SYNOPSIS of BIOLOGICAL DATA on SKIPJACK Katsuwonus Pelamis (Linnaeus) 1758 (PACIFIC OCEAN)

Home , Skipjack (cipher), Spratelloides, Striped bonito

Species Synopsis No, 22 FAO Fisheries Biology Synopsis No, 65 FIb/S65 (Distribution restricted) SAST Tuna

SYNOPSIS OF BIOLOGICAL DATA ON SKIPJACK Katsuwonus pelamis (Linnaeus) 1758 (PACIFIC OCEAN)

Exposé synoptique sur la biologie du bonite à ventre rayé Katsuwonus palamis (Linnaeus) 1758 (Océan Pacifique)

Sinopsis sobre la biología del bonito de vientre rayado Katsuwonus pelamis (Linnaeus) 1758 (Océano Pacífico)

Prepared by KENNETH D, WALDRON U, S. Bureau of Commercial Fisheries Biological Laboratory Honolulu, Hawaii

FISHERIES DIVISION, BIOLOGY BRANCH FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome, 1963

695 FIb! S65 Skipjack 1:1

1 IDENTITY 'Body robust, naked outside the corselet; 1, 1 Taxonomy maxillary not concealed by preorbital; teeth present in jaws only; dorsal fins with only a short space between them, the anterior 1, 1, 1Definition (after Schultz, et al, spines of the first fin very high, decreasing 1960) rapidly in length; second dorsal and anal each followed by 7 or 8 finlets; pectoral not very Phylum CHORDATA long, placed at or near level of eye, with Subphylum Craniata about 26 or 27 rays." (Hildebrand, 1946). Superclass Gnatho stomata The foregoing statement is applicable to Class Osteichthys mature adults of the genus. Subclass Teleostomi Superorder Teleosteica Order Percomorphida - Species Katsuwonus pelamis Suborder Scombrina (Linnaeus) (Fig. 1) Family Scombridae "Head 3. 0 to 3,2; depth 3. 8 to 4.1; D. XIV Genus Katsuwonus or XV - I,13 or 14 - VIII; A. II,12 or 13 - VII; Species pelamis P. 26 or 27; vertebrae 40 (one specimen dissected).rnote: Both Kishinouye, 1923 and Some of the larger taxa under which skip- Godsil and Byers, 1944 give the number of jack have been listed are shown in Table I. vertebrae as 4l,j The tendency over the past few decades has been to set the tuna apart from other scorn- "Body robust, its greatest thickness about broids, and further to set up monotypic genera. two-thirds its depth, tapering strongly pos- Within recent years this tendency appears to teriorly; caudal peduncle slender, depressed, be changing to one of re-including tuna in the with a strong, lateral keel, its depth 14 to 15,3 family Scombridae and doing away with some in head, head somewhat compressed, convex of the monotypic genera,Under this system above; snout long, pointed, 3. 3 to 3.7 in head; it is likely that skipjack will be included in the eye moderate, round, 5.7 to 6,1; interorbital genus Euthynnus, 3.7 to 4.0; mouth slightly oblique, terminal; maxillary reaching nearly or quite opposite 1. 1. 2Description middle of eye, 2.6 to 2.8 in head; teeth in jaws in single series, short and rather stocky; gill - Genus Katsuwonus rakers long, slender, about four-fifths length

Table I Classification of skipjack according to different authors Authority Group Regan KishinouyeJordan, Evermann Berg Schultz et al, 1909a, b 1923 and Clark 1930 1940 1960 PHYLUM - - - - Chordata SUBPHYLUM - - - - Craniata CLASS Pisces - Pisces Teleostomi Osteichthys SUBCLASS Neopterygíi - Actinopteri ActinopterygilT eleostorni SUPERORDER - - Acanthopterygil - Teleosteica ORDER Percomorphi Plecostei Percomorphi ThunniformesPercomorphida SUBORDER Scombroidei - Rhegnopteri - Scombrina SERIES, SECTIONScombriformes - Scombriformes - DIVISION FAMILY Scornbridae KatsuwonidaeKatsuwonidae Thunnidae Scornbridae SUBFAMILY - - - Auxidini - GENUS - Katsuwonus Katsuwonus Katsuwonus Katsuwonus SPECIES - pelamis vagans - pelarnis

696 FIb/S65 Skipjack

Fig. 1Lateral view of an adult skipjack, Katsuwonus pelamis (Linnaeus) (from Walford, 1937)

697 FIb/S65 Skipjack 1:3 of eye, 36 to 40 on lower and 15 or 16 (counted through the kidneys the arteries turn outward in only 3 specimens) on upper limb of first and forward, instead of turning more or less arch; lateral line missing anteriorly, curved backward as in the other plecostean fishes. downward at midlength, attaining a midlateral Each artery reaching to the myotorne of the position under about the first dorsal finlet, first rib is divided into two arteries, epaxial straight from thence to caudal keel; scales and hypaxial.The epaxial artery runs below present anteriorly in region of pectoral, the first rib, while the hypaxial artery runs extending on back to or beyond origin of above the rib.These two arteries, are first dorsal, reduced scales in lateral line, nearly equally developed, and are separated and a pointed area of scales extending about from each other at a distance of 6-8 times an eyets diameter beyond tip of pectoral the breadth of the blood-vessels. These fin; first dorsal composed of slender spines, arteries do not form a ioop at the caudal the anterior ones long, posterior ones short, region. The cutaneous artery and cutaneous scarcely extending above dorsal groove, the vein lie in juxtaposition, nearly flat at the first one 1.8 to ¿.0 in head, origin of fin a surface of the body. Arterioles and venules little behind insertion of pectoral, its connected with these cutaneous canals run in distance from tip of snout 2,6 to 2.9 in opposite directions, along the surface of the length; second dorsal somewhat elevated body and they are not so numerous as in the anteriorly, with deeply concave margin, the tunnies,The rod of the vascular plexus in posterior rays being very short, its origin the haemal canal is called kurochiai by fisher- about equidistant from origin of first dorsal men, and it is thicker than the diameter of the and base of next to last dorsal finlet; anal vertebral column." similar to second dorsal, its origin a little in advance of vertical from base of last ray The spinal column (Fig. 2) differs of second dorsal; ventral somewhat shorter sufficiently from that of other fishes to be than pectoral, inserted slightly posterior to worthy of comment. Godsil and Byers (1944) pectoral, its distance from tip of mandible state "The spinal column is extremely complex 2.7 to 2.9 in length; pectoral moderately in this species.In the posterior half of the pointed, 1.9 to 2.1 in head. precaudal, and the anterior part of the caudal region, haemopophyses arise at both ends of "Color very dark blue above, with each vertebra, with the anterior haemo- metallic reflections; silvery below; lower pophyses of one vertebra articulating with the part of head, chest, and area around ventral posterior haemopophyses of the preceding fins dirty white; lower half of side with two vertebra. These haernopophyses project far to four dark longitudinal strips; spines of below the spinal column and alternate with the first dorsal dusky, anterior margin of first longer haen-ial spines described below. An spine and membranes pale or white; second osseous bridge roughly parallel with the dorsal, dorsal and anal finlets, caudal, anal, spinal column unites the anterior and posterior and pectoral all more or less dusky; pectoral haemopophyses of each vertebra. Viewing the much darker on inner side than outside, the spinal column laterally, the bridge and the upper rays generally silvery; ventral fins circular opening it forms may be easily seen, white on outside, inner side dusky; inside of A branch arises from each bridge and extends mouth largely dusky." (Hildebrand, 1946), downward to unite in the median line with its In life, and in some preserved specimens, fellow from the opposite side. This forms the light vertical bars are evident along the haemal arch from the tip of which a single sides. haemal spine continues veritralward,This complex basketwork occurs only in the Anatomical descriptions of the species skipjack." and genus may be found in Kishinouye (1923), Godsil and Byers (1944), Schultz et al(1960). The skipjack lacks an air bladder. Kishinouye (1923, p. 452) states, "The Some of the organs not mentioned by Gods il cutaneous circulatory system is unique. A and Byers (1944) or not described in detail by pair of cutaneous arteries branch just behind Kishinouye (1923) have been described by other the insertion of the pharyngeal muscles as in authors, Among these descriptions are those the tunnies and other bonitos; but passing of Rivas (1953) and Rasquin (1958) concerning

698 1:4 FIbIS65 Skipjack the pineal region; tJchihashi (1953) on the brain of skipjack since it was first proposed in 1915 as related to ecology; Hanyu (1959) on certain by Kishinouye, although at that time he used structures of the eye; and Suyehiro (1941, 1942, the spelling TTpelamysht.The present tendency 1950) on the pituitary body, the Islets of appears to be towards replacing Katsuwonus Langerhans, arid the digestive system (it might with Euthynnus, retaining the specific name be noted that this author erred in ascribing an pelamis. air bladder to skipjack). 1.2.2 Other scientific names ascribed 1.2 Nomenclature to this species 1.2.1 Valid scientific name The following list of names, taken mainly from Rosa (1950), is arranged in alphabetical Katsuwonus pelarnis (Linnaeus, 1758) has rather than chronological order. Each name been widely accepted as the scientific name is followed by one of the early authors to use the combination.

Euthynnus pelamis Tanaka, 1912 Euthynnus (Katsuwonus) pelamis Fraser-Brunner, 1950 Euthynnus pelamys Jordan and Gilbert, 1882 Gymnosarda pelamis Dresslar and Fesler, 1889 Gymnosarda pelamys Barnard, 1925 Katsuwonis pelamis Herre, 1933 Katsuwonus pelamis Kishinouye, 1923 Katsuwonus pelamys Kishinouye, 1915 Katsuwonus vagans Jordan, Evermarin, and Clark, 1930 Orcynus pelamis Goode and Bean, 1879 Pelamys pelamys Bleeker, 1865 Pelamys thynnus Jenkins, 1925 Scomber pelarnides Lacpde, 1802 Scomber pelamis Linnaeus, 1758 Scomber pelamys Bloch and Schneider, 1801 Thynnus pelamis Hoek, 1914 Thynnus pelamys Cuvier and Valenciennes, 1831 Thynnus vagans Lesson, 1830

99 FIb/S65 Skipjack 1:5

1, 2. 3 Standard common names, tions of fish and the stocks of the Eastern and vernacular names Central Pacific also appear to be at least semi- independent, The following list of common names and vernacular names, the latter in parentheses, -Varieties and the countries in which they are used was taken from Rosa (1950).Other common names There are no records of varieties of skip- not included in the list, refer to skipjack of a jack. particularize, or to aggregations of skipjack as opposed to individual fish. -Meristic counts 1. 3 General variability Table II presents meristic counts listed in two publications. 1.3.1 Subspecific fragmentation (races, varieties, hybrids) 1. 3. 2 Genetic data (chromosome number, protein specificity) It is generally recognized that there is but one world-wide species of skipjack, which can- Little data concerning the genetics of skip- not be differentiated into subspecies.Working jack exist in the literature, and what can be with Pacific specimens, Hennemuth (1959) found are of recent origin.There are no data stated that, "The statistical analysis indicates concerning the chromosome number of skipjack - then, that the commercially available stocks of skipjack inhabiting the various fishing areas Considerable work is being done at present within the Eastern Pacific for which data are (1962) concerning the serological relations available represent semi-independent popula - between skipjack from the same and from

COMMON NAMES OF SKIPJACK FOR PACIFIC COUNTRIES (FROM ROSA, 1950) Australia Striped tuna Canada Skipjack (Oceanic bonito) Chile Atíín (Cachurreta, Cachureta, Cachorreta, Barrilete) China Tow chung (Chien) Hawaiian Is. Skipjack (Ocean bonito, Little tunny, Striped tuna, Aku, Aku kinau) Indonesia Bonito Japan Katuwo (Katsuwo, Katsuo, Magatsuwo, Mandagagatsuwo) Mexico Barrilete New Guinea (Neth.) Tj akalang New Zealand Bonito Peru Barrilete Philippines Skipjack (Striped tuna, Gulyasan, Pundahan, Bankulis, Oceanic bonito, Sobad, Bonito, Palawayan) Polynesia (except Tahiti Atu and Hawaii) Tahiti Auhopu United States Skipjack (Arctic bonito, Oceanic skipjack, Striped tuna, Watermelon, Victor fish, Striped bonito, Oceanic bonito, Skippy, Ocean bonito)

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Table II Meristic characters of skipjack from the Pacific

(I)

cG G)

Or-1 o cG .- 4O'i)

lstDorsal 15-16 14-17 15-16 15-16 2nd Dorsal 14-15 13-15 14-16 14-15 D. Finlets 8 8-10 7-8 7-8 Anal 14-15 13-15 15 14-16 Anal Finlets 7 6-9 7 6-8 Pectoral - 24-32 - - Ventral - - - - Gill Rakers 16-20+ 19 -20+ 18-22+ Upper + Lower 36-41 37-42 35-43 Total 44-61 Vertebrae 41 41 41 different locations. Most of this work is being could be detected by extracts of seeds of done in the Pacific. Glycine max, Phaseolus vulgaris, Virgilia divaricata, and-Garagana arborescens Gushing (1952) in one of his early studies respectively.Working with reagents of skipjack blood found that it contained an which detect A-positive, C-positive, and agglutinin specific for the human type B subS. Be-positive bloods it was possible to stance. Later he found (Gushing, 1956) that differentiate between groups of skipjack within a group of ten skipjack taken from one from Rangiroa, from the Marquesas Is., location near the Hawaiian Is. four and from the Hawaii-Johnston-Christmas different categories based on blood factors were Is. area. discernible. Group one had a strong affinity for antibodies of rabbit antiserums prepared Schaefer (196la) reported the discovery against the blood of yellowfin tuna, albacore, of possibly two blood-type systemsamong skipjack, and white croakers. Group two skipjack from the eastern Pacific through contained antigens with strong affinities for the use of a battery of phytohemagglutinins, the "natural" antibodies of 'normal" sera He expressed the belief that at leastone of (human, bovine, sheep), Group three con- the systems would be suitable for the study tained both of the above antigens, while of skipjack populations within thearea of group four contained neither. the eastern Pacific fishery. Recently Sprague (1961) and Sprague and Matsumoto (1960) reported on the Nakashima (1961) reported finding differences differentiation of tuna species by means in the agglutination by phytoagglutinins of of paper chromatography and found that blood of skipjack from different localities skipjack could be separated from yellowfin, in the Pacific. They used saline extracts bigeye, albacore, and two species of frigate of the seeds from various legumes to define mackerel on the basis of two-dimensional four blood factors which were named A- chromatogran-is. Fig. 3 shows the two- positive, D-positive, Be, and Bf, which dimensional chroniatogranis for adult and larval skipjack from central Pacific waters. 701 FIb/S65 Skipjack 1:7

Fig. ZCranium and spinal column of an adult skipjack (after Godsil and Byers, 1944)

Q G W

1: 0 Q L

-- I I

I IRRKRI 4" FIRST DIRECTION W- BUTANOL METRYLETRYLKETONE AMMONIA WATER- 24 HOURS fFIRST DIRECTION UK- BUTANOL METHYLETHYLKETONE AMMONIA WATER- 24 HOURS Fig. 3Two dimensional chromatograms for adult (left panel) and larval skipjack (right panel)(from Matsumoto, 1960)

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Z DISTRIBUTION Z.Z Differential distribution ¿.1 Delimitation of the total area of 2.2.1 Areas occupied by eggs, larvae, distribution and ecological charac- and other young stages: terization of this area (Fig. 4) annual variations in these patterns and seasonal varia- Rosa (1950) gives the following distributions for stages persisting tion for skipjack in the Pacific Ocean. over two or more seasons. Areas occupied by adult stages: 'South Pacific Ocean - Coast of Ecuador including Galapagos Is., off the coast of Peru seasonal and annual variations approaching it during certain periods of the of these. year in certain areas, coast of Chile as far south as Valparaiso, coast of Australia from - Eggs New Guinea to St. Helens in northeast The distribution of skipjack eggs has not Tasmania, coast of New Zealand. been described, because it has not been possible to differentiate skipjack eggs from "North Pacific Ocean - Seldom found on those of other tunas. the coast of Canada on Vancouver Is., and on the coast of the United States north of Point Conception in California, common from this - Yolk sac stage point to Mexico, entire coast of Mexico, including the Gulf of California, entire This developmental stage is sorare in coast of Central America; Marshall, Caroline, collections that it is not possibleto define the and Palau Is., Philippines, Formosa Is., distribution.With the possible exception of Ryukyu Is., Pacific coast of Japan as far the vertical distribution, it is probably found north as off the south coast of Hokkaido, In the same areas as the non-yolksac stages. Kurile Is., Bonin and Marianas Is., occasion- ally found in the Japan Sea as far as the coast - Larval stages of U.S.S.R. during the summer." Larval skipjack have been found in all the Skipjack are also taken throughout the major oceans of the world.In the Pacific, general Pacific areas of Polynesia including Kishinouye (1926) collected larvae from the Hawaii, Melanesia, and Micronesia. In the Ryukyu Is. area, and Yabe (1955) reported mid-North Pacific skipjack have been taken on larvae collected in the waters of the as far north as latitude 43° N. Caroline and Marianas Is. as wellas from the Ryukyu Is.Matsumoto (1958) and The area described above and shown in Strasburg (1960) reported on larvae collected Fig. 4 is characterized in general by in the central Pacific, both north andsouth temperatures at the surface of 15° C or of the Equator, Wade (1950) reportedon greater and lies roughly between latitudes older larvae from Philippine waters, 40° N and 40° 5. Where skipjack are found Schaefer and Marr (1948) reported thecapture outside these limits may indicate the presence of large larvae from the Marshall Is., of a tongue of warmer water. and Schaefer (1960) reported the capture of larvae and juveniles in the eastern Paòific. Blackburn (1961) has pointed out that skipjack may avoid areas with temperatures above Skipjack larvae have been taken thin 600 feet (= ca. 185 m)of shore in the 28° C. Hawaiian Is. (Strasburg, 1960), as well - Vertical distribution as in the open ocean far from land. While no direct evidence is available it is The north-south distribution of larvae generally considered that skipjack inhabit only appears to be somewhat more restricted the upper one or two hundred meters of the than does that of the adults.Matsurnoto surface waters. (1958) reported the capture of larvae to latitude 25° N and Strasburg (1960) records

703 2 :'2 FIB/S65 Skipjack

000 1100 200 300 400 J5Q0 600 1700 800 70° 60° 50° 40° 30° 120° 110° 00° 90° 80° 70° 60° 60° ______- 60°

40°

30°

20° 20°

loo 10°iÓb,U4U7iYOz iii ' W

p 0°r 10° 20° 20°

30°IF 30° 40°Utdk1UìÎIIIì...II 40°

50° 0° 120° 30° 40° 150° I 'auuuuiiuuui70° 80° 170° 60° 50° 140° 30° 20° 110° 00° 90 80° 70° Fig. 4Distribution of adult skipjack in the Pacific Ocean, and location of the commercial fisheries in the western, central, and eastern Pacific (Brock, 1959)

704 FIb/S65 Skipjack 2:3 larvae to latitude 18° Sin the mid-Pacific. - Juveniles However, this was also the limit of sampling, and so may represent an artifact of sampling By the time the young reach a length of rather than the limit of distribution of the about 20 mm they have acquired many of larvae.In the western Pacific Yabe (1955) the adult characteristics, although the body reported skipjack larvae from about latitude form is somewhat different.They may be 33° N and Brock (1959) shows laivae considered juveniles until they have acquired occurring off the Australian east coast at the coloration and body form of the adult, this about latitude 33° S.Fig. 5 shows locations occurring at a length of about 200 to 250 mn-i. in the Pacific from which larvae have been colle cted. Juvenile skipjack have been reported from much the same locations as have the larvae and, With respect to vertical distribution, the adults.Up to a length of about 45 mm 'Strasburg (1960) has shown that in the Pacific, they have been collected under night lights with larval skipjack (specimens less than 15 mm dip nets. Above that size most of the speci- length) are very rarely taken below 140 m mens collected have come from the stomachs depth. He further stated that 15° C appeared of larger fish, mainly other tunas and spear- to be a limiting temperature, and in the one fishes. exception, when larvae were collected below 140 m a tongue of warm water extended Most of the published records are from the below this depth. Pacific area, and Table III lists collections of juvenile skipjack up to 1951. Chapman (1946) reported schools of "8 -lO inch skipjack" in the Solomons Is, area, and skipjack of less than 250-mm size have been observed in Hawaiian waters.

Table III

Records of the capture of juvenile skipjack (Shirnada,1951) Author and date Locality of capture Length of Number of Date of of publication or specimens specimens capture LatitudeLongitude in mm

Kishinouye, 1923 Ryukyu Is. (Okinawa) 210 1 Aug. 1916 Kishinouye, 1924 do. 105, 125 2 July 1923 Do. do. 120, 153 2 June 1924 Do. 28°lO'N129°l5' E 58 1 May 1924 Do. 29°51'Nl29°52'E 60, 80 2 May1924 Do. 29°47'N129°25'E 63, 83, 85 3 May1924 Kishinouye, 1926 29°47'N129°25'E 26 1 April 1924 Do. Ryukyu Is. (Okinawa) 210 1 Aug. 1923 Do. do. 100-140 3 June 1924 Inanaini, 1942 Truk Is. 198 1 April 1939 Do. do. 45 1 May1940 Schaefer and Marr, 1948 9°22.5N85°47.5'W 21 1 Jan, 1947 Do. 9°lOtN85°20W 44 1 Mar. 1947 Marr, 1948 Bikini Atoll 45, 50 2 July 1947 Wade, 1950 6°37.2'N l2l°3lE 13-27 6 July1948 Eckles, 1949 20°30'N158°45tW. 113-118 6 July1948 Do. l9°33'N156°00' W 183 1 Sept. 1948 Shimada, 1951 3°50.5'S 171°48.5'W 35, 48 2 July1950 Do. 4°30' S172 °ll' W 20, 22, 36 3 Aug. 1950

708 2:14 FIb/S65 Skipjack

000 10° 20° 130° 40° 50° 60° 70° 80° 70° - 60° 150° 140° 30° 120° 110° 00° 90° 80° 70° 60° 60° w 600

§0° §0°

40° 40°

30° ____ íil. 30° Q e' HAWA/I 200 .111.L 'J 20° loo 10° Ii_I .0 . 0°

lOo j.: IO' o.I.

20' a 20°

30°UF ïaILi.__ h 30'

40' U1IIUUIUUU__I 40° ULi110° 120° 160° 180° 170 140 130 120 00° 30 40° 50° o 60 50 o 00° 90 80° 70° 60° Fig. 5Distribution of larval skipjack in the Pacific Ocean (Brock, 1959; Yabe, 1955; Schaefer, 1960)

7Q6 FIb/S65 Skipjack 2 :l

With the exception of the eastern Pacific August in the Bonin-Minamitorishima area specimens (Schaefer and Marr, 1948) all of south of Japan. the juveniles were collected between April and September. The eastern Pacific specimens - Adults were collected in January and March. Suda (1953) reported young skipjack most numerous At about 220 mm skipjack begin to be in stomachs of large predators during June and taken in commercial fisheries and may be considered as s exually immature sub-adults.

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3 ]3IONOMICS AND LIFE HISTORY 3.1,3 Mating (monogamous, poly- gamous, promiscuous) 3,1 Reproduction No information is available as to the 3.1.1 Sexuality (hermaphroditism, mating behavior of skipjack.In view of their heterosexuality, inter - known schooling behavior it is reasonable to sexuality) assume that mating is promiscuous. The skipjack is normally heterosexual. 3.1.4 Fertilization (internal, There are a few recorded instances of external) hermaphroditism (Nakamura, 1935; Uchida, 1961).It is not known if these examples were There are no publications describing functional hermaphrodites.In a specimen fertilization of skipjack eggs, but since males received at the Biological Laboratory, lack any intromittent organ fertilization is Honolulu, and described by Uchida (1961) the probably external. ovarian section contained ripe and resorbing ova, but the testicular portion showed no 3.1.5 Fecundity evidence of running milt. - Relation of gonad size and egg 3.1.2 Maturity (age and size) number to body size and to age There is no definite information Yabe (1954) found considerable variation in concerning the age at which skipjack the number of eggs in skipjack of similar and mature, and this can be attributed to different sizes.Table IV is modified from differences of opinion as to the age of skip.-. his table 6. jack in general. Brock (1954) used length frequencies occurring in commercial As can be seen from the table, skipjack landings to age fish and with respect to differing in weight by only 0.7 kg may differ maturity stated, "It therefore appears by over 700, 000m the number of ova likely, considering also the information on (specimens i and 2).Both these fish were age and rate of growth, that aku may presumably the same age, either age group mature at 1 year." Yabe (1954), following IV according to Aikawa and Kato (1938) or the aging method developed by Aikawa and 1-year-olds according to Brock (1954). Kato (1938), stated with respect to maturity that, "Providing that these represent the Yoshida (MS) made ova counts on four smallest mature fish, both male and skipjack from the Marquesas Is. area and female were age IV fish, according to the found the ripening eggs numbered 0.1 million age determination of Dr. Aikawa." Ripe in a 43-cm and 2,0 million in a 75-cm males 391 min and spawned out females 407 skipjack. mm in length have been reported by Marr (1948) from the Marshall Is, area.In Hawaiian waters Brock (1954) reported a ripe female 432 mm in length, and he also stated that the smallest fish with maturing ovaries were between 40 and 45 cm in length.Yabe (1954) reported the smallest skipjack with mature eggs measured 46.8 cm, Wade (1950) reported a ripe female only 340 mm in length from Philippine waters, although most of the ripe females which he reported were above 400 mm in length.

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Table IV Estimated numbers of maturing ova in skipjack of different size Fish Ovaries Total number Length Weight Weight Maturity of eggs (cm) (k) (gm)

46.8 ¿.55 106.5 mature 113,364 49.5 3.2 227 mature 856,180 54.5 4.5 119 immature 565,131 56.2 5.25 250 mature 609,730 61.0 5.96 148.9 immature 859,897

3,1.6 Spawning Yabe (1954) states that in the western Pacific - skipjack above 50 cm length show the greatest - Spawning seasons (beginning, gonad development between May and June, while end, peak) the smaller skipjack under 45 cm length show the greatest gonad development during June to August. The literature contains a number of references to spawning seasons, most of which are - Number of spawnings per year, based on the presence of young in collections or frequency on the stage of gonad maturity in the adult fish. Table V lists some of the more recent estimates There is no direct evidence as to the number of spawning seasons, and the basis of the eti- of spawnings per year, Brock (1954) and Yoshida mate, in the Pacific,It appears that in equato- (MS) have hypothesized, on the basis of multiple rial waters spawning occurs throughout the year, modes in ova diameter frequency distributions although there may be identifiable peak periods. that there is multiple spawning At increasing distances from the Equator the season becomes progressively shorter and occurs - Spawning time of day during the summer months. No information available nor does the literature contain any hypotheses concerning this as- The presence of larval skipjack in plankton pect of spawning. collections probably provides a more precise estimate of spawning seasons than does the 3,1,7 Spawning grounds presence of mature ovaries in the adults. Brock (1954), on the basis of gonad condition, - Coastal (surface, vegetation, shore placed the spawning season in Hawaiian waters shoal, sand, shelter); bottom between February and September, while Sherman No direct evidence exists as to the localized and Brown (MS), working with larval collections distribution of spawning. Larvae have been from Havaiian waters, have hypothesized that found in very close proximity to land, e.g. spawning activity peaks sharply in late spring Strasburg (1960) reported larvae in Hawaiian and early summer, i, e., May to July. waters collected within a few hundred yards of shore, However, spawning could have taken place at some distance offshore and the larvae have been carried inshore by currents.

709 Fth/S65 Skipjack 3:3

Table V

Skipjack spawning periods in the Pacific Ocean

Area Author Spawning period Evidence'

Central America Schaeier and Marr, 1948 January - March Gonad; juveniles Central America Schaefer and Orange, 1956 December - Marc1 Gonadal Revillagigedos Schaefer and Orange, 1956 Summer and Fall - Gonadal Hawaiian waters Brock, 1954 February - SeptemberGonadal Hawaiian waters Sherman and Brown (MS) May - July Larvae Central Equatorial Pac,Matsumoto, 1958 All months except Larvae April and December Marquesas -Society Is. Yoshida (MS) November to April Gonadal and larvae Marshall Is. Marr, 1948 April to August Gonadal and juveniles Philippine waters Wade, 1951 All year with peak Larvae September to April Japanese waters Yao, 1955 June - August Gonadal Japanese waters Yabe, 1954 May to August Gonadal

J Evidence: Gonadal = Presence of ripe or maturing ovaries in adults observed. Juveniles = Juveniles collected. Larvae = Larvae collected. J Near the Revillagigedos summer and fall would be from July to December approximately.

- Oceanic (surface, bottom) which in the most primitive stage appear as simpl&transparent cells. The larger ova in Again there is no direct evidence as to the this stage contain a relatively large nucleus. exact location of spawning, but most of the The mean diameter ranged from 0.16 to larvae collected have been taken in plankton 0. 33 mm. Ova in this category are present tows made at some distance from land, in all ovaries. The developing ova are Because most of the larvae have been completely opaque owing to the deposition collected from the upper 200 m of the ocean of yolk granules. The mean diameter of (Strasburg, 1960) it might be assumed that these ova ranged from 0.37 mm to 0.66 spawning takes place no deeper than this. mm. The advanced stage includes ova ranging Further, fishermen have reported sighting in appearance from those which were still spawning fish near the surface, however, the relatively opaque and containing a cluster of spawning activity was deduced from the small oil droplets, to those which were semi- behavior of the fish rather than from an transparent and containing a well-developed, actual observation of the spawning act. bright yellow oil globule.The mean diameter of these ova ranged from 0.49 mi-nto 0.74 3.1.8 Egg: structure, size, hatching mm." Yoshida goes on to describe the type, parasites, and predators running ripe eggs of skipjack as, "They were almost perfectly spherical and transparent, Yoshida (MS) gives the following descrip- with a distinct straw-colored oil globule. tion of the ovarian eggs of skipjack: "Four Fifty were measured. They ranged in diame- rather distinct developmental stages were ter from O.85mm to 1.12mm with a mean recognized and designated asearly develop- diameter of 0,96 mm. The oil globule ing: developingJ'advanced',and¶ripe'. measured about 0,14 mm in diameter," The early developing category includes ova

710 3:4 FIb/S65 Skipjack

Brock (1954) described running ripe are not yet fully developed at the posterior end skipjack ova as averaging 1.125 mm in of the body, but there appear to be forty-one diameter, spherical to slightly oval, with a of them." single oil globule which was between 0.22 and 0.45 mm in diameter. Yabe (1954) Wade (1951), Yabe (1955), and Matsunioto gave the size of running ripe ova as 0.80 to (1958) provide descriptions of larval skipjack 1,17 mm diameter (average 1.00 mm), less than 15 mm in length collected from the with a single oil globule measuring 0.22 to Pacific area. Two characteristics which 0.27 mm in diameter (average 0.23 mm appear in all of the descriptions are the large size of the head, and the presence on the mid- No information is available concerning ventral portion of the caudal peduncle of a the embryonic development, hatching type, melanophore or group of melanophores. These parasites, or predators of the skipjack two characteristics, together with the presence egg. of 41 myorneres, appear to distinguish skipjack larvae of about 4 to 5 mm length. 3.2 Larval history Matsumoto (1958) gives the following distin- guishing characteristics of skipjack larvae 3,2.1 Account of embryonic and between 3.7 and l4.5mm in length (Table juvenile life (prelarva, 1arva VI). post larva, juvenile) (Fig. 6) Kishinouye (1926), Marr (1948), Schaefer Accounts of the embryonic development and Marr (1948), Wade (19.50, 1951), and of skipjack are lacking from the literature. Shimada (1951) describe the juvenile skipjack, The first distinguishable skipjack are in the generally those above 15 mm in length.The post-yolk-sac stage at a length of about 3 most reliable characteristic used in identifying mm(Kishinouye, 1926).Considering that specimens above this size appears to be the the ripe ovarian egg is about 1. 0 mm in presence of 41 vertebrae and the complex diameter, the skipjack larvae must approach trellis or basket-work formed by the haernal 3.0 mm in length at hatching. As Kishinouye arches.In stained specimens these are himself points out, these 3.00 mmlarvae visible in juveniles as small as 27 min length must have been hatched very recently. The (Wade, 1950), although the haemal arches are first specimens which Kishinouye (1926) not completely closed. identified positively as skipjack were 4 mm in length. He described this specimen as Preopercular spines, which have been follows: "The head is greatly developed used in describing the larvae become buried and about 10 large sharp teeth appear on each in bone by the time juveniles reach 40 mm side of both jaws.The vomerine teeth are (Schaefer and Marr, 1948), while the palatine not yet developed. There are two spines on teeth do not become overgrown until about the surface of the preopercle and five or six 60-75 mm length (Kishiriouye, 1926).The spines on its edge. The spines on the adult fin ray-and-spine complement is posterior edge are long and stout, with the attained at a length of only 15 mm, except one at the angle being the longest. There is for the pectoral in which rays continue to only a shallow round depression at the develop until about 30 mm length is reached. location of the nasal cavity.The distribution The trellis formed by the haemal arches is of melanophores is roughly the same as in fully developed at about 27 mm, and the the specimens described above ote: these vertebrae are fully ossified at a length of 10.9 refer to the 3 mm specimenjl, but one min (Matsumoto, 1958).Certain charac- notable difference is the presence of a large teristics of juvenile skipjack are listed in melanophore slightly anterior and ventral to Table VII. the caudal peduncle. There are also many melanophores around the anterior end of the spinal cord, and the tip of the mandible is blackish.The posterior end of the spinal cord is perfectly straight.It is difficult to tell the number of myotomes because they

711 Flb/S65 Skipjack 3:5

- Feeding 3. 3. 3Competitors Little is known regarding the food of larval In the commercial and experimental and juvenile skipjack.Botta and Ogawa (1955) fisheries of the Pacific the following fish have found that the stomachs of juveniles which had been observed in or taken from feeding aggre- been consumed by adults contained fish and crus- gations of skipjack, and should probably be taceans, with a higher proportion of copepods considered competitors for food than had been found in adult stomachs, Yuen (1959) found that among the smaller adults the Whale Shark diet consisted of a larger proportion of Crustacea (Rhineodon typus) (Gudger, 1941) and: molluscs than that of the larger adults. Nakarnura (MS) found that the stomachs of juve- Yellowfin Tuna nile skipjack contained mysiids, euphausiids, (Neothunnus macropterus)(Broadhead & Orange, amphipods, megalops larvae, copepods, squid 1960) and fish. Little Tunny (Euthynnus yaito) (Data files, BLH) 3.3,Adult history Frigate Mackerel 3.3,1 Longevity (Auxis sp,) (Data files, BLH) No direct evidence exists concerning the Albacore greatest age attained by skpjack. Brock (1954), (Thunnus germo) (Data files, BLH) on the basis of modal groups in the Hawaiian skipjackcatch, hypothesized that the largest Dolphin group (fork length about 75 to 85 cm) was com- (C.oryphaena hippurus) (Data files, BLH) posed of skipjack in their 4th year.If his estimate of the age at which the fish entered the Rainbow Runner fishery was in error by 1 year, which he stated (Elagatis bipinnulatus) (U.S. Fish and was possible, the largest group would be in their Wildlife Service, 1961) 5th year. Aikawa and Kato (1938) used the growth rings on vertebrae to estimate the age of skipjack These are the adult fishes known to compete from Japanese waters.They estimated skipjack with skipjack.In addition sea birds may be of the 72 to 80 cm length group to be 7th-year considered competitors, for both birds and fish, or in age group VI. Recent tagging studies skipjack feed on the same aggregations of indicate that the growth deduced by Brock (1954) forage organisms at the sea surface. is more nearly correct than that estimated by Aikawa and Kato (1938).Skipjack larger than The literature contains no mention of those found generally in the Hawaiian fishery are small fish which may compete with juvenile known to occur, and presumably these fish have skipjack. also attained a greater age. 3,3,2 Hardiness The literature contains no references to the hardiness of skipjack living in the normal range of their environment, Under abnormal conditions such as when fish are caught, either in nets or by hook-and-line, skipjack are thought to die quicker than some of the other tunas, such as albacore and yellowfin,

712 3:6 FIb! S65 Skipjack

MM. Fig. 6aLateral view of larval skipjack, 5. 35 mm length (from Matsumoto, 1958)

I MM. Fig. 6bLateral view of larval skipjack, 10. 9 mm length (from Matsumoto, 1958)

713 Flb/S65 Skipjack 3:7

Fig. 6cLateral view of juvenile skipjack, 21 mm length (from Schaefer and Marr, 1948)

Fig. 6dLateral view of juvenile skipjack, 47 mm length (from Matsumoto, in press)

71- Summary of the more prominent diagnostic characters of skipjack larvae at various sizes (after Matsumoto, 1958, table 5) Table VI Length (mm) MyomeresP reTeeth opercular (1 side)spines Characters 3.7 5.35 7.1 8.75 10.9 14.5 Vertebrae ist dorsal spines ist dorsal fin Pigmentation1stMiddorsaloutline dorsal fin line 2/

Midlateral line

Midventral line

incomplete4 upper;-3 6 lower MidbrainAnalTip of opening pectoral girdle Forebrain -

- no- chrom,

no chrom.

13 upper; 11 lower 1 chrom, on 416 or- 42 no chrom. pedunclecaudal no chrom, no10 chrom.chrom, overlarge area -

- no- chrom,

no chrom.

i chrom. oncaudal 15 each jaw no chrom, peduncle no chrom, 7 no24 chrom. chrom, overlarge area 8;41 -longest spine < 1/3 1 chrom.- - interspacedorsal 2/ 1/

no chrom.

16-18 each jaw i chrom. on no chrom. pedunclecaudal no chrom. 7 50 chrom, over 41-42- 2 chrom. large area 8 or- 9 6-86 chrom. chrorn. on atdorsal;outerbase fin of i edgeat ist origin of 2nd no chrom, dorsal 3 chron-i. at1 onbase caudal of anal; 15 each jaw no chrom. peduncle no chrom, 13;20+219- longest (on a spinespecimen)9.85 < mm. 1/2 45+ chrom, ove1wide area 2-3 chrom, concave interspace-dorsal 1/ chron-i. on outer edge to chrom. to ist12th spine 6 chrom, for-wardfinlet of cau- 4 chrom. atbasesdal peduncle of anal and finlets; 1 1520+218 upper;- 17 lower on caudal 16; longest spine= 1/2 dorsal peduncle concaveinterspace» no chrom. Band6 or 7 of chrom, chrorn,fin ontoouter 5th edge finlet of 4no chrom.80 chrom. chrorn. overlarge area BandBand of of chrom. chrom.fromat5th bases 2ndfinlet to of anal and finlets; i on no chrom. caudal peduncle no chrom. 12numerous chrom, chrom.over large area Summary of the more prominent diagnostic characters of skipjack larvae at various sizes (after Matsurnoto, 1950, table 5) (con.) Table VI Mandible (otherthan at tip) Characters no chrom. 3.7 no chrorn. 5.35 1 chrom. 1/4distance from 7.1 Length (mm) 8.75 10.9 14.5 ¿I1/ distance between insertions of chrom.ist and =2nd chromatophores dorsal fins tip 1 chrom. 1/2distance from tip 1 chrom. chrom. on ¿/3 lengthof Spines Teeth Some characters of larval and juvenile skipjack Table VII Gill Hae- o Kishinouye 60-75(1926) Author Seci- Pre- lenh operclemm sur- edge face Each jaw Pala- ist D. 2nd D. D. finlets Anal- tine l0 Fin rays and spines A. finlets Caudal Pelvic Pectoral rakers mal trellis MarrSchaefer (1948) and(1948) Marr 45442150 2 or 3 0-- - -- Pres. - XVI - 15 - 8- 15 - 7 - - 26 68 + 3233 - Wade (1951)(1950) 13-27 75 69 3 lowerupper - 26- 21 10-12 8 Pres. --8 XVI 9 1115 - 7 or 8 14 or 15 5-- 10 5 7- 6+32 ShimadaMatsurnoto (1958)(1951) 35 3.79 381 2-3 upper - 4 16-18 - Pres. 14 - XVI no fin rays 1415 8 8 1513 - 77 6 27 6.75.35 76 3 3 lowerupper -- 6il-13 - - - - 12 + 11 18 14.10.9 5 8.757. 1 97- 3 3- lowerupper - 17 - 15 16-18 - 6 XVI Pres.1510 8 6-7Pres. 8 1410 - 6-7 7 2216 + +¿2 16 - 1+5I + 5 striations - ¿0 - -not- de- veloped - 2027 - - - XVI 15 8 15 7 22 + 22 - 20-21 -l3th-22n4 vert. Lflo t' Comments o partially pi - - l4th-33rdclosed presentAll spines at 15and mm. rays e o closed 'r- FIb/S65 Skipjack 3:11

3.3.4 Predators The literature contains references to the following animals as predators on either juvenile or adult skipjack: Katsuwonus pelamis Eckles 1949, Marr 1948 Neothunnus macropterus Kishinouye 1924, Reintjes and King 1953, Koga 1958 Thunnus sermo Koga 1958 Parathunnus sibi Koga 1958 Istiompax marlina Royce 1957 Makaira ampia Royce 1957, Koga 1958 Makaira audax Royce 1957 Makaira mazara Shimada 1951 Istiophorus orientalis Shimada 1951 Cybium chinense Imamura 1949 Alopias (sp.) Strasburg 1958 Acanthocybium solandri Iversen and Yoshida 1957 Gempylidae Nakamura (MS)

3.3.5 Parasites The skipjack is listed by various authors as host for the following parasites: Cestoda

Tentacularia coryphaenae (Bosc 1802) Yamaguti 1934 Cailotetrarhynchus specios us (Linton 1897) Yamaguti 1952 Pe]jchnjbothrium (larva) Yamaguti 1934 Rhyncobothrium Kishinouye 1923 Trematoda

Dinurus thynni Yamaguti 1934 Yamaguti 1953 Syncoeliam katuwo Yarriaguti 1938 Yamaguti 1953 Didymozoon filicolle Ishii 1935 Yamaguti 1953 Didymozoon longicolle Ishii 1935 Yamaguti 1953 Didymozoon minor Yamaguti 1934 Yamaguti 1953 Didymozoon auxis Taschenberg 1879 Dawes 1947 Didymocystis abdominalis Yamaguti 1938 Yamaguti 1953 Didymocystis bilobata Ishii 1935 Yamaguti 1953 Didymocystis dissimilis Yamaguti 1938 Yamaguti 1953 Didymocystis ovata Ishii 1935 Yamaguti 1953 Didymocystis simplex Ishii 1935 Yamaguti 1953 Didymocystis soleiformes Ishii 1935 Yarnaguti 1953 Didymocystis submentalis Yamaguti 1938 Yamaguti 1953 Didymocystis wedli Ariola 1902 Yamaguti 1953 Didymocylindrus filiformis Ishii 1935 Yamaguti 1953 Didymoproblenia fusiforme Ishii 1935 Yamaguti 1953 Neodiplotrema pelamydis Yamaguti 1938 Yarnaguti 1953 Lobatozoum multisacculatur-n lshii 1935 Yamaguti 1953 Kollikeria globosa Ishii 1935 Yamaguti 1953 Kollikeria orientalis (Yamaguti 1934) Yamaguti 1953

718 3:12 FIb! S65 Skipjack

Kollikeria reniformis Ishii 1935 Yamaguti 1953 Tergestia laticollis Manter 1940 Hirudinella clavata Manter 1940 Hirudinella marina Garcin 1730 Nigrelli and Stunkard 1947 Hirudinella ventricosa (Pallas 1774) Nigrelli and Stunkard 1947 Tristomurn laeve Verrill Linton 1901

N em atode s

Anisakis (larva) Yarnaguti 1941 Philometrojdes sp. Personal comm. J. Harada Acanthocephala

Nipporhynchus ornatus (Van Cleave 1918) Van Cleave 1940 Acanthocephala (as a group) Van Cleave 1940 Crustacea: Copepoda

Caligus aliuncus Wilson Wilson 1937 Calius bonito Wilson Wilson 1905 Caligus katuwo Yamaguti 1936 Caligus pelamydis Kroyer Wilson 1905 Caligus productus Dana 1854 Wilson 1905 Caligus tessifera Shiino Shiino 1952 Caligus thymni Dana 1852 Wilson 1905 Homoiotes bermudensis Heegaard 1943 Lepeophtheirus dis simulatus 1-Ieegaard 1943 Lepeophtheirus salrnonis Heegaard 1943

3.3,6 Greatest size the body temperature exceeded the water temperature by 2.1°C to 3.4° C. More recently In the literature the greatest size mentioned Fukushima (1953) found that the temperature of is a length of about i m and a weight of 25 kg; the body cavity of skipjack was 1.4° C higher Kishinouye (1923) states that this size is rarely and the so-called TTred muscle' was 590 C higher attained. than the surrounding water temperature, 3.3.7 Body temperature 3. 3. 8 Biochemistry (chemical composition, amino -acids, seasonal varia - Tuna as a group appear to have a body tem- tions) perature higher than that of the surrounding waters, and in this respect differ from many This section is not intended to cover exhaus- other fish.Because of the problems of obtain- tively the field of biochemistry of skipjack but ing accurate measurements of body temperature only to give a brief review of the chemical com- of living skipjack, the only measurements have position of the fish and some of its organs. been of recently killed or dying fish, Uda (1941) reported the body temperature of skipjack to be Dill (1921) analyzed 10 skipjack from southern 1° C to 3° C higher than the water from which California waters and gave the following composi- they were taken, while Watanabe (1941) found tion of the edible portion, i. e., the lateral muscu- lature:

719 FIb/S65 Skipjack 3:13

Moisture - 65.94% Horiguchi, Kashiwada, and Kakimoto (1953) Solids 34,06% found that the pyloric coeca contained the Ether Extracts 7. 37% cations Al, Zn, Fe, Ca, Mg, Na, K, and Ash L 30% traces of Ni, and Co, the anions S, P, Cl, and Total Nitrogen 4.07% traces of Si.This differs from the list of elements shown by Imanishi (l960a) in containing nickel and cobalt and lacking copper, Suzuki, et al(1909) found that the soft lead, manganese, and tin. tissues of skipjack contain i. 46% phosphorus on a dry weight basis. More rcent1y Imanishi In additional studies of the pyloric coeca in a series of papers (Imanishi l960a, 1960b, Kashiwada, Kakimoto and Horiguchi (1952) 1960d, 1960e, 1961a, l961b, 1961c) has described found a seasonal variation in the crude fat in some detail the inorganic chemical constitu- content of this organ, with the content highest ents of the various organs and regions of the in the spring, decreasing during the summer, skipjack body. He lists (1960a) the following increasing in early autumn, and decreasing elements as occurring in the ash of skipjack, slightly during late autumn. determination being by spectrographic analysis: Within the past two decades considerable Aluminum Manganese work has been done in describing the protein Calcium Phosphorus structure of skipjack, a major portion of this Carbon Potas sium work being done by Japanese scientists,Lukton Chlorine Silicon and Olcott (1958) described the occurrence of Copper Sodium free imidazole compounds in the muscle tissue 1ro n Sulphur of skipjack, including the amino acid histidine Lead Tin and the polypeptides carnosine and anserine. Magnesium Zinc Table VIII lists the amino-acids found in skipjack by Sugimura, et al(1954).Matsuura,

Table VIII Amino-acids found in skipjack (Sugimura, et al., 1954) 1-

Amino-acid Percent of total Amino-acid r Percent of total N in muscle N in muscle Arginine 8.7 Methionine 1,0 Aspartic acid 6.3 Phenyl-alanine 2.0 Glutamic acid 5.9 Proline 4.3 Glycine .5.3 Serine 2.6 Histidine 5.7 Threonine 2.2 Isoleucine 3.1 Tryptophane 0.6 Leucine 4.2 Tyrosine 1.5 Lysine 9.1 Valine 7.2

1/ In addition to the above, Kakimoto, Kanazawa and Kashiwada (1953) reported the presence of Alanine, Cystine, and Ornithine.

1/ Dill does not list moisture, but Vinogradov (1953) attributes this figureto Dill (1921).

720 3:14 FIb/S65 Skipjack

et al. (1954) listed most of these sanie amino- The vertical range over which skipjack feed acids, and also noted slight differences in the is not known. However, it is known that they composition of certain muscle layers. feed at and just below the surface, for it is at this time that it is possible to observe them. In other studies Kashiwada (1952) found a To a surface observer it appears that skipjack seasonal change in the proteolytic enzyme drive their prey to the surface, where both the activity in the pyloric coeca, with the highest fish and any nearby birds feed on the forage activity during the spring, a decrease in the organisms. Fishermen utilize this behavior summer, and a fluctuating degree of activity to locate fish schools, i. e. ,a flock of birds during the fall. He noted that the enzyme seen feeding at the surface may be taken to activity appeared to follow the skipjack catch indicate a fish school in the same location. in the southern sea of Kyushu. - Manner Other articles concerned with the biochemistry may be found in such journals as the The exact manner in which skipjack feed is Bulletin of the Japanese Society of Scientific not well known. Kishinouye (1923) and Imamura Fisheries. (1949) state that skipjack may surround and concentrate schools of small fish and feed on 3.4 Nutrition and growth the periphery, rather than make dashes into the center of this mass. 3.4.1 Feeding (time, place, manner, s e as on) Recently, during the course of experimental fishing in Hawaiian waters, it has been - Time possible to observe, through subsurface viewing ports, the actions of feeding skipjack. Very little is known concerning the feeding Certain of these observations, not yet pub- habits of skipjack, and most existing knowledge lished, will be reported here as personal has been deduced from variations in the catch communications from members of the Behavior rates of commercial fishing, or from varia- Program at the Bureau of Commercial tions in the volume of the stomach contents Fisheries Biological Laboratory at Honolulu. rather than from direct observation. Skipjack swim in a horizontal attitude with their mouths slightly agape while searching for food. tJda (1940) stated that skipjack feed most When taking prey of a size visible to an actively in the early morning and somewhat observer, the mouth is opened and then closed. less actively around noon and just before dusk. Food organisms may be taken while the fish is Nakaniura (MS) hypothesized a midmorning moving in any direction from vertically upward period of active feeding, followed by a midday to vertically downward. minimum and another period of active feeding just before sunset. Data from Yuen (1959) - Season indicate minimum feeding activity in early afternoon with higher degrees of feeding Hotta and Ogawa (1955) found seasonal activity in the morning and later afternoon, differences in the types of food consumed by although the differences are not statistically skipjack in some areas and no differences in significant. that consumed in other areas,In general the greatest differences were in the non-fish - Place portion of the diet (excluding larval fish).lt was felt that the diet was determined largely by Feeding skipjack are found in oceanic the availability of particular items, e.g., when waters, both near to and far from land, over larval forms were abundant, large quantities of shallow or very deep water. Aside from these larvae would occur in the stomach contents. rather general statements which may be found in Kishinouye (1923), Suyehiro (1938), Nakamura Nakarnura (MS) found little seasonal vari- (MS), and others, little is known regarding the ation in the amount of fish consumed, but the place in which skipjack feed. amount of Crustacea consumed was lowest in

721 FIb! S65 Skipjack 3:15 the winter and highest in summer, while the 3.4.3 Growth greatest amount of Mollusca was consumed in the winter and the least in the spring. Hayashi (1958) reviewed the age determi- nations of Pacific tunas, including skipjack. 3.4.2 Food (type, volume) He noted considerable disagreement between the only works describing growth and age of The major food items have been described skipjack, those by Aikawa (1937) and Aikawa as fish, crustaceans, and molluscs by various and Kato (1938) working with skipjack from authors (Kishinouye, 1923; Suyehiro, 1938; Japanese waters, and that by Brock (1954) Welch, 1949; Ronquillo, 1953; Tester and working with skipjack from Hawaiian waters. Nakamura, 1957; Nakamura, MS; Waldron and Hayashi attributed part of the difficulty in King, 1962).In addition there are numerous aging skipjack by means of rings on hard parts, other reports with brief mention of food as the method followed by Aikawa, to the slight fish, crustaceans, and molluscs or squids. seasonal change of the environment in which The most comprehensive studies to date are skipjack live. those by Hotta and Ogawa (1955), and unpublished manuscript by Nakamura (MS), and Waidron and In their study of the age of skipjack, using King (1962).Table IX compiled from the above vertebral rings, Aikawa and Kato (1938) gave sources, lists the food. organisms consumed by the following lengths, weights, and growth skipjack. rates for skipjack from Japanese waters. Hotta and Ogawa (1955) give the amount of Age group Length range Weight range Growth rate food consumed in terms of grams of food per cm kg kilogram of body weight.The greatest weight of any one organism is 81.5 gro 1kgbody 0 <27 < 0.4 weight, with other values ranging down to a few I 27 - 37 0.4 - 1.3 0.90 tenths of a gram or to zero. Nakamura (MS) II 37 -46 1.3 - 2.5 0.69 lists the average volume of stomach contents III 46-55 2.5 -4.1 0.53 for skipjack under 60 cm length as between 4 IV 55-64 4.1-6.5 0,46 and 24 cc,with considerable variation with V 64 - 72 6.5 - 9.5 0.37 time of day at which the fish were captured. VI 72 - 80 9.5 -13.0 0.31 Yuen (1959) lists values of up to 214 ccfor VII >80 > 13.0 7 large skipjack, i.e., above 60cm, and gives values of 9.1 rol,20.4 rn1 and 35.6 ml as This growth is considerably different from the mean volume of stomach contents for fish the growth as determined by Brock (1954) based of less than 50 cm,50 to 60 cm, and greater on the presence of modes in the size distri- than 60 cm. fork length respectively. bution of Hawaiian skipjack. Brockts growth curve shows that at the end of periods of 1,2, With respect to size of organisms consumed, 3, and 4 years skipjack may attain lengths of Waidron and King (1962) found that the average 41 cm,69 cm,79 cm, and 82 cm. volume for individual Crustacea was 0. 2 cc., respectively. His estimates are thus greater for Mollusca 3.9 cc,and for fish 3.7 cc.The than those of Aikawa and Kato by a factor greatest average size of organisms consumed approaching two. Kawasaki (1955b, his was 34.8 cc(a squid, Sepioteuthis sp.) and figure 8) shows that Japanese skipjack of the 34. 3 cc(a fish, Decapterus pinnulatus). "Archipelago group" grow about 18 cm during the third year and 12 cm during the fourth Rather striking differences are observed in year.This is much more rapid growth than the food of skipjack from different areas. that estimated by Aikawa and Kato and more Waidron and King (1962) reported that fish, nearly approaches the growth hypothesized by molluscs, and crustaceans make up Brock (1954) for Hawaiian skipjack. respectively 75, 20,and 4 percent of the diet of central Pacific skipjack, while Schaefer On the basis of growth of tagged skipjack (1960) reports these same groups contribute released at a size of about 2.5 kg,Yamashita about 33, 4, and 62 percent of the diet of and Waidron (1959) estimated an increase in eastern Pacific skipjack. weight of about 0.36 kg per month, or about

722 3:16 FIb/S65 Skipjack

Table IX A list of food organisms consumed by Katsuwonus pelamis

Lfl in o'

Arthropoda X X X X X Crustacea X X X X X C opepoda X Malacostraca X X XX Mys idacea X Mysiidae X Is opoda X Amphipoda X X X Hyperüdae X Phronirnidae X Phronirna sp. X Phrosjnjdae X Pronoidae X Oxycephalidae X Oxycephalus sp. X Euphausiacea X X X Euphaus iidae X Thysanopoda tricuspidata X Euphausia sp. X Decapoda X X X X X Natantia X X X X Pandalidae X Heterocarpus ensifer X Reptantia X X Scyllaridae X Paljnuridae X Phyllos orna larvae X Nephropsidae X Enoplometra occidentalis Brachyuran zoea larvae X Brachyuran megalops larvae X X X Crab larvae X Stomatopoda X X X X Squillidae XX Squilla sp. X Pseudosquilla sp. X Ps eudos quilla ciliata X Pseudosquilla occulata X Lysiosquilla sp. X Lysiosquilla maculata X Odontodactylus sp. X X

723 FIb/S65 Skipjack 3:17