues; the overhead pays for the deamination of proteins (IS’%) and the 

excretion of nitrogenous waste (Solo). This will add an additional requirement of 16 kcalld: 80 -- (80 x 0.2) = 64. 

The energy content for a whole skipjack tuna is approximately 1.46 kcallg, which for our 1,500 g example is 2,190 

kcal. Thus, a skipjack tuna swimming at 1.4 body lengthsls 

should lose about 3.7% of its total energy per day. 

How does this compare with a real, fasting fish? Work at 

the Kewalo Research Facility demonstrated that starving 

skipjack tuna lose weight at 1.8Vold. But weight loss is not 

the complete story. Fish utilize fat and hydrate as they starve, 

so weight loss underestimates energy loss (Kitchell et al. 1977). 

They estimate that skipjack tuna lose about 1.8% of energy 

content per unit weight per day. This change of caloric density is added to the absolute weight change to yield an independent estimate of energy loss of 3.6%, which compares 

favorably with the 3.7% figure estimated from respiration 

measurements. 

Gordon (1968) estimated metabolism from still another 

approach. Metabolic rates of unstimulated, minced muscle 

from skipjack and bigeye tunas were measured. Both red and 

white muscle preparations were employed. Values for red 

muscles averaged 2 mg O,/g per h, for white muscle, 0.4 mg 

O,/g per h. Tuna muscle is obviously a fast respiring muscle 

comparable with rat thigh muscle at its physiological temperature (Gordon 1968). Another interesting aspect of this work 

is the lack of much of a temperature effect upon respiration; 

a Q,o of virtually 1 is observed. Dizon et al. (1977) also observed this temperature compensation of swimming speeds. 

Perhaps this rapid temperature compensation, something observed in other animals whose habitats have rapid temperature changes, is an adaptation for their behavior of fast, continuous, vertical movements from the thermocline to the 

surface. 

One difficulty in determining respiration of tunas is to arrive at a common level of activity which produces a re\piration rate which can be compared with other fish. Utually 

this value is the SMR, which is defined as the energy requirements of a postabsorptive animal completely at rest. Tunas 

cannot stop swimming so this activity level is meaningless. 

Gooding et al. (1981) referred to the routine respiration rate, 

that is, the energy requirement of a tuna swimming at itr 

“characteristic” speed. A standard rate can be determined 

from the routine measurement by extrapolating the swimming speed to respiration rate relation back to zero activity. 

Another approach is a direct measurement of respiration on 

a paralyzed, restrained fish (Brill 1979). 

The SMR of 33 restrained fish ranging in size from 0.3 to 

4.7 kg was: 

log VO, 0.8987 ~ 0.437 log W 

where V02 = SMR (mg OJg per h) 

W = fish weight (8). 

How do these values compare for other fish and for other 

methods of determining tuna respiration? Again the weight 

exponent is different. Other teleosts range from 0 to about 

-0.350 (quoted in Brill 1979). The value of -0.437 indicates that as body size increases, the SMR decreases relatively 

faster than for other teleosts which contradicts Gooding et 

al. (1981). However, the values predicted for the SMR by 

Brill’s method compares favorably with the extrapolations of 

speed to zero of Gooding et al.’s regression. Compare 0.253 

mg OJg per h determined by extrapolation of the regression 

to 0 and 0.324 mg OJg per h determined by Brill (1979). 

Brill reported that SMR’s for tunas by either method exceed 

those reported for other teleosts by 5 to 10 times. 

A logical next step is to inquire if high respiration rates 

correspond to a sensitivity to low dissolved oxygen (DO) 

levels in the habitat. Work reported in Dizon (1977) and 

Gooding et ai. (1981) indicate that this is so. Skipjack tuna 

show definite signs of stress when DO levels fall below 3.5 

mg OJI. Both investigations report dramatic increases in 

swim speed around 4.0 mg OJI. Dizon (1977) failed to observe any increased swim speed responses in yellowfin tuna 

at oxygen levels as low as 2.5 mg OJI. The swim speed response seems to be an adaptation to remove the animal from 

stressful habitats. An alternate explanation, increased ram jet 

ventilation, is not indicated since metabolic O2 demands rise 

faster than the 0, delivery to the gills as swim speed increases. 

Although these data indicate a great sensitivity to low 0, 

environments, skipjack tuna can probably be conditioned to 

penetrate waters below 2 mg O,/I for a reward, as was done 
with kawakaua (Chang and Diron“). Hence the correlatioll 

of purse 5eining success uith a shallow, O:-deficient thermocline (Green 1967) is probably contradicted by the tunai’ 

willingness to penetrate toxic habitats for short periods. 

co 

cv 

r 

do 

PCV 

(3) Locomotion 

For many years, the biochemiitry of the skipjack tunas’ 

unique intermediary metabolism har been researched by P. 

Hochachka and his students. The following is from Hochachka et al.’s (1978) excellent summary of the skipjack tunas’ 

unique “pouer plant and furnace.” Skipjack tuna, as other 

tunas, have an unusually large and distinct red muscle mass 

(8%) (Magnuson 1973), which is distinct and separable from 

the white (Fig. 53). This fact makes the tuna an excellent 

I I 

K peloms 

T. albacores 

Figure 53.-Transverse section\ of four luna species \ho*ing the positions of Ihe red muscle (shaded areas), Ihe central and lateral rete mirabile (r). 

and the major blond vesscIs suppl>ing Ihr Telia; dorsal aorta (da). posterior cardinal vein (pvc). cutaneous arleries (ca), and veins (01. Note (hat Ihe 

position nfculsneous artcrieI and \pin) In tuihw?tru\ linmru~ is reversed compared with that in other specie, and that onl) an epaxial pair is present. 

Alw. Ih~iiii~ii~ rhi~iinrn dnr, not hate 3 p#nIerior cardinal bein lfmm Graham 1975). 

56 
subject for muscle biochemistry studies. Previous data on 

teleosts supported separation of function for the two types of 

muscle: red is aerobic and is thus used for basal level swimming; white is anaerobic and is used for high speed and burst 

swimming. Recent workers suggest that there might be more 

overlap of function than heretofore suspected (sources cited 

in Hochachka et al. 1978 and in Brill and Dizon 1979). In 

general, the red muscle is highly vascularized and contains 

significant amounts of fat and glycogen, high amounts of 

enzymes of aerobic metabolism, and tremendous numbers of 

mitochondria. Red muscle thus appears totally aerobic, burning either carbohydrate, fat, or both. This is the typical teleost pattern. White muscle is, however, unusual. It contains 

high amounts of glycogen and concomitantly high glycolytic 

enzyme activities. But the enzymes for aerobic metabolism 

also occur in large quantities. Mitochondria exist in far 

greater numbers than in typical teleost white muscle, and fat 

droplets also occur. While white muscle contributes to burst 

swimming, supported by the “most intense anaerobic slycolysis thus far known in nature” (Hochachka et al. 1978: 

154). most white muscle functioning is aerobic-based. This 

fact will be important when we later discuss thermoregulation. 

The biochemical and physiological adaptations of the skipjack tuna characterize a high performance animal. This is obvious to anyone familiar with the morphology of the tunas. 

Strong selection pressures have operated to create body and 

fin configurations, swimming movements, and musculature 

to minimize drag and maximize thrust production per unit of 

input energy. Adaptations for reducing drag are more extensive in the tunas than perhaps in any other group. The skipjack tuna has an almost perfect body shape for longitudinal 

streamlining (Magnuson 1978). Drag reaches a minimum 

when maximum body thickness is 22% of length (Webb 

1975). In contrast, the wahoo has a ratio of 12% (Magnuson 

1973). 

Turbulence is reduced by the presence of longitudinal, 

horizontal keels on the caudal peduncle (Webb 1975; Magnuson 1978). Similarly, the midline finlets across the dorsal 

and ventral portions of the posterior quarter of the body 

direct the water in a smooth flow as the tail beats in rapid, 

but low amplitude oscillations. The avoidance of turbulent 

flow around the fish results in lower drag. The oval peduncle, 

faired with the lateral bony keels, slips back and forth through 

the water with a minimum of resistance and yet provides a 

strong link with the high aspect, lunate tail. Drag producing 

structures are absent or reduced or can be removed from the 

water stream. No scales are present, save at the corselet, so 

the body is smooth. The eyes, nares, and mouth structures 

are almost perfectly faired into the body surface. The dorsal 

and pelvic fins are used only during turns so in straight-line 

swimming, the dorsal fin is slotted into the back and the 

pelvics are appressed into grooves. The pectoral fins provide 

lift to the negatively buoyant body and must remain in the 

water stream. However, when the skipjack tuna turns, the 

outer fin is appressed into a lateral groove on the body surface and its body pivots. Magnuson (1978) stated that during 

burst swimming, both pectoral fins are appressed into their 

respective body depressions, thus presenting a smooth body 

surface. Tuna fins have high aspect ratios; the length is long 

with respect to the width. Induced drag is relatively smaller 

for this fin shape (Magnuson 1978). 

Yuen (1966) has recorded swimming speeds from skipjack 

tuna using cine techniques from a ship’s observation chambers at sea. Speeds ranged from 0.5 to 14.4 body lengthsls. 

Dizon et al. (1978). employing ultrasonic tracking methods. 

recorded swimming speeds that clustered around 2 body 

lengthd-not exceptionally fast, but they observed that 

sometimes the tuna would maintain speeds above 6 body 

lengthsls for over 30 min, and over 3 body lengthsls for 

over 240 min. Trout, in contrast, can only maintain 5 body 

lengthsls for < 5 min (Brett 1973). But we doubt that tunas 

are the fastest fish; Fierstine and Walters (1968) compared 

acceleration times of wahoo and yellowfin tuna and found 

that wahoo could accelerate around six time\ faster. b’hilc 

this is not surprining in a “lurking” predator like the \\ahno, 

it provides insight into tuna design criteria. The impres5ion 

that emerges is that the skipjack tuna should be viened as a 

sustained performer, continually swimming at \peed\ considerably above most other teleosts’, but perhaps not capable 

of the high speed bursts of the lurking predators. 

Small tuna species (at least as adults) are totally lacking in 

a swim bladder and are, as a result, negatively buoyant 

(Magnuson 1973). Perhaps this is an adaptation to allow 

them to rapidly forage throughout the water column without 

danger of embolism. The ultrasonic studies reported by 

Dizon et al. (1978) showed continuous vertical movement 

that ranged between the surface and 273 m; these were sometimes completed in less than a minute. 

Because the fish are negatively buoyant. lift must be provided to maintain hydrostatic equilibrium. The pectoral fins 

provide most of this lift (Magnuson 1973). Since lift is generated from the forward motion of the fish, the necesity of 

maintaining hydrostatic equilibrium imposes a minimum 

swim speed on the skipjack tuna. If speeds fall below, this 

minimum, insufficient lift is generated and the flsh either 

sinks or must resort to tail-down-head-up swimming behavior 

or a kind of flapping of the pectoral fins. The minimum 

swimming speed of skipjack tuna (Fig. 54) falls as size increases, from above 3 body lengthsls for a I2 cm animal to 

below 1.5 for a I00 cm one (Magnuson 1973). Incidentally. 

Magnuson (1973) reported that the minimum speeds predicted for skipjack tuna are identical to those predicted by 

Shuleikin (cited in Magnuson 1973) as most efficient for 

migration by aquatic species. 

Measurements of drag by the various body and fin components can be used to estimate power consumption. Magnuson (1978) and Magnuson and Weininger (1978) probided 

accurate estimates of body dimensions and drag estimates 

for the various tunas. Iliron and Brill (1979) and Gooding 

et al. (1981) have used these to determine power required and 

heat generated at various swim speed\. An estimate for the 

1,500 g example skipjack tuna’s theoretical power uptake 

can be calculated by the following. 

1) Total input power to swim is the sum of the power required for caudal thrust divided by the muscle efficiency of 

the propulsion system (20‘70) (Webb 1975) and added to the 

power required for nonswimming processes (Brill 1979; 

SMR, see Section 3.61(2)). 

2) Thrust power must be equal to drag force multiplied by 

velocity (power equals force times velocity) and is estimated 

by: 
tipre 54.--F~iimated minimum swimming qpeeds el se\en rromhroid fi\hr\ 

for mainlahing hldroslalir equilihrium in (a) rm/wr and (h) hod? Imglhs/\rc 

(from Mapnuson 1978). 

P=(O.~XQXSX VxCdx 10 ’)x Vx253 

where P = required thrust power (mg O,/h) 

e = water density (1.0234) 

S = surface area (0.4 L’, where L = fork length) 

V = velocity (centimeters/s) 

Cd = coefficient of drag (dimensionless number 

253 = a multiplier that converts matts to nig O,/h, 

relating surface area and velocity to dras force) 

assuming an oxycalorific equivalent of 

3.4 cal/mg 0,. 

3) The coefficients of drag and body surface area have been 

calculated for skipjack tuna and others by %lagnuson and 

Weininger (1978). For the purpose\ of this treatment, a 

simple 0.4 L’ a<\umption is sufficient for bod! \urface and 

an estimate of’ drag coefficient of: 

where p = uater \isco\ity (0.0096). 

The 1,500 g (43 cm) example skipjack tuna s\+imming at 1.4 

body lengthsls should theoretically have a coefficient of drag 

of about 0.02, require 0.028 mg O,/g per h of thrust and 

thus five times as much input power to the propulsion system 

to “pay” for the 20% efficiency of the system (80rn is lost as 

heat, some of which warms the body). Input required is thus 

0.14 mg O,/g per h, which must be added to the pouer required for nonsuimming functions such as osmoregulation. 

Using Brill’s (1979) SMR relationship (0.35), the total required power should be 0.49 nig 02/g per h. which compare 

\\ell with Gooding et al.’s (1981) measured value of 0.498 mg 

O:/s per ti. 

3.62 Therrnoregulaticin 

Stevens and Neil1 (1978) characterire the life history strategy of the tunas as energy speculators. Tunas gamble relatively large amounts of energy to quickly move through their 

forage-poor enbironment and anticipate correspondingly 

large energy returns. Adaptations for energy speculation have 

produced many of the unique specializations that we have so 

far described for fast suimming and rapid energy overturn. 

It has also presumably brought them to the verge of homeothermy. Tunas, and some pelagic sharks, maintain their body 

temperature above that of the surrounding water. As indicated in earlier sections, tunas, particularly skipjack tuna, 

have exceptionally high metabolic rates. Because the propulsion system is only 20% efficient, 800’0 of the input power to 

the system is available to heat the tissues. In most teleosts 

this excess heat is carried from the trunk muscles by the 

venous system to the gills, where the heat is lost to the water. 

Thermal equilibrium occurs at a much faster rate than the 

mass transport of 0: and C0:. so the heated venous blood 

thermally equilibrates uith the seawater. As a result, excess 

temperatures of most teleosts never exceed a feb tenths of a 

degree. 

The thernioconserving mechanism taxonomically distinguishe the 13 species of true tunas from the rest of the scombrid family (Klaue 1977; Collette 1978). Heat is transferred 

from the venous to the arterial sides of the circulatory system 

in a system of countercurrent parallel arterioles and venules 

in close contact (Fig. 53). These are the rete mirabile (“wonderful nets”). Tunas have four different types: 

I) Cutaneous retia ser\ed by cutaneous arteries and veins 

which are unique among teleosts. This system is very large in 

bluefin tuna and provides all of the muscle blood tlow. In 

the skipjack tuna, however, it is small and of preturnably 

minor importance (Stevens and Neil1 1978). 

2) Visceral retia are found within the bluefin tuna, albacore, and bigeye tuna but are absent in the skipjack tuna. 

3) Eye and braii? retia are found in the bluefin tuna and 

albacore but unknoun in the skipjack tuna, although they 

exhibit elevated hrain temperatures. 

4) Central rete are well developed in the skipjack tuna and 

other small tunas. It is located directly beneath the vertebral 

column. Steven5 et al. (1974) has described the retia in great 

detail (Fig. 55). 

Because of the countercurrent rete, metabolic heat is 

trapped within the muscle mass of the body with excess temperatures ranging from Io to 21 above the Furrounding 

water (Barrett and Hester 1964; Carey et al. 1971; Stevens 

and Fry 1971; Graham 1975; Dizon et al. 1978; Stevens and 

Neil1 1978). 

Tunas have several thermoregulatory options (Dizon and 

Brill 1979): 

58 
needs and alter the effectiveness of thermoconserving ctructures. 

It is difficult to use field data as evidence for phy5iological 

thermoregulation. Barrett and Hester (1964) determined a regression between sea surface temperature (Tu) and body temperature (Tb): Tb = 0.58 Tu + 16.39. Pre\umably, if Tb 

was constant over a great range of Tu’s, one could assume a 

thermoregulatory ability. In skipjack tuna, the slope is different from unity implying some thermoregulatory proceLs. 

However, other workers (Carey and Teal 1969; Stevens and 

Fry 1971) observed muscle temperatures quite different from 

Barrett and Hester’s (1964), although the fish were taken in 

waters of the same temperature. Still, Stevens and Fry (1971) 

concluded that skipjack tuna could maintain a fixed Tb in 

waters ranging from 25“ to 34°C; that is, that they do regulate. Conclusions drawn from such field evidence, however. 

are not justified because the thermal histories of the fish 

prior to capture were unknown. Very cool water is available 

to the fish within less than a minute before the time it is 

Figure 55.-Schemalic drawing illustrating the major features of the vaxular 

heal exchanger of skipjack tuna. DA. dorsal aorta: K. kidne): M. muwle: 

caught, boated, and measured. Consequently, sea-surface 

temperature does not necessarily represent the real habitat PCV. posteardinal vein: V, vertebra. Left inset shows pattern of arterial blood 

flow; right inset shows patterns of venous blood flow: mall arrows indicate 

heat tlsnsfer from venules to arlerioles in the exchanger (From Stevens et 81. 1974). 

temperature, Moreo.uer, thermal inertia causes 73 to lag behind apparent changes in Tu, and activity of the fish is a 

critical determinant of heat production and presumably Tb. 

1) Behavorial thermoregulation 

Since tunas are mobile and live in a heterothermal environment, they can behaviorally select optimal thermal habitats. 

Except for bluefin tuna, the distribution of the others is 

narrowly circumscribed by temperature (Sund et al. 1981). 

And because they “store” heat, they can behaviorally regulate activity to alter heat production: less activity means less 

heat produced. 

2) Passive thermoregulation 

Any process that stabilizes body temperature without requiring nervous system intervention can be (haracterized as 

passive thermoregulation. 

Water temperature- and swim velocity-related heat production have been suggested as affecting body temperature. 

Water temperature affects water viscosity and density and 

thus influences drag forces which, in turn, alter the energetic 

requirements of swimming. 

Thermal inertia might explain much of what was considered 

physiological thermoregulation in the past (Carey et al. 1971; 

Neil1 and Stevens 1974; Neil1 et al. 1976; Dizon and Brill 

1979). Because of the retia, heat is exchanged with the environment at a much more reduced rate in tunas than in 

similar-sized teleosts; body temperature changes in tunas 

thus can significantly lag behind environmental changes. 

As swim velocity increases, the surface heat dissipation 

rate is increased. However, it does not appear that this process is sufficient for stabilizing body temperatures in response 

to increased heat production as tuna swim faster (Dizon and 

Brill 1979). 

3) Physiological thermoregulation 

Activity-independent thermoregulation requires that the 

nervous system has the ability to recognize thermoregulatory 

All these affect the thermal history of the fish before they 

are captured. 

To differentiate between thermoregulatory options, experiments must be conducted that measure Tb and activity at 

specified, constant Tu’s. Dizon et al. (1978) and Dizon and 

Brill (1979) investigated thermoregulator:. performance in 

skipjack and yellowfin tunas,.where Tu could be fixed and 

Tb measured by ultrasonic telemetry. Swim speed in an 

annular tank was constantly monitored with photocells. 

Under these conditions a simple Tb versus Tu regression 

reveals little temperature adjustment. Body temperature is 

clearly dependent upon Tu: Tb = 3.14 + 0.97 Tu. Dizon et 

al. (1978) estimated heat production (H,,) using the same 

techniques outlined in the previous section for determining 

total power input. Heat production is the input power, minus 

the output power dissipated as thrust, plus the SMR. Use of 

these relationships allows comparisons to be made between 

fish of different sizes swimming at different speeds. An index 

of whole-body thermal conductance can be developed, 

H, k(Tb ~ TU) 

where HL = steady state heat loss 

Since the fish is assumed to be in steady state, that is, Tb is 

constant, then H, must equal H,,. In contrast to yellowfin 

tuna, skipjack tuna were found to exhibit a great deal of 

variability in the whole-body thermal conductance over the 

temperature range from 20” to 30°C. (If regulation were occurring, increases in Tu should be accompanied by increases 

in H, so that Tx would get smaller where Tx = Tb - Tu.) 

Nevertheless, even though H,, is tightly linked to swim speed, 

changes in swim speed were not accompanied by changes in 

body temperature. Clearly, some mechanism existed in skipjack tuna to alter either the pattern of H,. heat loss, or both. 

Dizon and Brill (1979) extended the above experiment by 

forcing tunas to swim at high speeds in water temperatures 

approaching their upper lethal limit. Under these conditions, 

59 
if physiological regulation was possible, the tuna would 

demonstrate it. Swim speed was increased by force feeding 

plastic-coated weights to 23 fish. Only three survived the 

treatment long enough to collect data. Two of the fish were 

able to significantly increase whole-body thermal conductance 

and one with no decrease in swim speed. Dizon and Brill 

(1979) consider this true physiological thermoregulation. 

There are two situations in tunas which require some thermoregulatory adjustments: I) Tx should be increased or decreased when ambient temperatures approach lethal limits, 

and 2) extremely rapid increases in heat production accompanying increases in swim speed must be dissipated to prevent 

muscle overheating. The tuna could meet these two exigencies 

with I) some physiological process that would alter the effectiveness of the heat exchanger, and 2) changes in the relative 

contribution of the red and white muscle fibers to swimming. 

Prevention of overheating presents no problem. There has 

been much in the literature on the overheating of large, active 

tunas in warm water, notably Barkley et al. (1978) and Sharp 

(1978), to define inhabitable regions in the ocean. However, 

there is no physiological data indicaticg that tunas can overheat under free-swimming normal conditions. If the notion 

that the white muscles are only used for high speed swimming 

is discarded, a way out of the overheating conundrum is 

apparent. In the section on energetics, the argument was 

made that white muscle fibers are graded into contributing to 

swimming propulsion at a relatively low swim speed (Hochachka et al. 1978; Brill and Dizon 1979). White muscle fibers 

are vascularized by pathways which bypass the vascular heat 

exchanger (Kishinouye 1923; Godsil and Byers 1944) so the 

heat or a portion of it is lost at the gills, like other nonthermoconserving fish. 

Control of the relative contribution of the two fiber types 

to swimming might also serve to adjust Tb in response to the 

first exigency. At high environmental temperatures, the tuna 

might rely more on the white muscles, which add to the body 

heat burden to a lesser extent than rhe red. In addition, 

there might well be some regulation of the effectiveness of 

the heat exchanger itself. 

In summary, tunas have relatively large numbers of processes 

to deal with thermal challenges. They obviously can employ 

behavioral responses of seeking new habitat or reducing or 

increasing swim speed. Thermal inertia helps stabilize body 

temperatures during periods of rapidly changing environmental temperatures. Physiological mechanisms exist as seen 

in the observation that heat production and body temperature 

are not always linked. 

As to the question, “Why be warm in the first place?’ 

many ideas have been suggested, but no real answers have 

emerged. Because of the fast swimming, energy speculative 

nature of the tuna, most suggestions have centered on swimming. The original assumption (Carey et al. 1971) is that 

higher temperatures increase muscle power. However, Walters 

and Fierstine (1964) found that a yellowfin tuna, whose body 

temperature was twice that of a wahoo, exhibited speeds only 

slightly greater and acceleration times considerably less. 

Neil1 and Stevens (1974) suggested that recovery time from 

anaerobic activity might be lessened because of the increased 

temperature. Perhaps energetic efficiency is enhanced, or 

there are biochemical advantages to the thermoconserving 

structures. Hochachka et al. (1978) felt that the tuna has 

greater control over its highly charged metabolsim. Give. il 

Q,o of 2, a twentyfold change in metabolic rate can be converted to a fortyfold change, if the change is accompanied 

by a IO” rise in temperature. Perhaps the retia temperature 

function is only secondary; its main one may be to retard 

mass transfer of lactic acid, the byproduct of anaerobic 

glycolysis. Then lactate from the white muscle would pass to 

the red muscle, where it would be metabolized by that tissue’s 

liverlike enzymes (Stevens and Neil1 1978). But much of the 

white muscle circulation bypasses the rete mirabile entirely. 

There also might be benefits occurring simply from stability 

of the body temperature. Neil1 and Stevens (1974) and Neil1 

et al. (1976) suggested also that because of the thermal inertia, 

perception of very weak horizontal gradients could be accomplished. But because of the skipjack tuna’s habit of 

constantly making vertical migrations (Dizon et al. 1978), we 

wonder if the horizontal gradients have much meaning to the 

tuna. 

3.63 Hypothetical habitat of skipjack tuna 

Barkley (1969) has hypothesized that the habitat of the 

adult skipjack tuna coincides with the area where a shallow 

salinity maximum occurs seasonally or permanently, whereas 

that of larval skipjack tuna coincides with the area having a 

permanent shallow salinity maximum. Thus, a hypothesized 

maximum depth and distribution can be mapped (Fig. 56). 

Temperature and oxygen data can also be utilized to delimit 

distributions based upon size (Figs. 57-59). Barkley et al. 

(1978), based upon information from Neil1 et al. (1976). considered three environmental conditions that would be likely 

to operate to determine distribution of skipjack tuna. These 

are: 1) A lower temperature limit around 18°C. 2) a lower 

dissolved Oi level around 3.5 p/m, and 3) a speculative upper 

temperature limit, ranging from 33°C for the smallest skipjack tuna caught in the fishery to 20°C or less for the largest. 

According to the analysis, skipjack tuna larger than 11 kg 

should find relatively few habitable regions within the eastern 

tropical Pacific Ocean (Fig. 60). While this agrees with fishery 

data, the third condition does not seem to hold (see Section 

3.62). The authors arrived at this condition by extrapolating 

from data obtained from restrained fish and by predicting 

temperatures for a fish swimming with red muscle only. 

These conditions could then result in thermal runaway; that 

is, a positive feedback system could be generated with muscle 

temperature increases causing faster swimming, causing increasing muscle temperatures. But as indicated in the previous section, tunas do not overheat even though they are 

very active in warm water. Some process intercedes to reduce 

temperature excess (Dizon and Brill 1979). 

3.64 Responses to stimuli 

(I) Vision 

Nakamura (1968) studied the visual acuity of skipjack tuna 

and kawakawa using conditioning techniques. Tunas were 

taught to respond to targets bearing either vertical or horizontal stripes in order to receive food and avoid electric 

shock. At lower light levels, visual acuity of the two species 

was similar but at higher levels, the skipjack tuna did better 

(Fig. 61). 

60 
UNITED STATES 

Figure %.-Depth in meters of the shallow salinity maximum in the Pacific Ocean, based on all available oceanographic 

station data, averaged within areas of 2" of longitude by 2" of latitude. Regions where a shallow maximum is present during one or more quarlers but not all year are indicated by hatching. A region analogous to the halched area in Ihe North 

Pacific must also be present in the South Pacific but cannol be shown due lo sparsily of data (from Barkley 1969). 

Figure 57.-H)pothetical maximum depth (meters) of the skipjack tuna habilat 

in the eastern Pacific Ocean, a$ determined by the deplh of the 18°C isotherm 

(hatched area) or the 3.5 ml/l(5 p/m) isoplelh of dissolved oxygen (crosshalched 

area). Contour interval is 50 m except for a few areas near the coast. where a 25 

rn contour interval is used (from Barkle) el al. 1911). 

The role of visual acuity in the life of pelagic fishes may 

involve, in addition to the detection of prey and predators, 

the recognition of transient body marks, such as the vertical 

bars exhibited by the skipjack tuna duriiig feeding (Strasburg 

and Marr 1961; Nakaniura 1962; Nakamura and Magnuson 

1965) or during courtship (Iversen et al. 1970). as well as 

other permanent body markings. Nakamiira (1968) calculated 

the distances of resolution of prey or body marks a\ 36 m, if 

the skipjack tuna were in waters having [he same visibility 

61 

Figure 58.-HypotheIical minimum deplh of the skipjach tuna habitat in the 

Pacific Ocean east of the 180°meridian, fur fish Neighing about 6.5 kg (I4 Ib) 

which are limited lo water cooler than 24°C. Contours show the depth. in meters, 

of the 24OCisotherm (from Barkle) et al. 1978). 

conditions as that in the experimental tank. He also determined (hat the skipjack tuna could prey on object5 as sinall 

as 0.9 mm, based on the minimum sire of food particle that 

could be retained by the gill rakers, and that it would be 

able to resolve prey of this size at a maximum diytance of 

54 cm. The transient vertical bars on the flanks of skipjack 

tuna, being about 2 cm wide, would he resolved at a maximum distance of 12.4 m, whereas the permanent black longitudinal stripes on the belly, being 0.5 cm wide, would be re- 
Figure 59.-lhickness ef Ihe h\polhelical habitat la!er (meters), for 6.5 kg skipjack luna in Ihe eastern and cenlral Pacific Ocean. Contour, were obtained b? 

rubtracting depths of Ihe upper habitat limit (Fig. 571 from Ihe lower one (Fig. 

561. In the crosshatched areas off Mexico and Peru. there should be no habitat 

wilable for fi5h of lhis or larger sizes. In Ihe hatched ares. water warmer lhan 

24°C i5 present above the habilat lajcr. Immedislcl) be)ond this area. Ihc 

habitat (dashed depth conloursl extends to the sea surface. Out5ide of the 18°C 

surface isotherm, Ihe Haler is probably too cold for %kipjack tuna (from Barkle) 

CI al. 1978). 

LOG OF LUMINANCE OF WHITE STRIPES (FOOT-LAMBERTS 1 

Figure 61.--\isual acuity curbe, of skipjack luna and kawakawa. turhvlnrn 

ntth7ir. at adaptive illumination of 170 luxes (from Xaknrnura 1968). 

Figure 60.-Water temperalure al thorc depths where Ihe concentration of dis5ohed oxtgcn is 3.5 mlbl. Ikeper water is cooler and lower in oxkgen, shallower 

water is warmer and has more oxjgcn. Skipjack tuna weighing more lhan 11 kg 

require well-oqgenated water cooler than ZIT; they should find all the shaded 

area5 stressful. Similarl). 4 Lg skipjack tuna should find Ihe area enclosed b) the 

26'C conlour either loo warm or loo poor in dissolved oxygen. Smaller skipjack 

tuna would seldom enco~nler thermal sire% in the easlern lropical Pacific, and 

can inhabit all area5 b? ,la?ing in the shallower. o\)ecn-rich la)m (from Barkley 

et al. 1978). 

solved at maximum distances of 3.1 m, again in waters having 

the same visibility conditions as those in the experimental 

tank. These conditions would be met in pelagic waters typical 

of the tropical ocean with an unobstructed sun at an altitude 

of 65 '. 

The presence of color vision improves perception, but 

tunas are color blind. Work by Tamura et al. (1972) and 

Kawamura et al. (1981) failed to find chromaticity-type Spotentials. This is a technique where electrical potentials are 

measured from the isolated or in situ retina. The chromaticity 

S-potential is considered by visual physiologists as a definitive test for color vision. The latter authors looked for but 

failed to find these potentials in yellowfin tuna: bigeye tuna; 

albacore; striped marlin, Terruprurus audax: blue marlin, 

Mukaira nigricans: and black marlin, M. indica. Tamura et 

al. (1972) recorded S-potentials from skipjack tuna, kawakawa, and frigate tuna and failed to find the chromaticity 

type. That these fish are color blind is consistent with the 

findings by Tester (1959) that tunas exhibited no color preference in behavior tests. Also, most wavelengths of light are 

rapidly attenuated with depth, so that the principal habitat of 

the pelagic fish is a monochromatic one. Munr and McFarland (1977) found only a single visual pigment in retinal extracts of wahoo, yellowfin luna, kawakawa, black marlin, 

and blue marlin. This is also a strong indicator of the lack of 

color vision. 

(2) Temperature 

Skipjack tuna have thermal receptors and can make behavioral decisions based on Zemperature stimuli. Dizon et al. 

62 
(1974, 1976), using heart-rate conditioning techniques, demonstrated temperature perception in skipjack tuna. Steffel et 

al. (1976) demonstrated that free-swimming, captive kanakawa could discriminate temperature thresholds of 0. IO” to 

0.15”C. We suspect that skipjack tuna can do as well because 

this degree of sensitivity has been demonstrated in other teleosr species. Even though this seems like a high degree of 

sensitivity, horizontal gradients in the ocean are never likely 

to exceed O.OOOIo to 0.001 “C/m. This fact, and the tunas’ 

habit of making continuous vertical movements through 

gradients of several degrees, makes it unlikely that they use 

the weak horizontal temperature changes as orienting cues. 

(3) Mechanical 

Sound does not seem to affect the behavior of skipjack 

tuna at sea (Strasburg and Yuen 1960). Noise produced by 

hammering on the hull failed to alter the behavior of the 

tuna being fished. HowJever, it is doubtful that skipjack tuna 

are deaf. lversen (1969) determined threshold curves (thresholds versus frequency) for kawakawa and yellowfin tuna. 

Thresholds were not remarkable when compared with other 

teleosts. 

York (1974), on the other hand, found that sudden or irregular noise, as caused by worn bearings, a bent propeller 

shaft, a slack rudder, or changes in throttle with a “high 

revving” engine, caused schools of skipjack tuna as well as 

yellowfin tuna and albacore to sound. In addition, York reported some success in attracting skipjack tuna schools by 

playing back recordings of anchovies “breezing” and gannets 

diving. 

(4) Chemical 

Tests at sea to attract skipjack tuna using extracts from 

skipjack tuna, yellowfin tuna, and anchovy were either negative or inconclusive (Tester et al. 1954). Other substances, 

such as skipjack tuna blood sprayed in the water or skipjack 

tuna blood and body slime allowed to drain from the scuppers 

into the water failed to affect the behavior of the skipjack 

tuna (Strasburg and Yuen 1960). 

4 POPULATION 

4.1 Structure 

4.1 1 Sex ratio 

Most of the sex ratios reported in the literature are byproducts of studies on maturation and fecundity. No real use 

was made of the sex ratio information including attempts to 

relate changes in hex ratio with maturation, size of fish, 

spawning season, or changer of availability or migration 

associated with attainment of maturity. The discussion below 

will proceed by ocean and all ratios are presented as number 

of males to number of females. 

For the Atlantic Ocean, Batts (1972~) presemed data obtained from recreational catches off tw~ locations in North 

Carolina, U.S.A., in 1964-66. Over all years and both areas, 

the sex ratio favored females in all but one case and ranged 

from 1.06:l to 0.58:1, averaging 0.86:l. Cayre (1979) found 

a ratio of 0.95:l for a small sample collected in Dakar from 

Senegalese purse seiners fishing in the Gulf of Guinea. Chur 

et al. (1980) likewise reported that the sex ratio of fish captured by purse seiners in the eastern tropical Atlantic from 

1969 to 1977 did not differ from I:l. 

Fur the Indian Ocean, Raju (1964a) found males predominating in most months, especially for older age groups in 

samples collected in the Minicoy Islands. Marcille and Stequert (1976) and Stequert (1976) presented monthly data on 

the sex ratio of fish collected from Japanese bait boats fishing off the northwest coast of Madagascar from February 

1974 to March 1975. There was no apparent trend in the 

monthly data and all but one of the values fabored females. 

For the entire period, the sex ratio uas 0.83:l and ranged 

from 1.01:1 to 0.72:l. 

For the Pacific Ocean, Schaefer and Orange (1956) found 

that more females (0.73:l) occurred in the samples from the 

young fish taken in the eastern tropical Pacific fishery; however, sex could not be determined for a large proportion of 

the samples. Yoshida (1960) reported that the sex ratio was 

1.05:l for samples collected in the central South Pacific. In 

Hawaii, Brock (1954) showed that males predominated in the 

catches during most time periods (1.16:l overall) and differed 

significantly from 1:l in the September-December period in 

1949 and 1950 (1.64:l and 1.32:1, respectively). .4lso, catches 

from experimental trolling in inshore Hawaiian waters 

(Tester and Nakamura 1957) showed a predominance of 

males (1.57:l). For the northern Marshall Islands area (Marr 

1948). males again predominated (1.60:l). For fish taken off 

Taiwan, Hu and Yang (1972) reported a preponderance of 

males (1.31:l) in the 50-59 cm FL interval, while the ratio 

was 1:l in the adjacent IO cm FL intervals. Monthly data 

showed considerable variability, for example, from 3.01: 1 to 

0.64:1, but there was no apparent trend. Off Japan, the ratio 

was reported to be 1.09:l (Waldron 1963). Wade (1950) reported that females dominated in the Philippine catch with a 

ratio of 0.86:l. Likewise in New Zealand, females are reported to dominate the catch, with the ratio being 0.76:l 

for fish caught by purse seine over the period December 

1976-March 1978 (Habib 1978). 

These statistics suggest that those fisheries known to depend on young, immature fish have sex ratios favoring females Mhile those fisheries harvesting predominately older, 

spawning age fish exhibit sex ratios favoring males. 

4.12 Age composition 

Few, tables of age composition have been presented in the 

literature because of the inherent difficulties in aging skipjack tuna as pointed out in a previous section. For the most 

part, age composition has been determined from tables of 

sire composition using various growth equations. Until skipjack tuna can be aged with certainty and with greater ease 

than is currently possible using otoliths or other hard parts, 

little use is likely to be made of tables of age composition. 

4.13 Size composition 

Since all fisheries capture skipjack tuna over a restricted 

sire (age) range and most over a limited geographical range, 

the size-frequency distributions and other measures of size 

composition given below are in all likelihood not representative of any complete stock or population. Length-frequency 

63 
distributions have been published most commonly to simply 

describe the sire composition of the catch and document 

seasonal changes in a fishery. They have been used less frequently to study recruitment, relationships among areas or 

fisheries, growth, and gear selectivity. From another perspective, Sharp (1976, 1978, 1979) and Barkley et al. (1978) 

proposed that the temperature and oxygen structure of the 

oceans in interaction with the physiological properties of 

skipjack tuna limits the distribution of larger individuals. 

This subject is discussed more fully in Section 3.63. 

(I) Length composition 

All length measurements are fork length unless otherwise 

stated. Modes were determined by visually inspecting the distributions in all but one case. 

(a) Pacific Ocean 

For the eastern tropical Pacific Ocean, Broadhead and 

Barrett (1964) presented quarterly length-frequency distributions for four northern areas of the fishery in 1954-60. While 

there was considerable variability among years, the distributions for 1956 (Fig. 62) show that the four areas generally 

had a single mode, though of different size. The Baja California area tended to have consistently smaller fish (40-50 

cm), while the Gulf of California, the Mexican coast, and the 

Revillagigedo Islands tended to have more fish between 60 

and 70 cm. Data presented for more recent periods, for example. 1967-79 for the entire Commission’s Yellowfin Regulatory Area (IATTC 1980), likewise show considerable variability among years and, probably because several areas were 

combined, the distributions tended to show multiple modes. 

Diaz” showed that the length-frequency distribution from 

purse seine catches frequently shows more large fish than do 

those for bait boats. His data also indicated that fish off 

Ecuador were mostly between 50 and 60 cm. While the skipjack tuna taken in the northern area of the fishery are generally considered to comprise a single stock, the variability 

shown here as well as the morphometric and genetic findings 

of Sharp (footnote 4) suggest that a considerable amount of 

heterogeneity may exist. 

For Hawaiian waters, Rothschild (1965) compared the 

third quarter length-frequency distributions for Baja California (taken from Broadhead and Barrett 1964) with those 

from Hawaii (Fig. 63) and found a remarkable similarity for 

the small modal group in 1954 and 1960. In 1959, however, 

the two distributions differed considerably. Rothschild (1965) 

inferred from this figure and other evidence that the small 

fish in the Hawaiian and eastern Pacific (at least Baja California) fisheries have a common origin and that the origin is 

in the central equatorial Pacific Ocean. Houever, from Figure 63 and other evidence, it is apparent that there are differences in size distributions between the eastern tropical Pacific 

and Hawaiian fisheries. The Hawaii size frequency is bimodal 

or trimodal, including small fish as in the east but large fish 

in addition. Summarizing over years for the winter months in 

“U\ai. E. L. 1’166. Growth ot \kiplack tuna. liarslrunnlrcprlalll/r. in theraiiern Pacilic Ocean. Inter-Am. Trop Tuna C‘omm , lnlernal Rep. 2. 18 p. InterAmcncan Tropical Tuna Conirnirilon. c/o Scrlpps In\iliuimn of Oceanopapli), 

La Jolla. CA 92037. 

the Hawaiian fishery, modal groups are found at approximately 35, 50, and 75 cm, while in the summer months they 

are found at approximately 45 and 70 cm. Whereas Brock 

(1954) found progression of modal groups and based a growth 

model on them, Rothschild (1965) found the progression to 

be irregular with instances of regression. 

Iat QUARTER 2d OUARTER 3d OUARTER 4th QUARTER 5, t 

0 GULF OF CALIFORNIA 

5 

0 D 

0 5 10 

E5 

s 

1 

3 ID 

YO 

REVILLAGIGEW ISLANDS 

5 

0 1 

151 A BAJA CALIFORNIA 

LENGTH tmm) 

Figure 62.-Length distribution of skipjack luna For 1956 by quarter of the year 

for the Gulf of California, Mexican waters, the Hevillagigedo Islands, and coastal 

areas of Kaja California (modified from Broadhead and Barretr 1964). 

I I 1960 15 

LENGTH [MM 1 

Figure 63.-Length-frequenc) distributions for skipjack tuna taken during the 

third quarter of the )ear from fiaheries off Kaja California and the Hawaiian 

Islands (from Rothrchild 1965). 

64 
For the equatorial region in the central Pacific Ocean, experimental longline catches showed the presence of fish from 

47 to 84 cm (Shomura and Murphy 1955). While most of the 

fish were between 70 and 80 cm, longline catches are commonly believed to be biased toward larger individuals. 

For the South Pacific, most data have resulted from exploratory fishing rather than from commercial or subsistence 

fisheries. Rothschild and Uchida (1968) found that experimental catches in the Marquesas Islands showed a trimodal 

distribution with mode\ at 47 (dominant), 67, and 75 cm. 

Bayliff and Hunt'h prewnted data assembled from various 

sources for the Marquesas and Tuamotu Islands by various 

months from 1957 to 1959 and 1977 to 1980. In most cases, 

there were one or two size modes with the major one at approximately 50 cm and a lesser mode sometimes occurring 

around 70 cm. For the Society Islands, Bayliff and Hunt 

(footnote 16) again presented data assembled from various 

sources by various months from 1973 to 1978. Some of these 

data were collected from the subsistence fishery in Tahiti. 

There appeared to be three modes, which in many cases fell 

roughly in the 40-50, 60-70, and 80-90 cm intervals; the total 

range was 32-91 cm. They also found some evidence of progression of modes which they felt indicated growth, but Josse 

et al. (1979) concluded otherwise. Moving slightly to the east, 

a single mode was found at 45-50 cm for samples taken at 

Pitcairn and Gambier Islands in 1980. For American Samoa, 

Kearney and Hallier (1978~) presented data for May-June 

1978 showing a range of 39-64 cm with modes at about 45 

and 60 cm. The sizes of fish collected in West Samoa in July 

1978 were somewhat smaller, 36-58 cm, and showed only one 

mode around 49 cm (Kearney and Hallier 197%). Moving 

slightly westward into Tuvalu, the length frequency of fish 

collected in June-July 1978 had a range of 35-66 cm and 

exhibited three modes at around 38, 52, and 61 cm (Kearney, 

Hallier, and Kleiber 1978). Slightly to the north in the Gilbert 

Islands (Kiribati), the range of fish taken by exploratory bait 

boat in July 1978 was 38-62 cm, with modes around 40 and 

47 cm (Kearney and Gillett 1978). In Fiji the FA0 bait boat 

development project obtained fish ranging in size from 29 to 

84 cm in 1972, 1973, 1974 (FA0 1974a). Two modes were 

apparent at approximately 46 and 54 cm. Length-frequency 

distributions from the developing purse seine fishery in New 

Zealand (Vooren 1976; Habib 1978; Habib et al. 1980a, b, c) 

indicated a size range of 30-70 cm, but generally there was 

only one strong mode at 45-50 cm with minor modes around 

35 and 55-60 cm appearing in some month/area strata. At 

Norfolk Island, exploratory bait boat catches in March 1980 

showed a size range of 42-68 cm and a mode at around 64 cm 

(Kearney and Hallier 1980a). 

For the southwestern Pacific, Kearney and Gillett (1979) 

reported that off Queensland, Australia, the size range of 

fish taken by exploratory bait boats was 35-70 crn with 

modes at 48 and 62 cm; off New South Wales, the range was 

39-66 cm with modes at 46, 51, and 60 cm. In New Caledonia, 

data from experimental trolling in 1956-60 showed a range of 

40-75 cm standard length (SL) uith a strong mode at 50 cm 

SL and possibly minor modes at 54, 60, 65, and 72 cm SL 

(Legand 1971); whereas exploratory bait boat catches in December 1977-January 1978 showed a range of 38-74 cm, with 

modes around 43, 50, and possibly 55 cm (Kearney and Hallier 

1978a). In New Hebrides data from exploratory bait boat 

catches in December 1977-January 1978 indicated a size 

range of 41-78 cm with modes near 45 and 51 cm (Kearney 

Lewis, and Hallier 1978). In Papua New Guinea, exploratory 

bait boat catches showed a range of 36-66 cm with a mode at 

56 cm (Kearney and Hallier 1979). whereas the size range for 

commercial bait boat catches was 46-69 cm (Wankohski 

1980). For the Japanese purse Feine fishery to the northeayt 

of Papua New Guinea, Honma and Suzuki (1978) presented 

data showing catches ranging from 29-30 to 73-74 cm in 

1970-74 with a single mode ranging from 44 to 53 cm. There 

was no indication of a progression of modes with time. For 

Indonesia, Simpson (1979) presented information on the size 

composition of the bait boat fishery in Ambon, Bitung, 

Sorong, and Ternate. In February 1977 and January-May 

1978, the size range was 30-72 cm with a single mode at 

around 50 cm. A more ambitious sampling project is no& 

being conducted by Indonesia (and Philippines) in cooperation with the U.N. South China Sea Fisheries Development 

and Coordinating Programme and a report on the first year'\ 

sampling (1979-80) will be available shortly. 

For the northwestern Pacific, we will first discuss the Trust 

Territory of the Pacific Islands, then Philippines, Taiwan, 

and Japan. Within the Trust Territory for exploratory Japanese longline operations south of lat. 10"N in 1950-51, the 

size range was from 47 to 82 cm with a single mode at 57 cm 

(Murphy and Otsu 1954). For exploratory bait boat catches 

along a cruise track from the northern Mariana Islands 

through Truk and Ponape to Kosrae in November 1979, the 

size range was 27-60 cm with modes at 30 and 52 cm (Kearney and Hallier 1980b). For the northern Marshall Islands, 

experimental trolling catches had a size range of 34-74 cm 

with modes at 45, 55, and 65 cm (Marr 1948). The size composition of the catches made by the Japanese distant water 

bait boat fleet in an area encompassing the northern Mariana 

Islands and Palau Islands during June 1964-April 1966 shoned 

a range of 35-75 cm (Kasahara and Tanaka 1968). The distributions were multimodal in most months with modes falling 

in the 40-50, 50-60, and 60-70 cm intervals. Progression of 

the smallest mode was quite apparent. Data collected during 

1965 from the bait boat fishery based in Palau Islands indicated a range of 30-74 cm, and a prominent mode at 50 cm 

in June and 58 cm in October (Higgins 1966); whereas data 

collected in 1967 and 1968 showed evidence of at least two 

modes, one between 45 and 50 cm, and the other around 60 

cm (Uchida 1970). Since 1964 the Tohoku RFRL, Japan, has 

been issuing reports on the Japanese southern uater fishery 

(Tanaka undated a, b, c, 1978, 1979, 1980; Tohoku Regional 

Fisheries Research Laboratory undated a, b, c, d), which uas 

initially limited to the area within the Equator and lat. 24"N. 

and long. 125" and 15O"E. Initially, the data were obtained 

only from bait boats: the season usually started in June and 

was over by April of the following year, and the area firhed 

was from the northern Mariana Islands to Palau Islands. 

Now, purse seiners have entered the fishery and the fishing 

area has expanded to well belou the Equator and east to 

long. 150"W. A few generalizations can be gleaned from 

these reports. First, it is apparent that catches from shoala\sociated school1 generail! had a \ingle niodc around 3 cni. 

65 
uhercas open ocean catches were multimodal. Second, purse 

seine catches generally exhibited only two modes around 40 

and 50 cm whereas bait boat catches exhibited several modes. 

Third, there were great variations in the frequency distributions among months, years, and areas as well as variations in 

the amount of fishing effort expended in these timelarea 

strata that make interpretations difficult. 

For the Philippines, length-frequency distributions appearing in the literature have been derived primarily from experimental fishing operations. The troll-caught fish from October 

1947 to November 1948 had a range of 34-65 cm with a single 

mode at 50 cm (Wade 1950). Chikuni (1978) presented lengthfrequency distributions for fish caught by exploratory purse 

seine fishing primarily at night in assxiation with rafts 

(‘‘payao” or “payaw”) from November 1974 to June 1977. 

In Mor0 Gulf, nearly all of the fish taken were between 40 

and 55 cm from January through July. Small fish, 25-35 cm, 

entered the fishery in May and became more prominent in 

October and December. In the Sulu Sea, the size range &as 

30-70 cm in July and a broad mode was centered at 52 cm. 

The size range was much smaller in May and September, 

with modes at 62 and 52 cm, respectively. Evans et al.’7 

reported length-frequency distributions for both small-scale 

and commercial fisheries based on Camiguin Island in the 

Bohol Sea. For handline and various sustenance gear, the 

size range in March-November 1977 was 37-84 cm TL, with 

possibly two modes at 66 and 72 cm TL. For commercial ring 

net boats during the same period, the size range was 18-63 

cm TL, but the sample size was too small to determine modes. 

For Taiwan, Hu and Yang (1972) collected samples in 

fresh fish markets from skipjack tuna caught by trolling off 

the southwest coast between August 1970 and November 

1971. The samples ranged from 23 to 71 cm and exhibited 

modes at 30 and 49 cm. 

For Japan, Kawasaki (1952, 1955~1, b, 1964, 1965b) used 

length-frequency distributions for skipjack tuna from various 

areas in the coastal fishery to separate hypothesized nonmigratory and migratory stocks. The distribution varied considerably among months and areas with respect to both the 

size range and the number of modes. The total range was 

from 25 to nearly 80 cm and a distinct mode appeared in 

every IO cm interval in at least one time or area stratum. 

Modes within the 40-60 cm range occurred commonly and 

often showed a monthly progression in average size. Iwasaki 

(1976, 1980) determined that the size of fish varied with the 

means of capture and whether the school was associated with 

banks, floating debris, birds, and sharks or whales. For fish 

caught in Japanese waters by bait boats, schools associated 

with birds had a size range of 20-80 cm with modes at 32, 

45, 58, 67, and 75, cm; those associated with banks had a 

range of 42-62 cm and a single mode at 49 cm; those associated with floating debris had a range of 25-61 cm and modes 

at 34 and 52 cm; and those with sharks and whales had a 

range of 33-61 crn and a mode at 50 and possibly 40 cm. For 

purse seine-caught fish, schools associated with birds had a 

range of 41-54 cm and a single mode at 45 cm while those 

associated with floating debris, sharks, or whales had a range 

of 4-18 cm with modes at 45, 62, and 70 cm. 

(b) Indian Ocean 

Just as the fisheries are relatively underdeveloped in this 

ocean compared uith the Pacific and Atlantic Oceans, there 

is likewise less information available on the length composition of the stocks. Marcille and Suzuki (1974) summarized 

length statistics obtained from Japanese longline operations 

during the period 1967-71. The size range and modes for 

each area in the Indian Ocean were as follows: South of 

Indonesia, 36-86 cm with a single mode at about 50 cm or 

less; eastern central area, 36-88 cm with two modes at 55 and 

64 cm; western central area, 44-78 cm with a mode at 50 cm 

in the first and second quarters and at 60 cm in the fourth 

quarter; Bay of Bengal area, 38-84 cm with a single mode 

generally above 60 cm; and northwestern area, 42-84 cm also 

with a single, but strong mode usually above 60 cm. Generally there was no indication of a progression of modes with 

time. With the data combined over all areas, two modes at 

55 and 68 cm were apparent. 

For the waters of Sri Lanka, Sivasubramaniam (1965) presented figures showing the size composition by coastal area 

and gear type. For the west coast, troll-caught fish had a 

range of 20 to 70 cm, with a single mode centered at the 50- 

55 cm size class. On the east coast, catches by bait boats and 

trollers had a range of 16 to 70 cm, while for gill net catches, 

the range was 26 to 70 cm. A single mode occurred in the 

51-55 cm interval for all gear types. On the south coast, which 

is the primary tuna fishing area, the size range was 21 to 80 

cm for all gear types, and a single broad mode generally 

occurred in the 41-45 to 46-50 cm interval. For fish taken in 

several areas and by various gear types, Sivasubramaniam 

(1972) found five modal groups at 34-36, 42-44, 52-55, 62-64, 

and 72-74 cm intervals by using the probability paper method 

of Harding (1949) and Cassie (1954). For experimental Japanese bait boat fishing, the size range was 28 to 72 cm with 

modes occurring at 32-34, 44-46, and 58-60 cm intervals 

(Sivasubramaniam 1975). 

For the Indian fishery in the Laccadive lslands from 1958 

to 1961, Jones and Silas (1963) reported a size range of 32-72 

cm. During the December-April period, two modes at about 

48 and 60 cm were observed, while during other months, 

only the 48 cm mode was apparent. The authors stated that 

the samples indicate size specific recruitment to the fishery. 

For the waters off Madagascar, samples collected during 

August 1973-March 1975 had a size range of 32-71 cm and a 

single prominent mode at 49 cm, as well as a less well-defined 

mode in the 50-55 cm range in some months (Marcille and 

Stequert 1976). The authors noted that the smaller mode progressed from < 45 cm to just over 50 cm from September 

1973 to May 1974, but no progression of modes was evident 

in the remainder of the time period. 

(c) Atlantic Ocean 

With the ICCAT emphasizing the description of tuna fisheries and the assessment of the tuna stocks in the Atlantic, a 

number of papers are now available describing size composition of skipjack tuna. For catches by U.S. purse seiners in 

the eastern tropical Atlantic during 1968-74, the size range 

66 
was 25-70 cm (Sakagawa et al. 1976), and a single mode 

usually occurred at 45-50 cm, though at times two modes at 

45 and 50 cm were indicated. For catches by Korean bait 

boats in the same area in 1975, Choo (1977) presented data 

showing the ranges of 33-65, 32-62, 31-75, and 36-54 cm in 

each of the four quarters, or 31-75 cm overall. Quarterly 

average sizes were stated as 49.8, 48.5, 45.6, and 44.1, or 

47.6 cm overall. The size ranges of fish taken by Japanese 

longline boats in the Northern and Southern Hemispheres 

were 44-84 and 48-86 cm, respectively, while the only obvious 

mode occurred at 72 cm in both hemispheres (Kume 1977). 

For Japanese bait b.oat catches in the Gulf of Guinea during 

1974 and 1975, the size range was found to be 37-62 cm, and 

few fish below 40 cm were captured (Kikawa and Higashi 

1979). Then for Japanese purse seiner catches in the same 

area during 1967-74, the size range was greater, 22-75 cm, 

and fish below 40 cm were well represented. The authors 

indicated that recruitment of 30 cm fish occurred continuously throughout the year. Modal groups w'ere not clearly 

defined nor was any modal progression apparent. Experimental U.S.S.R. purse seine catches in the eastern Atlantic 

from 1969 to 1977 consisted of fish with a sire range of 28- 

63 cm and modes occurring in the intervals 36-37, 44-45, and 

52-53 cm (Chur et al. 1980). In waters off Brazil, Zavala 

Camin (1978) reported a size range of 54-78 cm with a mode 

at about 66 cm for fish taken on tuna longline gear in MayAugust 1977. 

Coan (1976) constructed composite length-frequency distributions for fish caught by longline and surface fleets of the 

participating states over the period 1968-73 (Fig. 64). These 

distributions were all unimodal with a peak at approximately 

50 cm. Sakagawa and Murphy (1976) produced length-frequency distributions comparing the United States purse 

seine, French-Ivory Coast-Senegalese bait boat, French-Ivory 

z 4001 1969 400 

6001 , 6001 1 

1972 

11 I I 1 

OL 30 (40 50 60 ' 

FORK LENGTH(cm) 

Figure 64.-Length compoaition of the skipjack tuna catch from the caslern 

Atlantic Ocean (fmm Coan 1976). 

Coast-Senegalese purse seine, and Japanese purse seine fleets. 

In 1969-71, the distributions for all of these fleets were unimodal with a peak at about 50 cm. In 1972-73, each individual distribution was still unimodal, but the fleets were fishing 

different sized fish, thus giving the overall distribution a bimodal character with modes at approximately 40 and 55 cm. 

Length-frequency distributions for specific fleets may be 

found in Pianet (1974). Bour (1976). Sakagawa et al. (1976). 

and Ansa-Emmim (1977). For the northwestern Atlantic, 

Batts (1972a) reported a size range of 26-76 cm for fish taken 

by charter sportfishing boats off North Carolina, U.S.A., in 

the summers of 1964 and 1965. He presented length frequencies by age groups (up to five) based on examination of 

growth rings in the dorsal fin spines, a method generally 

considered least reliable by recent investigators (see Section 

3.43). 

(2) Size at first capture 

The size at first capture of fish amounting to significant 

numbers in most years in most fisheries, is about 40 cm. Recruitment into the fishable stocks generally takes place at a 

smaller size, around 30 or even 25 cm for many fisheries 

(Kawasaki 195Sa, 1964; Sivasubramaniam 1965, 1972; Hu 

and Yang 1972; Iwasaki 1976, 1980; Marcille and Stequert 

1976; Sakagawa et al. 1976; Choo 1977; Habib 1978; Honma 

and Suzuki 1978; Kikawa and Higashi 1979; Habib et al. 

1980a. b, c; Tanaka undated a, b, c, 1978, 1979, 1980; 

Tohoku Regional Fisheries Research Laboratory undated a, 

b, c, d). For the ring net and purse seine fisheries in the 

Philippines, the size at first capture is around 25 cm and the 

size at recruitment is around 15 cm (Evans et al. footnote 17; 

Choo'n). 

(3) Size at maturity 

See Section 3.12. 

(4) Length-weight relationships 

Length-weight relationships have now been published for 

all oceans (Table 17), but they are not available for all fisheries. Neither have the papers cited below included studies of 

sexual, seasonal, or long-term variations. 

All of the studies cited in Table 17 fitted the allometric 

growth equation W = 6L" where W is weight in grams, kilograms, or pounds, L is usually fork length in centimeters or 

millimeters, a is the coefficient of allometry, and 6 is another 

constant. In all but one of the cited studies, the log-linear 

transformation of the equation, 

log,W = log)^ + a logrL 

was used to estimate the parameters using predictive (linear) 

regression analysis. Today it is generally believed that the 

length-weight relationship is intrinsically nonlinear, meaning 

67 
lahk 17.--1 rngth-uciaht relatinnrhip\of $kipjack tuna. 

Calculated 

ho ’we \reteht of 

of range Con~iant 01 allometr) Viephi Length 55 em iish 

Coeilicleni 

S<,Llrcc 1 (ICBIIt\ tIIh (en11 h a unit unit (kp) C omnienti 

20 10.60 11.01 I3 

2hR 3’)-x3 I.X7X* IO 

YZJ iy-71 i ~S:X in’ 

1.282 19-71 3 763 x IO ’ 

1x9 34-66 3.267 x IO 

I.2YX 33-xx 4 546X 10~’ 

M4 16-76 2 5mx 10.” 

2.554 36-64 5 hlI x 10‘ 

520 411-73 J I IS x io 

x4x 41-62 1 lil x 10 

lix) 3s-SJ 6 ?Oh x 10.’ 

200 JX-X’ 5.‘00, 10 a 

4.1 17~62 3 727x 10 

I20 38-71 3 475X 10.” 

3.67 e 

3.16 p 

3.2164 Ib 

3.403 lb 

3 265 Ib 

3.0569 Ih 

2 96482 Ib 

3.36836 Ih 

3 3533 kg 

3.31497 ke 

3 409 kg 

3.15X ke 

3.19 e 2 20 kg 

I 70 kg 

3.29 e 

cm Ft~ 

cm FL 

em PL 

mm FI. 

mm FL 

cm TL. 

em TL 

mm FL 

mm FL 

em FL 

em FL 

cm FL 

mm FL 

cm TL 

CIII TL 

mm Ft 

149 9 Column, 4and Y ?\timated Sroni Lda’\ 

Tip. 6. Kzgression 

~tatt\iu w\pv:t. 

3.57 Bajt hoat catrhei. 

3 31 Sue ranprcalculaied. 

3.50 Bait boat and purse 

seine catche\ 

3.58 Includes data from 

Chatuin 11959) 

3.40 Malesonl) Data from 

Ronquillo‘5 fig. 25 and 

regrewon equations 

3.33 Female\ onl?. Data from 

Ronquillo’s he. 25 and 

regrexsnn equations. 

3.50 Bait hoat catches. 

3.33 Troll carchec 

3.30 Lop linear reeresion 

anal)\,? 

3.35 Linear reerewon 

analy\ts. 

3.54 Ban boat catcher. 

3.35 Punt wne catchec 

3.71 Handlinecalche5. 

3.39 Artisanal tichine 

sears 

3.60 Pune seine carche,. 

that parameters estimated from a log-linearized relationship 

are biased. Corrective procedures are available, but none 

were employed in the papers cited. In addition, Ricker (1973) 

has pointed out that the predictive regression method (uhich 

asumes a dependent-independent variable situation) tends to 

produce inflated estimates of a, especially when the sample 

range truncates that of the tru: population; a geometric 

mean functional (reduced major axis) regression (which does 

not assume a dependent-independent variable situation) producer less biased results. Pianet (1974) used the latter procedure but performed it on log-transformed data. 

A summary of these studies is presented in Table 17, and a 

typical curve descrihinx the relationship is given in Figure 65. 

The table is an updated venion of a similar table presented 

by Nakamura and Uchiyama (1966), and the figure, based on 

data from the central Pacific containing the largest sample 

tize and widest size range of all studies cited, was taken from 

the same reference. The calculated weights of fish of a common size, based on the regession staristics of the various 

authors, show close agreement except for that obtained by 

Uda (1941). The estimates of the coefficients of allometry 

are amazingly similar, except for those published by Ebans et 

al. (footnote 17), and no effect of ocean or gear type is 

apparent. 

T 

FORK LENQTI IYY ) 

Figure 65.-I,ength-~eight relation of skipjack tuna: lug Heieht = ~ 8.34241 + 

3.36836 log length lparameters not corrected for tran\formation bia3): number 

of observations. 1.298: size range. 327-877 mm: data collection period F-ebruar) 

1948-December 1960 irom filer of Southwest kirheries Center lfmm Vakamura 

and Cchi)aina 1966). 

5 

68 
4.2 Abundance and density (of population) 

015 

010- 

Since some fishing effort directed primarily at yello\vfin 

tuna is included in the calculation, this index is biased. Several 

attempts have been made to improve the index. Calkins (IY61) 

and then Joseph and Calkins (1969) calculated relati\e density 

after eliminating those areas of low skipjack tuna catch \\here 

the fishing effort was directed primarily at yelloufin luna. 

The resultant index of relative abundance again did not shoa 

a long-term trend over the 1951-65 period. RelatiLe density 

was highest in the third quarter, and there appeared to be no 

relationship between the indices in the areas north and south 

of lat. 15"N. Subsequently, Pella and Psaropulos (1975) adjusted the fishing effort statistics by simulating the effect of 

the following factors on the fishing power of vessels: Vessel 

speed, probability of capturing sighted tuna schoolz, time 

spent in a set, and portions of schools retained that are set 

upon succestfully. Using a new standardized fithing effort 

statistic based on their simulation, they found a different history of relative stock abundance over the years of the fishery 

(Fig. 68). There seems to be some indication of a decrease in 

abundance of skipjack tuna in the northern grounds and a 

definite decrease in the southern grounds. It is not knoun at 

this time which index of abundance is the better descriptor of 

the stock(s). Later, the IATTC computed relative abundance 

using only those 5' latitude by 5 O longitude areas and 3-mo 

periods where 200 short tons (181 t) or more of skipjack tuna 

were caught and 100 or more standardized fishing days were 

expended. This procedure retained 89% of the catch and 

eliminated 43% of the purse seine fishing effort in 1961-79. 

The resulting indices of fish density for the years 1951-79 

(Fig. 69) show no clear trend in the areas north of lat. 15"N 

and between lat. 15" and 5"N, whereas south of lat. 5"N the 

trend has a convex shape (Forsbergh 1980). 

For the bait boat fishery in Hawaii, a number of estimates 

of relative abundance have been made. These indices, measured in catch per trip or per effective trip (as in Fig. 70), 

are highly correlated with catch and show no long-term trend 

(Yamashita 1958; Shippen 1961; Uchida 1967, 1976). A 

, , , , , , , , , , , , , 

, SOUTHERN AREA 

0-0 ALL CL1SSES INCLUDED 

\ ', - CLASSES 3 a+ OMITTED - 

... CLASS 3 OUlTTED 

(I) Pacific Ocean 

Forsbergh (1980) has recently reviewed the history of the 

measurement of apparent abundance of skipjack tuna in the 

eastern tropical Pacific. Shimada and Schaefer (1956) produced the first measure of relative density by comparing the 

daily catch rates of bait boats standardized by vessel class. 

They concluded from the trends shown in Figure 66 that the 

fishery was having no measurable effect on the stock. Since 

purse seine vessels have almost entirely replaced bait boats in 

the fishery, the measure used today is reported catch divided 

by reported fishing effort standardized to class 3 purse seine 

vessels (101-200 short tons or 94-181 t of carrying capacity). 

For a later period, 1960-79 (Fig. 67), the index in terms of 

purse seine units shows considerable annual variation but 

again no discernible trend. 

I 

Figure 66.-Catch per standardized-da? fished, total catch. and calculated relative fishing intensit) For skipjack tuna in the ea9tern Pacific Ocean, 1934-54 

(from Shimada and Schaefer 1956). 

1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 

I I 

'1 t 

it6 

19TC 1971 1972 1973 1974 1975 1976 1977 1978 1979 

Figure 67.-Monthl) and annual catch per standard da)'a fishing (CPSDE) for 

skipjack tuna. in class 3 purse seine units, in the eastern tropical Pacific during 

1W-79 (from Forsbergh 1980). 

04 biL CLASSES INCLUDED 

NORTHERN AREA CLASS 3 OMlTTED 

CLbSSES 384 OMITTED 

y 005 

oca 

Figure 68.-Annual hiomn\\ indices of Aipjarh tuna in the Irlditional fi4iinu 

arw\ oP the eastern Pacific Orran wing different ramhinationr nt PI da*w*. 

1960-72 (from Pella and Pvaropaln\ 1975). 

69 
R 

z 

C6 

W 

z 

L" 

m5 

i2 

5 

(1: 

a 

I 

3 

Figure 69.-Indice\ 01 abundance of *kipjack tuna (in ton, per da)', fkhing 

standardized lo class 4 hail hoats and c1a11 3 purw \einer\l for lhree area5 nf the 

eastern Paritic Orean (from Eor\hcrgh 198111. 

-- 7- 

VI 

L1 

28 N 

IE 

12 

14 

24 4 

2 

- TOTAL CATCH 

CATCH/STANDARD DAY FISHED 

26 0 

LL 

v) 

122 z 120 2 

- 

t 

+ 

r 

I8 

I 

Y 

- 

16 - I 

0. 1950 1955 1960 1965 1970 

YEAR 

Figure 7O.--Total calch. calrh per slandard da! fished. and Ihe relative 

fishing inlensits of skipjack luna laken in Ihe Hawaiian fisher?. 1948-70 

(from Uchida 19761. 

marked seasonal trend is quite apparent with the highest 

values occurring in May-September (Shippen 1961). 

For the purse seine fishery in New Zealand, relative abundance in terms of catch per set, catch per effective set, catch 

per day, and catch per season-day shows no long-term trend 

over the 1975-76 through 1979-80 seasons (Habib 1976, 1978; 

Habib et al. 1980a, b, c). As in Hawaii, there is a marked 

seasonal trend, but with the highest density occurring in the 

January-February period. 

In Papua New Guinea for the locally based bait boat fishery, Wankowski (1980) presented relative abundance indices 

showing the highest values occurring in May-September and 

the lowest values occurring in December-February. No longterm trend for the period 1970-79 was apparent. 

In the Japanese southern water fishery (routh of lat. 24"N 

and from long. 125"E to 150"W), catch per vessel increased 

both for the 1958-68 period in the Bonin-northern Mariana 

Irlands area (Iwasaki 1970) and for 1964-75 over the entire 

fishery (Kasahara 1977). Hornever, this trend probably reflects the evolution to larger boats in the tleet rather than 

changes in relative population density. While catch per day 

for some areas in the southern water bait boat fishery (Fig. 71) 

has shown a declining trend (Kasahara 1971), a similar index 

derived from Japanere purse seine operations in tropical 

waters shows no such trend (Honma and Suzuki 1978). 

For the bait boat fishery off the northeast coast of Japan in 

1951-62 (Kawasaki 1964) and off the south and southwest 

coast in 1957-69 (Kasahara 1971), no trend in relative abundance was evident. The same was true for the purse seine 

fishery in coastal waters from 1971 to 1974 (Honma and 

Suruki 1978). However, Kasahara (1975) showed that the 

relative density has declined with increasing fishing effort in 

the Japanese coastal fishery during the period 1958-72. There 

is a regular seasonal cycle in relative abundance with the 

highest values occurring in June-August (Anraku and Kawa- 

. Lastly, lshida (1975) showed that there was no 

~ 

01 1963 1964 I965 1966 1967 1968 1969 

15'-24'N 

0, 147'-155'E \ 

'0 

5"s- IO"N \ 1350-1550E \ \ 

141'-147'E 

Figure 71.-Indices of relative abundance for the fishing 5eason Ma)- 

October of skipjack luna in the Japanese southern water fisher). 1963-69 

(from Kasahara 1971). 

0.2 I 

Figure 72.--Catrh per da! for Japanese bait hoah of various size classes in the 

weCtern Pacific (from lshida 1975). 

70 
trend in the catch per day of Japanese bait boats from 1957 

to 1973, apparently over all western Pacific fishery grounds 

(Fig. 72). 

8 60 

9 

-. 

I U 

% 40- 

(2) Atlantic Ocean 

A number of estimates of relative abundance have been 

determined using data from the surface fishery in the eastern 

tropical Atlantic. As is the case with the eastern tropical 

Pacific fishery, this is a two-species fishery and similar problems exist in separating fishing effort directed at skipjack and 

yellowfin tunas. For the local bait boat fishery in Angola, de 

Campos Rosado (1971) presented data showjing a steady 

index of relative abundance for both species combined, while 

the index for yellowfin tuna has declined proportionally to 

the increase in the index for skipjack tuna. The relative density derived from purse seine fishing showed an increase in 

the last half of the year for both United States and Japanese vessels (Sakagawa 1974; Kikawa and Higashi 1979). 

However, no consistent seasonality in the catch per day statistics for Japanese bait boats was apparent (Kume 1978; 

Kikawa and Higashi 1979). None of these studies, nor Fonteneau and Cayre's (1980) study on the French-Ivory CoastSenegalese purse seine fleets, found a long-term trend in relative abundance. However, a study by Pianet (1980) utilizing 

data from French-Ivory Coast-Senegalese, United States, and 

Japanese bait boat and purse seine fleets did find a slight decline in relative density over the 1969-78 period (Fig. 73). 

tI 

7" t 

i6 

12 

has been voiced about recruitment into the population(s), but 

little actual research has been reported in the literature. Recently Forsbergh (1980) described work carried out on this 

subject by the IATTC in the eastern tropical Pacific. An 

index of relative abundance of young skipjack tuna in the 

fishery was tested against three factors: Sea surface temperature in an hypothesized spawning area in the central tropical 

Pacific (long. 130°W-180") approximately 1.5 yr earlier, 

barometric pressure differences at stations in the Indian and 

Pacific Oceans, and a mixing index in the spanning area. 

While a significant relationship was found between relative 

abundance of skipjack tuna in the fishery and temperature 

in the central tropical Pacific (IATTC 1972, 1973), no obvious biological explanation for this relationship was apparent. 

Subsequently it was found that the temperature variations in 

the spawning area were correlated with the southern oscillation index. This index, which is defined as the difference 

between sea-level atmospheric pressure at sites located in the 

Indonesian equatorial low region and those in the South 

Pacific subtropical high region, has been used to study largescale variations in meteorological and oceanographic conditions occurring over the equatorial Pacific, including the El 

Nix0 phenomenon off the northwest coast of South America 

(Quinn 1976, 1977). In turn, the southern oscillation index 

has been found to be related to wind conditions in the central equatorial Pacific. During the same period Lasker (1975, 

1978, 1981) showed that the survival of anchovy larvae in the 

California Current was dependent in part on the availability 

of sufficiently concentrated food and that this occurred with 

stable structuring of the upper mixed layer of inshore waters. 

During periods of high wind or strong upwelling, this stable 

structuring is destroyed and food organisms are dispersed 

throughout the upper mixed layer resulting in too low a concentration for survival of anchovy larvae. Hypothesizing a 

similar phenomenon with respect to the survival of skipjack 

tuna larvae in the central equatorial Pacific spawning area, a 

significant correlation was found between the index of abundance of fish between 12 and 24 mo of age in the eastern 

tropical Pacific fishery and an index of mixing (the curl or 

cube of the wind speed) in the central Pacific approximately 

1.5 yr earlier. While the relationship is statistically significant, 

it accounts for only 47% of the variation and is thus a poor 

predictor of catch. 

4.4 Mortality and morbidity 

Most estimates of mortality coefficients appearing in the 

literature have been calculated for the stock in the eastern 

tropical Pacific. Using tagging data from 1957, Fink (1965) 

estimated the total mortality coefficient Z to be 6.98, the 

fishing mortality coefficient F to be 2.10, and, using a value 

-:i

0 10 20 30 40 50 60 70 

STANDARDIZED EFFORT I 10" 

of 1.06 for mortality due to tagging from work on yellowfin 

tuna, the coefficient of natural mortality M to be 3.82, all on 

an annual basis. Joseph and Calkins (1969). using tagging 

Figure 73.-Relation between ratch and CPUE \ersus standardized effort 

in the eastern Atlantic (m = 2. K = 1) (from Pianet 1980). 

data from a later period, estimated M on an annual basis as 

1.68. Again utilizing more tagging data for the eastern tropical Pacific, Bayliff" estimated 2 to be 0.58, F to be 0.35, 

4.3 Natality and recruitment 

various factors relating to natality were discussed in settion 3.1. Reproduction, and more specifically fecundity, was 

covered in Section 3.15. Recruitment into a fishery, in terms 

of migration, was discussed in Section 3.51. 

Since recruitment into the established and major fisheries 

around the world is extremely variable, much speculation 

"Baylilt. W. H 1977. Estimair\ 01 ihs raic~ (>I nmialii~ (11 *hipi.wh lu11~1 111 

the eastern Pacific Ocean denled from iaepiiig s\prrinirni*. InIcI--\Ill rinp 

T~~~ ~omm.. lnlernai R~~. IO. 59 

C/O Scrippc Insitrution olOceaii<,raph!, I ii lolla. C')t 92017 

Inter-Anwrtcari T~O~IC'II TU~M < WWWWW. 

71 
and M to be 0.23, but in this case, on a monthly basis. Using 

a completely different approach, Sillirnan (1966) simulated a 

catch history for the eastern tropical Pacific fishery by allowing M to vary from 0.2 to 0.7 and holding Z constant at 0.8. 

The best fit was obtained with M at 0.3, but this method 

does not provide a unique solution. Most authors indicated 

that their estimates should be considered as preliminary and 

acknowledged that the estimates of M included emigration 

and losses due to tagging. Considering the lifespan of skipjack tuna to be about 5 yr, Murphy and Sakagawa (1977) 

stated that the published estimates of M were either too ION 

(0.3) or too high (1.68 and 3.82). 

Using tagging data for 1973-74 in Papua New Guinea. 

Lewis (1980) estimated 2 on a monthly basis as being equal 

to 0.257 overall and 1.285 when the stock was exposed to the 

fishery. Lewis also pointed out that the estimates of 2 included emigration. 

4.5 Dynamics of poQulation (as a whole) 

Assessments of the state of the stocks are currently available for the Atlantic and the eastern Pacific, while estimates 

of potential production are available for the western Pacific 

and the Indian Oceans. Most of the results must be viewed as 

preliminary because of the highly variable relationship between catch per unit effort and effort, as well as the low reliability of estimates of population parameters. 

(I) Atlantic Ocean 

Two estimates of the potential yield are available. First, 

Shomura (1966b) argued that since the Atlantic Ocean is half 

the size of the Pacific Ocean, the potential yield for the Atlantic should be half that for the more well-developed fisheries in the Pacific Ocean. Using a catch of 234,100 t in 1962 

for the Pacific, he estimated the potential yield for the Atlantic to be 117,000 t, whereas using the largest reported catch 

for the Pacific, 662,000 t in 1978 (FA0 1980), results in an 

estimate of 331,000 t for the Atlantic. The harvest reported 

for the entire Atlantic Ocean was 87,714 t in 1979 (FA0 

1980). [Japan.] Fisheries Agencyzo estimated a total production of 100,000 t for the North Atlantic and 100,000-150,000 

t for the South Atlantic. The method was based on the abundance of skipjack tuna larvae compared with that for other 

commercial tuna species. 

A yield-per-recruit analysis of the eastern tropical Atlantic 

fishery by Hayasi (1974) indicated an optimum size at first 

capture of 0.6-1.3 kg (about 33-40 cm). Hayasi concluded 

that there was no evidence that skipjack tuna in the Atlantic 

Ocean was fully exploited. Sakagawa and Murphy (1976) examined the relationship between total catch and normalized 

catch per unit effort for several gear types in the eastern 

tropical Atlantic. They concluded also that the stocks were 

not being fully exploited since there was no consistent trend 

in the data. Recently, Pianet (1980) found a slight but apparently significant decline in catch per unit effort versus 

effort for standardized French-lvory Coast-Senegalese, United 

States, and Japanese bait boat and purse seiner data from 

1969 to 1978 (Fig. 73). A production model fitted to these 

:"[Japan.] Fisheries Asency. 1%8. On the powbiticy 01 developing rnarlnr fisher) resources. Information, cited in Kawasaki 1972. 

data resulted in an estimate for maximum sustainable yield of 

92,500 t for the eastern tropical Atlantic. 

(2) Pacific Ocean 

In the eastern tropical Pacific, Joseph and Calkins (1969) 

estimated yield per recruit, using the Ricker (1958) model, 

for both the northern and southern skipjack tuna fisheries 

(Fig. 74). Their model assumed that all individuals in the 

stock make two passes through the fisheries, and it included 

von Bertalanffy growth parameters and mortality estimates 

derived in the same paper. From Figure 74, it can be seen 

that the yield per recruit for the northern fishery could be 

increased only slightly by a doubling of the fishing effort (a 

multiplier equal to 1.0 represents the level of fishing mortality 

in 1963), while that for the southern fishery could be increased 1 to 1.5 times by increasing fishing effort 2 to 2.5 

times. 

Rothschild (1966) and Silliman (1966) both estimated the 

potential yield for the combined eastern Pacific stocks if 

they were harvested beyond the range of the existing fishery. 

Using the Beverton and Holt (1957) yield per recruit model 

and lengths of time exposed to the existing coastal fishery of 

2 and 6 mo (Fig. 75). Rothschild estimated the yield could be 

increased between 2 and 17 times the then existing catch. 

Silliman (1966) estimated the potential yield using a population simulation model which held the total mortality coeffiFigure 74.-Yield per recruit of skipjack luna at age of entry into the northern 

fishery (lop) and southern fi$her) (bottom). eastern Pacific Ocean, using yon 

Benalanff) growth parameters from ungrouped data (VBCF 1) and grouped 

data (VBGF 2) (from Joseph and Calkins 1969). 

72 
z 

Figure 75.--Values of R, (ratio of yields under nhole-life exploitation to lield 

under limited-life exploitation) for skipjack tuna. as a functinn of X belween I 

and 2. lpper curve is for assumed sojourn time of 2 mo, lower curve for 6 mo 

(from Rolhschild 1966). 

cient Z constant at 0.8 and allowed the natural mortality 

coefficient M to vary from 0.2 to 0.7. Maximum sustainable 

yield was estimated as 225,000 t with M equal to 0.3 (Table 

18). If Rothschild's (1966) estimate of maximum length of 

skipjack tuna is conservative and if the escapement stock 

from the eastern Pacific fishery is only a segment of the total 

fish available in the central Pacific, both of which are likely, 

then both Rothschild and Silliman underestimated the potential yield from the central Pacific. While the results from 

both these authors suggest that yield could be increased by 

harvesting the escapement stock from the coastal fishery, 

Joseph and Calkins (1969) observed that yield per recruit 

could be increased by harvesting fish younger than were then 

available to the fleet in the eastern tropical Pacific. 

Table IX.--Ealimale~ of sustainable yield for the ea\tern-central 

Pacific stock of skipjack tuna at 11of0.03 and al selected \aluc5of 

F(from Silliman 1966). 

~~ ~ 

Effort 

1,ooO hoatAkerage days from 

sustainable yield = 

F (1.wOt) 4 Tonc/boar-day 

0.5 180 33 5.5 

0.8 220 53 4.2 

0.9 220 60 3.: 

I .O 225 67 3.4 

1.1 215 73 2.9 

t4 = 0.01501 from F= 0.5 and,/ = 33.3. for 1952-56. 

0.6 200 40 5.0 

[Japan.] Fisheries Agency (footnote 20) estimated potential 

yields for various oceans and subareas based on the abundance of skipjack tuna larvae and juveniles and their occurrence in the stomachs of predators. These estimates are: 

Japanese coastal area 

Entire Pacific Ocean 

Increase of 200,000-400,ooO t 

lncrease to 800,000-1,~.ooO t 

Indian Ocean 200,000-300,OOO t. 

Suda (1972) elaborated on the above procedure by saying 

that since about twice the number of skipjack tuna larvae 

were taken in plankton tows as larvae of all other commercial 

tunas, the spawning stock of skipjack tuna was also roughly 

two times that of other commercial tunas. Therefore, the potential production of skipjack tuna should be at least two 

times more than that of all other commercial tunas. Both 

Kawasaki (1972) and Matsumoto (1974) noted that Suda's 

projections of adult skipjack tuna catch from larval abundance was oversimplified. By citing differences in fecundity 

and average body sizes between skipjack tuna and other 

tunas, Matsumoto showed that the conversion ratio of 2:l in 

favor of the skipjack tuna was not logical. One other irnportant factor not taken into account by Suda was the difference 

in the efficiency of the different types of fishing gear used in 

taking the various species of tunas and the effect this would 

have on the conversion ratio. 

4.6 The population in the community and the ecosystem 

4.61 Physical features of the biotope 

of the community 

Skipjack tuna inhabit the epipelagic zone of the worlcfs 

major oceans and apparently only make forages into deeper 

zones. As such, they generally live in the upper mixed layer 

where temperature and salinity variations are small and 

where oxygen is at or near saturation. Latitudinal and longitudinal variations in temperature and depth of the mixed 

layer determine to a great extent the distribution of the several 

races of skipjack tuna. The greatest concentrations of fishery 

vulnerable skipjack tuna are limited to the temperature range 

17"-28"C. The major water masses and their associated currents also influence the distribution of the species, not only 

through drift of the larvae and possibly the adults as well, 

but also through the availability of food associated with current boundaries and areas of upwelling. Except for these 

latter areas of higher productivity, the epipelagic zone is 

characterized by low productivity. 

4.62 Species composition of the community 

The most important or obvious members of the community 

are the permanent members of the nekton that feed and reproduce in the epipelagic zone. These include some sharks, 

Lamna nasus, L. ditropis, Isurus glaucus, Cetorhinus maximus, Prionace glauca, Pterolamiops longitnanrts, and others: 

many flying fishes, Exocoetus volirans, E. obtusirostris, E. 

monocirrhus, Hirundichthys speculiger, H. ronde!etii, H. 

albimaculatus, Prognichthys gibbifrons, P. sealei, and others; 

all the Scomberesocidae; several Scombridae, Casterochisma 

melampus, Allothunnus fallai, Thunnus alalunpa, T. albacares, 7. obesus; marlins, Tetraplurus audax, T. albidus, T. 

angustirostris, Makaira ~i~qricans; swordfish ~ Xiphias gladius; 

the dolphin, Coryphaena eyuiselis; opah, Larnpris regius; all 

the Bramidae; many Nomeidae and Centrolophidae, Nomeus 

albula; species of genera Psenes, Cubiceps, Schedophiliis, 

Centrolophus, and Ichichthys; Molidae; and some others. A 

second group of nekton includes those fish that spend their 

adult stages in the epipelagic zone but whose egg and juvenile 

stages are found elsewhere. These include the whale shark, 

73 
Rhincodon typus; the dolphin; bluefin tuna, Thunnus thynnus; some halfbeaks (Family Hemiramphidae) of the genus 

Euleptorhamphus; needlefishes (Family Belonidae) of the 

genera Platybelone and Ablennes; and flyingfishes (Exocoetidae) of the genera Cypselurus and Cheilopogon. A third 

group includes the eggs, larvae, and juvenile stages of fishes 

that inhabit the neritic (pelagic and littoral zones) and the 

abyssal zones. Among these are some Atherinidae, Scombridae, 

Carangidae, Gonorhynchidae, Synodontidae, Holocentridae, 

Stichaeidae, Mullidae, Scorpaenidae, Hexagrammidae, Cottidae (Hcmilepidotinae), Nototheniidae, Chaenichthyidae, 

Acanthuridae, Bothidae, Cynoglossidae, Pleuronectidae, 

Gonostomidae (Cyclothone, Vinciguerria), anglerfishes 

(Ceratioidei), and apparently some lanternfishes (Myctophidae). Also present are some mesopelagic species that migrate 

into the epipelagic zone during the night. These include the 

sharks, Euprotomicrus bispinatus and probably Isistius 

brasiliensis; the snake mackerel, Gempylus serpens; subsurface lanternfishes of the genera Myctophurn, Symbolophorus, 

Centrobranchus, Gonichthys, Tarletonbeania, Hygophum, 

Loweina; a few members of Families Melanostomiatidae 

(Photonectes, Echiostoma); and Astronesthidae (Astronestha); and others (Parin 1968). 

4.63 Interrelations of the population of the 

species in the community and ecosystem 

Waldron (1963) has adequately described the position of 

the skipjack tuna as follows: "Within the marine community 

the skipjack may be listed both as prey and predator, although 

as a large adult it may be only one step removed from a 

climax predator. In its larval and juvenile stages it serves as 

food for larger fish, including adult skipjack. As an adult it 

serves as food for larger tunas and spearfishes, and at the 

same time preys upon small to moderate-sized pelagic crustaceans, molluscs, and fish. Skipjack compete for food with 

tunas and other fish of similar size and habits. They might 

even be considered as competing with sea birds, for the food 

organisms are often apparently driven to the surface of the 

water, where they are preyed upon by both skipjack and 

various sea birds." 

4.64 Hypothetical habitat of skipjack tuna 

Metabolic and physiological properties of skipjack tuna in 

relation to the environment have been discussed in Section 

3.63. 

5 EXPLOITATION 

5.1 Fishing equipment 

The two most productive means of catching skipjack tuna 

on a wjorldwide basis have been pole-and-line and purse 

seine fishing. Other methods include gill netting, trolling, 

trap fishing, harpooning, and beach seining. Recently the importance of flotsam or man-made aggregation devices has 

increased greatly. Likewise, aerial spotting has become common in some fisheries, and the application of remote sensing 

is being investigated. 

The pole-and-line, bait boat, or live bait fishing method is 

or has been employed by Japanese fishermen throughout 

the Pacific and off both coasts of Africa, and by the United 

States, Ecuadorian, Maldivian, Korean, and Indonesian 

fishermen, to namc only the most obvious instances. Generally, the gear consists of a bamboo pole varying from 3 to 6 

m although fiberglass is sometimes used now. The length of 

the line varies depending on the distance of the fishing location from the sea surface. Generally, a barbless, artificial lure 

is used which is designed to imitate either a baitfish or a 

squid. The design of the bait boats varies considerably depending on the nationality and much effort has been expended in recent years, particularly by the Japanese, in 

modernizing the boats. Cleaver and Shimada (1950) and 

Yoshida (1966) provided descriptions of the boats that were 

in use a number of years ago in Japan and the United States. 

Descriptions of 200 and 250 gross tons (GT) Japanese bait 

boats appear in Kearney and Hallier (1980b). More modern 

bait boats are equipped with baitwells with recirculating or 

even refrigerated water, automatic fishing poles, and fish 

finders. 

Because the Japanese style bait boat carries more fishermen than other types (over 35 compared with as few as 6) 

and, thus, experiences problems in obtaining enough qualified 

fishermen, the Japanese have been experimenting with automatic gear, such as a continuous circulating mainline with 

hooks on short droppers (Inoue 1966; Igeta et al. 1978) and 

mechanical pole machines (Iwjashita et al. 1967; Suzuki 

Tekkojo Kabushiki Kaisha"). The automatic pole machines 

are now used on 80% of the Japanese bait boats larger than 

200 GT, but not on the narrow but highly productive bow 

area (Maruyama 1980). These machines have also been tried 

in Australia, as well as in Hawaii and the west coast of the 

United States. 

As the name implies, bait boat fishing includes the chumming of live bait to attract and hold tuna schools close to the 

vessel. Thus, bait boat fishing is essentially a double fishery, 

where suitable baitfish resources must be found, captured, 

hardened for future use, and transported to the fishing 

grounds in baitwells of the fishing boats. In addition to these 

technical aspects of baitfishing, the ability of baitfish stocks 

to withstand harvesting pressure has been investigated by 

Bayliff (1967) for the eastern tropical Pacific, Wetherall 

(1977) for Hawaii, and Dalzell and Wankowski (1980) for 

Papua New Guinea. Because of the limited availability of 

natural baitfish supplies in small island areas, some interest 

has been shown in raising or culturing baitfish (Baldwin 

1975, 1977; Gopalakrishnan 1979). A workshop on baitfish 

has covered many of these subjects (Shomura 1977). 

Advantages of bait boat fishing are the relatively small 

capital investment involved, ability to harvest small schools 

of fish, ability to operate out of small ports with a minimum 

of technical support, and ability to utilize unskilled labor, 

The major disadvantage is that it is a double fishery. Other 

disadvantages include the size of the fishing crew, dependence on the biting behavior of the tuna schools, and dependence on good weather conditions to find and fish tuna 

schools. 

"Suruki Tekkojo Kabushiki Kaisha. 1970. Report on the development of an 

automatic Tkipjack fishing machine. From a mimeographed report by the Suruki 

Tekkojo Kabushiki Kaisha (Suzuki Ironworks Co.. Ltd.). Ishinomaki, Miyagi, 

Japan, Sept. 30, 1970. [Engl transl. by T. Otsu. 1970. Southwest Fnh. Cent. 

Honolulu Lab., Natl. Mar. Fish. Sew, NOAA. P.0 Box 3830, Honolulu. HI 

968 12.1 

74 
Purse seine fishing for tunas began in the eastern tropical 

Pacific in the 1950’s when many of the bait boat clippers 

began converting to purse seine gear (McNeely 1961). The 

development of the Puretic power block. which lifts the net 

with its float line and purse rings, thus enabling the net to be 

stacked on the deck for the next set, made this possible. 

Technical aspects of the purse seine are described in McNeely 

(1961) and Green et al. (1971). Newer developments include 

lighter fabrication of the net to allow for greater length and 

depth and other modifications to increase the survival of 

porpoise (Coe and Vergne 1977). The purse seine net is basically a long net that is set around a school. The ends are 

brought together and the purse line is drawn until the bottom 

of the net is pursed or closed. While the first U.S. purse 

seiners in the eastern tropical Pacific were as small as 130 GT, 

they now are much larger, commonly around 1,ooO GT and 

as large as 2,000 GT. Modern purse seiners carry a full complement of electronic gear including fish finders and satellite 

navigation equipment. Similar vessels are now being built in 

several American countries (e.g., United States, Mexico, and 

Peru), Spain, France, Poland, and Japan. Fishing operations 

have expanded from the eastern tropical Pacific to the western Pacific including off Japan, Trust Territory of the Pacific 

Islands, Philippines, Papua New, Guinea, and New Zealand. 

The more successful boats in the open ocean or small island 

areas of the western Pacific tend to be smaller, averaging 

around 500 GT, than in the eastern tropical Pacific. A substantial purse seine fishery takes place in the eastern tropical 

Atlantic, and the principal countries in the fishery are France, 

Spain, United States, Japan, and U.S.S.R. Exploratory fishing has taken place in the Indian Ocean. 

Both bait boats and purse seine operations have rraditionally depended on the spotting of free-swimming, bird-associated, or flotsam-associated tuna schools from the bridge or 

Crow’s nest of the fishing boats. Many large purse seine boats 

now carry helicopters to search for fish schools, and landbased airplanes are commonly used in some fisheries, e.g., in 

New Zealand (Bell 1976; Habib”). Attempts are now’ underway to use remote sensing of sea surface temperature gradients from satellites or aircraft as an indication of likely fishing areas (Evans 1977: Stevenson and Miller 1977; Eggleston 

and Paul 1978; Noel 1978; Petit”,”; Petit et al. ”). 

While the association of skipjack tuna schools with tlotsam (including whales and large sharks) has been known for 

some time (Inoue et a!. 1963, 196% b; Greenblatt 1979) and 

exploited by both bait boats and purse seiners, the new purse 

seine fishery in the equatorial western Pacific is very depen- 

%. Habib, Forum Fnheries Agenc). P.O. Box 627. Honiara. Solomon Island\, 

pers. commun. 26 June 1981. 

“Petit, M 1979. Compte-rendu \uccinct des trdvaux accomplis lor, de la premiere, periode de l’operation radiometrie arrienene et prospection thoniere (06 

fevrier 1979-11 avril 1979). Ref. Article 3 - paragraphe 3-2 du contract No. 1 

DOM-TOM-ORSTOM.O.R.S.T.O.M.,CentiedeNoumea,nopa@ 0.R.S T.O.M., 

B. P. AS, Noumea-Cedex, Neu Caledonia. 

“Petit. M. 1979. Compte-rendu des travau~ accomplis lors de la deuxieme 

periode de l’operation radiometrie aerienne et prospection thoniere (16 juin 1979- 

14 juillet 1979). Ref. Article 3-2 du contract No. 1. DObl-TOM-ORSTOM 

O.R.S.T.O.M..CentredeNoumea,nopag.O.R.S.T.O.M.. B. P. A5, NoumeaCedex. New Caledonia. 

”Petit. M., J. Muyard. and F. Marsac. 1980. Compte-rendu des tra\auh accomplis lors de la troisierne periode de l’operation radiometrie aerienne et 

prospection thoniere (25 septemhre 1979-11 fehvier 1980). Ref. Article 3-2 du 

contract No. I. DOM-TOM-ORSTOM. O.R.S.T.O.M.. Centre de Nouniea, 17 

p. O.R.S.T.O.M., B. P. AS, Noumea-Cedex, New Caledonia. 

dent on this association. Most purte 5eine sets take place 

either at dusk or dawn. Also, anchored rafts are now employed on a commercial scale in the Philippines (Aprieto 

1980, 1981; Matsumoto footnote 12) and in Hawaii (Matsumot0 et al. 1981), and they are being deployed experimentally 

throughout the South Pacific, e.g., in American Samoa, 

West Samoa, northern Marianas, Guam, Palau Ihnds, 

Cook Island, and Tonga (Matsumoto footnote 12): in the 

eastern Pacific (Guillen and Bratten”); and in the Indian 

Ocean, for example, in the Maldives: and probably in other 

areas as well. In the Philippines, small scouting boats or 

canoes are used to determine whether tuna have been attracted to the rafts. If so, a light skiff ties up to the raft (or 

payao) to concentrate the fish for seining. The lights used 

vary from gasoline lanterns to powerful electrical lights, 

either submerged or above the surface, depending on the (ize 

of the operation. When it is determined that the fish are 

sufficiently concentrated around the light boat and the raft, 

the light boat is allowed to slowly drift away until there is 

room to set the purse seine around the light boat without 

becoming entangled in the raft. A given purse seiner, which 

can be anything from a converted tuna longliner to a 2,000 

GT purse seiner, will use 15 or more rafts, setting up to 3 

times a night and averaging a little over I set per night. Tuna 

are caught on over 90% of the sets. 

Advantages of purse seine fishing are independence from 

baitfish resources, lower requirements for shipboard manpower, and highly efficient catch rate per man per vessel. 

Disadvantages include the size of the capital investment, inability to fish in rough waters, inability (currently) to fish 

during the day in waters with a deep thermocline, the requirement for sophisticated unloading and support facilities, 

and large fuel consumption except when using anchored rafts. 

Gill nets are the primary gear used to catch tuna in Sri 

Lanka and Somalia. The gear used in Sri Lanka is described 

in de Bruin (1970) and Pajot (1978). Skipjack tuna are quite 

commonly caught in traps and drive-in nets in the Philippines 

([Philippines.] Bureau of Fisheries and Aquatic Resources 

1980). 

5.2 Fishing areas 

The major fisheries have developed adjacent to continents 

or large islands where upwelling or other oceanographic features lead to a concentration of skipjack tuna near the surface (Fig. IO). The shaded areas around various islands have 

been exaggerated while subsistence fisheries in the South 

Pacific Ocean and the small but developing fishery off Australia have not been indicated. Generally, the surface fisheries 

extend from the neritic-pelagic environment into the adjacent, 

oceanic-pelagic environment. Reports of skipjack tuna being 

taken in estuarine environments (river plumes primarily) have 

been received from the southwestern Pacific Ocean. Open 

oceanic surface catches are generally associated with upwelling zones between opposing currents, such as along the equatorial currents, the Kuroshio, and the North Pacific subtropical countercurrent. 

“Guillen. R.. and D. A. Braiten. 1981 Anchored raft experiment 10 aggregate 

tunar in the eastern Pacific Ocean. Inter-Am. Trop. Tuna Comm , Internal Rep. 

14, 10 p. Inter-American Tropical Tuna Commission, c/o Scripps lnititutinn of 

Oceanography, La Jolla, CA 92037. 

75 
5.3 tisliing \eason\ 

Gerierally. fisheries exi\ting in the equatorial, tropical area5 

do not sliow much 5easonal variation except possibly a tlight 

increase in catches during both the northern and southern 

summers. Fisheries situated more poleward in the subtropics 

or bordering on the temperate zone \how more seasonality 

with tlie peak catctie5 occurring in the summer months for 

the respective hemisphere\. 

Specific details on seasonality in various fi\heries for the 

w'estern Pacific and the Indian Oceans may be found in 

Uctiida (1975), for Neu Zealand in Habib (1976, 1978) and 

Habib et al. (1980a, b. c), for Papua Neu Guinea in Wankowski (1980), for the eastern tropical Atlantic Ocean in 

Sakagawa and Murphy (1976). and for French Polynesia and 

the eastern tropical Pacific in Forsbergli (1980). 

5.4 Fishing operations and re\ul[\ 

5.41 Ettort and intensitv 

Various means of measuring fishing effort directed at skipjack tuna were discussed in Section 4.2. Changes in the efficiency of the gear were discussed in Section 5. I. 

In tlie eastern tropical Pacific. reported effort is 22 to 26% 

less than the total effort; however, there is no apparent trend 

in the rate of reporting (Forsbergh 1980). The amount of 

fishing effort increased from I934 to the 1950's. remained 

fairly constant during the 1960'5, and then increased again 

through most of the 1970's. 

In Hawaii, estimates of relative fi5liing intensity (Fig. 70) 

fluctuated substantially Irom year to year and were at a 

somewhat higher level during the 1948-56 period than in subsequent years (Ucliida 1976). 

In the Papua New Guinea bait boat fishery. the amount of 

fishing effort has increased from some 500 d iii 1970 to 8,000- 

9.000 d by 1979 (Wankowki 1980). In the New Zealand 

purse seine fishery, about 400 d of effort have been expended 

in each fishing season starting in 1976-77 (Habib 1976, 1978; 

Habib et al. 1980a. b). 

For the Japanese bait boat fishery, there has been a slight 

increase in the number of trips from around 7,000 in 1970 to 

10,000 in 1976 by all size classes of boals in the coastal fishery (norrti of lat. 15"N). while for boats between 50 and 100 

gross metric tons (GMT). the number of trips increased from 

about 1,400 to 8,000 (Bour and Galenon 1979). For the 

southern water fishery, Kasahara (1977) showed that the 

total number of fishing days for all size classes of bait boats 

increased from nearly 9,000 in 1970 to over 21.000 in 1975, 

while the increase for 250 GMT boats was from around 

2,600 to nearly 20,000 d during tlie same period. Honma and 

Suzuki (1978). in their preliminary description of the Japanese 

purse seine fishery, showed that the number of single-boat 

purse seiners increased from 16 in 1969 to 52 in 1974, whereas 

the number of double-boat purse seiners declined from 55 to 

I2 over the same period. The number of trips for the singleboat purse seiners varied from some 390 to 940 and for the 

double-boat purse seiners from 1.500 to about 340 over the 

same period. In the period 1971-74 the number of sets for 

single-boat seiners in the coastal fishery tluctuared between 

1.000 and 2,000 sets, whereas for double-boat seiners the 

number of sets declined from around 1.300 to 400. In the 

southern water fishery, the number of sets by single-boat 

seiners increased from 60 in 1968 to 428 in 1974. 

5.42 Selectivity 

Pole-and-line gear (as used on bait boats) tends to take 

smaller and fewer Size classes of skipjack tuna than does 

purse seine gear. The selectivity of pole-and-line gear can be 

modified by changing hook size, weight of the pole, or bait 

size, but most significantly by merely selecting for size of fish 

in a school. Likewise, the size selectivity of purse seine gear 

can be varied by selecting schools and by adjusting mesh size 

to retain fish only of the desired size or larger. Longline gear, 

which catches skipjack tuna only incidentally, is known to 

take mostly large individuals as discussed in Sectinn 4.13. 

5.43 Catches 

(I) Total annual catches 

From 1961 through 1979, the nominal catches of skipjack 

tuna from all oceans have more than tripled, reaching 698.5 t 

in 1979 (Table 19). While the catch of all tunas has increased 

varying amounts over this period, that for skipjack tuna has 

increased more rapidly than the other principal market species 

of tunas (yellowfin tuna, albacore, bigeye tuna, and bluefin 

tuna). More specifically, the proportion of skipjack tuna 

catches has increased steadily from 24.5% in 1961 to around 

40% in the late 1970's (Fig. 76). Catches of skipjack tuna 

first surpassed those of the traditionally dominant yellowfin 

tuna in 1965, fell below in 1968 and 1969, and then remained 

above after 1970. Clearly the Pacific Ocean is the most important area for skipjack tuna fishing with catches there 

accounting for 80 to 90% of the world catch. Also, the fisheries of the western Pacific alone accounted for 67.1% of the 

world skipjack tuna catch in 1979 (Fig. 77). The developing 

fisheries in the Atlantic and the Indian Oceans accounted for 

12.5 and 4.7% of the world skipjack tuna catch in 1979. 

(2) Total annual yields from different 

fishing grounds 

A more detailed breakdown of catches is provided in Table 

20 for selected countries in the most productive skipjack 

tuna fishing areas. In the eastern central Atlantic Ocean, 

Spain and France have traditionally been the major producers 

but Japan, and more recently Korea, have become significant 

participants in the fishery. In the Indian Ocean, the production of the two major fishing countries, Maldives and Sri 

Lanka, has declined. In the northwestern Pacific Ocean, 

Japan is essentially the only producer, and their fishery reached 

a maximum of some 107,000 t in 1973. In the central western 

Pacific Ocean, the Japanese distant water fishery is the major 

producer and has shown a large increase from around 35,000 

t in 1966 to 198,000 t in 1978. Production by the Philippines, 

Indonesia, Papua New Guinea, and the Solomons has also 

shown marked increases in the 1970's. In the eastern central 

Pacific Ocean, the United States has been the leading fishing 

nation, producing up to around 100,OOO t. Ecuador has been 

the second largest producer, with a maximum of around 

17,000 I. Mexico has recently been adding significantly to its 

purse seiner carrying capacity and should be expected to con16 
Table 19.-Annual landings (in 1,OOO 1) of skipjack tuna from the major oceans by FA0 statistical areas (FA0 1968, 1971, 3971, 1976, 1977, 1978, 1979. 1980). 

Year 

Area 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 

AtlanticOcean' 0.3 0.3 2. 

Northwest 21 

Northeast 27 

Western 

central 31 

Eastern 

central 34 

Southwest 41 

Southeast 47 

No breakdown by 

area 

14.3 17.9 25.7 22.7 36.2 32.0 51.6 67.9 73.1 71.8 119.0 67.8 73.5 97.6 95.6 87.6 

0.5 0.0 0.0 0.0 1.8 0.4 - 0.4 0.0 0.1 - 0.0 0.2 0.0 0.4 0.3 

2.6 

1.1 1.0 1.1 1.2 1.8 1.3 1.6 1.7 1.6 2.1 4.1 2.9 4.7 6.0 1.9 2.1 

10.8 16.4 24.0 20.0 31.8 29.9 49.5 61.9 70.4 67.0 101.1 63.1 66.5 86.9 88.3 77.5 

_______________ 

0.4 0.5 0.6 1.5 0.8 0.4 0.4 0.8 1.0 0.6 0.5 1.1 0.6 0.6 1.4 1.4 

1.5 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 2.0 13.3 0.7 1.5 4.1 3.6 3.7 

lndianOcean - - - 0 13.2 16.0 18.1 16.6 18.7 29.7 31.4 20.1 24.2 41.3 36.2 38.4 30.3 30.4 32.6 

Western 51 [No bre;&:aown by ] 13.2 16.0 18.1 16.5 18.6 27.4 29.0 16.3 20.1 36.7 31.7 32.4 26.2 25.3 27.0 

Eastern 57 0.0 0.0 0.0 0.1 0.1 2.3 2.4 3.8 4.1 4.6 4.5 6.0 4.1 5.1 5.6 

PacificOcean 219.0 252.4 200.8 252.4 235.9 308.6 335.9 273.6 269.6 304.0 328.2 330.1 434.3 510.0 436.3 554.3 504.8 665.8 578.3 

Northwest 61 187.2 145.8 208.4 176.0 147.6 163.1 166.2 119.8 157.1 202.6 127.2 135.5 151.1 130.0 157.3 143.0 

Northeast 67 - 0.2 - 0.1 - 0.1 0.2 0.3 0.0 0.1 0.5 0.1 6.7 0.4 5.8 0.5 

35.7 35.2 42.5 41.5 70.0 114.8 119.9 173.0 303.0 205.4 252.9 272.5 359.7 316.9 

w:?:::a171 [ Nobre;;;yby ] - - 

Eastern 53.4 76.3 54.2 106.9 74.5 51.5 58.9 87.4 50.1 52.0 75.3 84.5 134.2 91.4 130.0 107.7 

Southwest 81 1.5 0.0 0.0 0.2 0.1 0.0 0.0 0.2 0.5 1.8 1.7 6.4 6.3 64 9.8 8.5 

Southeast 87 10.3 13.6 10.3 17.5 8.9 13.4 8.7 5.7 2.5 4.8 2.3 4.4 3.1 4.1 3.2 1.7 

Total 219.3 252.7 203.0 266.7 267.0 350.3 376.7 326.4 320.3 385.3 427.3 423.3 530.3 670.3 340.3 662.2 632.7 791.8 698.5 

central 77 

'Landings occur in the Mediterranean but in very small amounts. 

Figure 76.-Annual nominal catches of the principal market species 

of tunas from all oceans (FA0 1968. 1973, 1975. 1979, 1980) and 

catches of skipjack tuna as a percent of the total. 

PKIFIC OCEAN ITOTALI 

ATLANTIC OCEAN 

Figure 77.-Percent of skipjack tuna catches by ocean areas. 89% ' iesa r&o ' 8972 I&+ ' I& #&e ' 

1977. A retie\\ on the use 01 he haittiche\ to capture ikipjack tuna. 

tiotcu~ontrsprlonttr. 111 the tropical Pacific Ocean uith empha\i\ on their 

hehakior. sur\i\al. and a\ailahtlit). In R. 5 Shomura (editor). Collection 

01 tuna haitfi\h paper\. p. 8-35. ti.S. Dep. Commer.. NOAA Tech. 

Rep. NMFS Circ. 408. 

BARKLEY. R.A. 

1969. Salinity nidmnia arid the 5hipjach tima, ~unu~~~~~ti~c~~~Iuirtt~ Bull 

Jpn. Soc. ti\h. Oceanoer.. Spec. No. (Prof Uda'\ Coniniemorau\e 

Papers), p. 243~248. 

1972. Johnston Atoll'\ uake. J. Mar. Kc\. 30:201-216. 

1978. 

BARKLFY. R. A , \\ H. NEll I.. and K. hl. GOOUlNCa. 

Skipjack tuna. Korruwonur pdomrs. habitat had on temperatitre 

and oxygen requirements. Fi\h. Bull , U.S 76:653-662. 

BARNARD, K. H. 

1925. A nionoeraphnf thc marine li\hei 01 Sonth Alrica. Ann. S. Aft. 

hlus. 21, 1065 p 

BARNHART. P. S. 

1936. Marine h\hec (11 wuthern California Uni,. Calif he\\, Brrlele!, 

209 p. 

BARRETT. I., and F. J. HESTER. 

196.1. Bod? teinpcrature of~ello\\lin and shipjack tuiiar ti1 relation to \ea 

wrface temperature. Nature (L.und.)203:Y6-97 

BARRETr. I., and H. TSUYUKI. 

1967 Serum tran\terrtn polymorphism m \om? \conihroid li\he\. Copsia 

1967.551-?57 

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The trends in the southern water sktpjack tuna fishery (Nanpo katsuo 

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Flsh. Serv.. NOAA. Honolulu. HI 96812.3 

968 I 2.1 

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1975. 

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1977. 

KASAHARA. K ,and T. TANAKA. 

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Bull. Tohoku Reg. Fish 

KAWAMURA. G.. W. NISHIMURA, S. UEDA. and T. NISHI. 

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Jpn. Soc. Sci. Fish. 47:481-485. 

KAWASAK1.T. 

1952. On the populaiions of ihe \kipjack. Karsuwonus pelamis (Ltnnaeus). 

migraring to the north-easiern \ea area along the Pacific coast of Japan. 

[In Jpn.. Engl. summ.1 

On the migration and the grouih of the \kipjack, Kai.suwonus 

pelamis (Linnaeus). in the \outh-ue*tern sea area of Japan. [In Jpn.. Engl. 

siinim.] 

On the migration and the grouth of the skipjack. Karsuwonus 

pelanits (Linnaeus). in the ILU and Bonin Sea areas and the north-eastern 

sea area along the Pacific coast of Japan. [In Jpn.. Engl summ.] Bull. 

Tohoku Reg. Fish. Res. Lab.4:101-119. 

On the lluctuation of the fisheries condttlon\ in the livebait fishery 

of skipjack m water? adjacent 10 Japan. 11. [In Jpn , Engl. summ.) Bull. 

Tohoku Reg. Fi\h. Re\. Lab. 11.65-81. 

On the qructure of "fish $chool" of tunas. [In Jpn.. Engl. abstr.) 

Jpn. J. Ecol. 952-54. 

The grnuth of skipjack on the northeastern Sea of Japan [In Jpn.. 

Engl. summ.] Bull. Tohoku Reg. F!*h Res. Lab. 23:44-M) 

Population structure and dynamlcs 01 sklpiack in ihe North Paclfic 

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1965a. Ecology and dynamics of the \kipjack populat~on. 1 Clasrtfication. 

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Ecology and dynamic, ot the \kipjack population. II. Resources 

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Ser. 8:49-108. [Engl. tranrl. 1967. 7Yp.: b.S. Joint Publ. Re\ Sen.] 

The cklpjack tuna remurce (har\uo no shlgen ni tiuliel. [In Jpn.] 

Suisan Shuho (The Fnhmg and Fond Indwtry Meekl!) 660:32-38. July 15. 

1972. [Engl. tran\l 1973. In U.S. Dep. Commer.. Mar. tish. Re\. 

35(5-6):1-7 1 

KAYA. C. hl., A. E. DIZON. S. L). HtNDKIX. T K. KAZAXlA. and \I. K. K. 

Rapid and spontdneou, maturation. ovulat~on. and *pauninguf ma 

hy newly captured \kipjack tuna, Katsuwonus pelamis. Flsh. Bull., U.S. 

80:393-396. 

KEARNEY. R. E. 

Bull Tohoku Reg. Fish. Res. Lab 1:1-14. 

1955a. 

Bull. Tohoku Reg. Fish. Re?. Lab. 4:83-100. 

1955b. 

1958. 

1959. 

1963. 

1964 

1Y65b. 

1972. 

Qti E E: NTH 

1982. 

1975. The stock rtructure of rhipiack rewurces and the posslhle Implwalions on the de\elopment of skipjack Ii\heriet in the central and uesfern 

Pacific In Studies on \kipjack ~n rhe Pacilic. p. 89-69. FA0 Firh. Tech. 

Pap. 144 

Some li!poilicw\ on \hiptack. lhot\inwnrrc peIomi0 in the Pacillc 

Ocean. 

An o!er\leu (11 recciii change\ in ihe tiiherhe\ lor highl) mlgraiory 

\pecie\ ~n ilie ue\iern Paulic Ocean and projection\ lor futurc de\rlopmentr 

1978. 

lY-/Y. 

Souih Pac (omr . Occa.. Pap. 7. 23 p. 

South Pac. Bur. Ecnn. Cw7p ,SPEC (7Y)17, YY p. 

KtARNt1. K. E., atid K I). 611 I ET r 1978 liiterm report 01 the acrk\ii!e\ 01 the Shlplaih 5unc) and A\\e\\- 

m~nt Programme In ilw udicn 01 rlie Ciilhert I4andr (5 - 25 Jul! IY7X) 

Kep 11, Ilp 

lnterini rrpori (11 the dci~\~tic\ (It the Skipjack Sur\c) and ,\\w,. 

inen1 Programme in ihtl uater\ 01 Au\irdlid (I April ~ 13 >la\ 1979). 

South Pac. Conim , Shiptach 'iun 4\w\\. Programme. Pielliil < ouniry 

Rep. 17. I5 p 

KEARNFY. R. E., and J.-P HA1.L IFK 

South PdL. ro111m . SAlpl'lck \Ur\ 4 Progr'l,,,"le. prel,,,,. C,,unlr) 

19%. 

1978a. Inierinl repori ofthc d~i~\iim of ihc Sklpjdch Sune) and A\\e\\- 

ment Programme in ilie uarer, 01 Neu Caledonm (13 December 1977-19 

January 197XI. South Pac. Comm.. Skipjack Sun A5\nr. Programme, 

Prelim. Country Rep 3, 20p. 

Interim report ofthe acti\~t~es ofthe Skipjack Surbe! and Assessment Programme In the uaten of Western Samoa (6 - 14 June 1978). 

South Pac. Comm., Skipjack Sur\. Asses\ Programme, Prelim Country 

Rep. 8, lop. 

Interim report of rhe act~vities of ihe Skipjack Surrey and Assescmen1 Programme In the watery o1 Amerlcan Samoa (3 I May - 5 June, 15 - 

21 June 1978). South Pac. Comm.. Skipjack Surv. Assess. Programme, 

Prelim. Country Rep. 9. 9 p. 

Second interim report of the actik~ties of the Skipjack Survey and 

Assessment Programme in the Haters of Papua New Culnea (14 May - 2 

July 1979). South Pac. Comm., Sklpjack Surv. Assess. Programme, 

Prelim. Country Rep. 18. 14p. 

Interim report of the actIvLties ot the Skiplack Survey and Assecsmen1 Programme in the waters of Norfolk Island (26 - 30 March 1980). 

South Pac. Comm.. Skipjack Sur\. Asses Programme. Prelim. Country 

Rep. 24, 9 p. 

Second interim report of the act~\ities of the Skipjack Survey and 

Assessment Programme in the waters of the Trust Territory of the Paciflc 

Islands (2-21 November 1979). South Pac Comm., Skipjach Surv. 

Assess Programme. Prelim. Country Rep. 19, 14p. 

1978b. 

1978~. 

1979. 

1980a. 

1980b. 

KEARNEY, R. E., J:P. HALLIER,and P. KL-EIBER 

1978. Interim report of the aciiwties of the Skipjack Survey and Awssmen1 Programme in the uaieri of Tuhalu (25 June - 4 July 1978). South 

Pac. Comm., Skipjack Sur\ Asses\. Programme, Prelim. Country Rep. 

10.911 

KEARNEY, R. E.. A. D. LEWIS. and J:P. HAL1 IER. 

1978. Rapport interimaire \ur les activ~tiec au titre du programme d'etude 

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Skipjack Surv. Assess. Programme. Prelim. Country Rep. 4. 18 p. 

KIKAWA. S. 

1953. Obser\ations on the spauning of the big-eyed tuna (Purathunnur 

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KIMURA, K. 

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KIMURA, K.. M. IWASHITA. and T HATTORI. 

1952. lmagrnfrklpjack and tuna recorded on echo soundtng machine. [In 

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KOJIMA, S. 

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IW. 

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Change in catch per unit of effort for skipjack and yelloufin tuna 

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,Aiicii\l 211. IYSII. I2 p Iimnimhcrcdl II "$1 twm h\ 1 Otw. Y p 

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IY5X. I>e\cripiion and ili\irLhution of I.ir\ae of four \pecie\ 01 tuna in (eniral ~ac111c ~aier,. U.S FI~I \wciI. ~cr\ , FI\~ BUII. 5x:31-72. 

1961 (~olleci~on and dc\cripiion\ of luiciiilc luna\ from ihe central 

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1970. A convderaiion on ihe *pawning 01 ihc luna\. c\peciall> of ihe 

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