SPAWNING GROUNDS OF TUNA AND SKIPJACK SPECIAL SCIENTIFIC REPORT: FISHERIES No. 18 UNITED STATES DEPARTMENT OF THE INTERIOR FiSH AND WILDLIFE SERVICE SPAWNING GROUNDS OF TUNA AND SKIPJACK SPECIAL SCIENTIFIC REPORT: FISHERIES No. 18 UNITED STATES DEPARTMENT OF THE INTERIOR FISH AND WILDLIFE SERVICE Explanatory Note The series embodies results of investigations, usually of restricted scope, intended to aid or direct management or utilitation practices and as guides for administrative or legislative action. It is issued in limited quantities for the official use of Federal, State or cooperating agencies and in processed fonn for econc^y and to avoid delay in publication* Washington, D. C* April 1950 Uhited States Department of the Interior Oscar L. Chapman, Secretary Fish and Wildlife Service Albert M. Day, Director Special Scientific Report - Fisheries No. 18 SPAWNING GROUNDS OF TDNA. AND SKIPJACK Translated from the Japanese language by SCAP translators Edited by 6« M* Shimada and W. G» Van Cam pen Rioifie Oceanic Fishery Investigations CONTENTS Preface Page 1* A symposium on the investigation of tuna and 1 skipjack spawning grounds l/.*..* 2* A contribution to the study of tuna spawning grounds by Kenso Ikebe 2j % .•••• 12 l/ From South Sea Science [Kagaku Nanyo] Vol, 4, No. 1, pp. 64-75. July 1941. _ 2/ [From South Sea Fishery News Nanyo Suisan Joho] Vol. 5, No. 4, pp. 9-12 September 2, 1941. Participants (December A, 194.0, University Alumni Association Hall) Oshima, Masamltsu: Professor, Tokyo Prefectural Higher School Kawahara, Shlchiro: Chief of the Katsuura Branch, Chiba Prefectural Fisheries Experimental Station Kawakanl, Zenkuro: "^anko T'lshing Coc, Ltd, Takatsuki, Shunlchi: Professor, Tokyo University of Literature anr? Science Takeda, Shigeo: Japan Tune and Skipjack Fisheries Association Okada, Yalchlro: Professor, Tokyo Higher Mormal School Marukawa, Hisatoshi: Advisor, Marine Chemistry Institute Anemiya, Ikusaku: Professor, Tokyo Imperial University Seno, Hidemi: ''Ilppon Women's College SumlnOy Isamu: 'lippon Fishing Co., Ltd, Sponsors Hatal, Shinkishl! Director, Palau Trotilcal Biological Station; Professor Umeritus, Tohoku Imperial University Hanedaf Tata: Researcher, Palau Tropical Biological Station; Member, Dept. of Wediclne, Tokyo Charity Hospital Kedical College Kanai, KlyosM: Councillor, Pacific Society Takahashi, Keizo: Researcher, Palau Tropical Biological Station; Dept. of Zoology, Tokyo University of Literature anrt Science Nilya, Buel: Depto of Science, Japan Society for the Promotion of Science Hirano, GitarO: Pacific Society Fjitai: I vyir:h to 3r.pr5s;n my deop appreciation to you for s. crifioing your -"aluable time to asseiQble h3re» You have already been informed as to the purpose of to- night's meeting so I believe it is unnecessary for me to rev.ew it in detail. How- ever, as there are a few persons present tonight who are not acquainted with the objective of this gathering, I would like to say u few ?yords .''or their benefit. There exist an extremely large niunber of marine fish wh )se spawning grounds have not yet been determined. In fact, I feel I do not exagg3rate when I say that almost none have been delineated. Furthermore, notliing has been confirmed with respect to the skipjack and tuna, which are of great economic importance, especially to the Japanese fishing industry. It ia to be regretted that so little is known about the spawning grounds, spawning habits, and development of these fish. This difficult problem, however, is not limited to Japan alone - it is universal in na- ture. In Japan, such persons as the lete Kamakichi Kishinouye worked assiduously in collecting skipjack and tuna juveniles. Technician Marukawa, who is present to- night, and others have also been deeply concerned with this problem. They are now conducting a careful investigation but results are as yet unknown. Scientists from various foreign countries have also worked hard on this subject, but despite all this, their situation remains the same as that in Japan in that no conspicuous con- tributions have been made towards its solution. Due to these circumstances, each Pacific Science Congress usually takes up the question of skipjack and tuna spawn- ing and submits measures to undertake its solution through joint action of various countries. Although many helpful discoveries and studies pertaining to fisheries are reported on at these conferences, held at intervals of three or four years, the problem concerning the spawning habits of skipjack and tuna remains the same as when it was first presented more than ten years ago. No new facts have yet been ascertained. However, in generalizing on the common efforts and observations of fishermen and ichthyologists, the spawning grounds of skipjack and tuna are, to all appear- ances, in tropic seas. For example, information such as the capture of skipjack ready to spawn or collection of juveniles believed to be those of skipjack is usually received from the tropics. Fortunately, our country today has extensive tropical possessions. There is also the Tropical Biological Station sponsored by the Japan Society for the Promo- tion of Science. It has been my wish for many years to gather various Japanese scientists in an attempt to solve this important problem of spawning, using this Station as a ba^e for operations. The opportunity to realize this desire was made possible through the positive support given by the Pacific Society, I have, there- fore, requested the presence here tonight of you authorities on the tropics and those of you who are actually active in the area, to hold a discussion on skipjack and tuna with the main emphasis being placed on spawning. With this symposium as a foundation, I thought we could take up the formulation of a definite plan as to how this undertaking should be started and how it should be effected. Your presence here despite the pressure of other duties is gratifying indeed., Since we do not have too much time, this will serve as an introduction. Now we would like to hear your opinions. I would like first to call upon Mr, Marukawa. Marukawa; In Japan, research on skipjack and tuna was conducted qhiefly by Dr.. Kishinouye. Although there are Mr. Kit,ano*s report and a few others in taxonomy, I believe that Dr. Kishinouye was the first to make a scientific study of these fish^ The question as to the location of the spawning grounds of skipjack and tuna still is definitely a mystery; I can only conceive it to be in the South Seas, I believe that research soon to begin at the Palau Tropical Biological Station in the South Seas is most ideal. At this point, I will go back into the past for information \ cor-cerning the cuy.jecb. In 1907, I made a tour of about two weeks aboard the Ka^^-abusa f'iaru of the Fisheries College, which made a hydrobiological sui'vey of f5.sh- dng £i'ounds centered around Izu Oshima, At that time, a small fish was obtained from the stomach of a dolphin. This was reported as a young skipjack, but Dr. Kishinouye late? stated that the specimen was probably a young frigate-jnackerel. Dr. Kishinouye subsequently started to study skipjack young and requested all fish- eries experimental stations in the coujitry to forward to him stomach contents of skipjack. JuTfeniles were caught in larval nets used near the Toshima Islands of Kagoshina, These specimens, said to be most likely the young of skipjack, have al- ready been reported upon by Dr. Kishinouye at the Pan-Pacific Congress. I believe it can be said that no further scientific studies have been conducted on skipjack and tuna since Dr. Kishinouye' s work, I, too, was constantly on the lookout foi; young fish and had skipjack juve- niles recovered from the stomachs of skipjack being processed into dried sticks un- der the supervision of Mr. Kawakami of the Nanko Fishing Co., Ltd. About 20i4 specimens have been gathered to date and are repoaited with the Fisheries Experi- mental Station in Tolcyo, These specimens (shown in figure) vere found in skipjack taken e t Amarai -Oshima in 1922. About half of these were collected at the Nanko Fishing Co., Ltd. Juve- niles of big-eyed tuna measuring 3.5 to 3.6 sun (4.2 to 4.3 inches) were found in- side yellowfin tuna taken by longlines in the Tokobei area last year, while I was in Palau, by a ship of the Fisheries Experimental Station. In May, 1934, a report of a large school of blue-fin tuna was received from the Shoy5 Maru of the Kagoshima Prefectural Fisheries Experimental Station. Eggs were collected with a plankton net and samples were forwarded to us. These eggs were presumed to be those of tvina and upon examination, were found to be pelagic eggs covered with an oily layer. A large number of blue-fin tuna are caught in the Luzon Straits each year around May. The fishing season is from April to June. When the stomachs of fish caught in May are cut to remove the livers, careful observations of the reproductive organs reveal the ovaries to be of considerable size. The ovaries are extremely well developed in the early part of May, but are said to become smaller between the middle of May and the early part of June together with a loss of weight in the fish. This is based on statements made by members of the Foraosa Fisheries Experimental Station. In tVakasa Bay, the gonads are large in June; in the Japan Sea, spawning probably takes place in June. Technician Shimoda of the Ministry of Agriculture and Forestry visited Java, the Sulu Sea, and the Celebes area eight years ago and returned fdth ovaries of yellowfin tuna from this region. They measured from five to six sun (6 to 7.2 Inches) in length and from one and a half to two sun (1.8 to 2.4 inches) in diameter. The fish from which these ovaries were taken were caught In May and June. I thought that the ovaries were fairly well developed and this led me to wonder whether spawn- ing takes place In May or June in the Java, Sulu, and Philippines area. In 1913 and 1914 the Fisheries College conducted an experiment using purse seines to catch tuna near Hahajlma in the Benin Islands. At that time Mr. Kamishiro Kumada squeezed the bellies of tuna caught by the net and obtained eggs, these were placed in a cup for artificial fertilization, but the experiment failed because of the inability to cap- ture a mature male. However, the eggs were retained and brought back. Mr. Kumada' s experiment was the only attempt at artificial fertilization of mature eggs. It was decided recently to conduct similar tests under Director Hatai, but I believe that the experiment cannot be carried out without difficulty because of its complexities. I am billing, however, to give all possible assistance to this project, I wanted to go to Palau last year to study the spawning season of skipjack, but I arrived there between November and January so that I was unable to find a mature ovary among those which I examined. I requested Mr, Kawakami to gather data on the length of the fish, the condition and size of the gonads, the number of vertebrae, etc. In addition, 1. also requested a record of stoaach contents, the isize of fish, and daily catch. In looking at a table of measuremGiits, the lengths and weights of skipjack caught by pole and line are small. The -^ir^ller ones vreighed about 200 momme (1.65 lbs,) and the larger ones, from 300-4C0 momniQ (2,4. to 3.3 lbs.). Among the larger ones, there were fish that v;eighed or;e kan (8.27 Ibs.)^ The catch in May was extremely large so I thought that skipjack spawn possibly in April and May« Not too many of the larger skipjack were caught by pole and line, and I believe that this is prob- ably due to the fact that as the ovarj' enlarges, it presses against the internal organs causing a loss in appetite. Furthermore, 1 believe this to be the solution to the question why the possibilities of catching large mature skipjack decrease during the spawning seasono I believe that research would be expanded if nets as well as hooks were used to capture fish. At any rate, it would confirm the fact that ovaries of skipjack are comparatively large from April on, reaching maturity In May, In searching for spawning grounds, juveniles are collected and the sector from which these are abundantly recovered is assumed to be the spawning area. If a thorough investigation were made around Palau, I feel that the problem of skip- jack spawning grounds would shortly be solved. Hatai: Thank you for your information. Next, 1 shall call upon Mr. Oshlma for dis- cussion. Oshlma; I am an Ichthyologist, but until recently ray work has been devoted prima- rily to research on salmon and trout« I regret to say that my knowledge of skip- jack and tuna is very limited outside of the few observations I made In the South Seas when I was fonnerly in Formosa » At that time Dr. Kishlnouye, who was special- izing on these fish, had his eyes set on the southern areas as spawning grounds for skipjack and tuna. He requested that we watch for Juveniles during our study of other families of fish. At one time we foi-warded some specimens to him which were taken off the coast of Keelung, but these were all Scomberoides, more specifically, a species of Chorlnemus. The salmon and the eel stop feeding entirely when they begin their spawning migration. Consequently, eels descending towards the sea are not attracted by bait placed in bamboo baskets set out to capture them. Because of such exaii4>les, Ur. Manikawa*s statement that skipjack and tuna cannot be caught during the spawning season may be true. The reason why skipjack with matured eggs cannot be caught in May may also originate from such circumstances. However, according to recent re- ports, skipjack of various sizes are taken throughout the year by the fishery in British North Borneo, Numerous skipjack weighing about 100 momme (.82 lbs.) are caught in the Sulu Sea around November; these constitute a nuisance because they are not of commercial size. About 60^ of those skipjack caught In April and May, however, are adults and the roe and testes from these fish are often canned, l&r. Tsuji, the present managing director of the Borneo Fishing Co., Ltd., had these products on hand at the time of my visit and he showed us a sample can containing a large testis. These facts are clearly contradictor^' to the aforementioned concep- tion that skipjack cannot be caught during the spawning season. It is possible that juvenile and mature skipjack and tuna are not caught in waters adjacent to Japan because this area is not a migratory sector for these particular sizes of fish. Upon closely correlating the flow of the Kuroshio to the migration of skipjack and tuna, it may be assumed that the originating point of the Kuroshio, wAiere the Northern Equatorial Current touches the Philippine Islands and changes course to \the northeast, is the spawning ground for migrator}' fishes. For example, the Japanese mullet, found frequently along the coast of Formosa, descends southward from the South China coast along the western coast of Formosa, spawns in the Bashl Channel and returns north. The sane species found In Nagasaki Prefecture, from which the noted product of that prefecture, dried mullet-roe, is manufactured, journeys to the Sfikishima Islands in Okinawa to spawn in the Kuroshio. Therefore, since skipjack and tuna are fish which inhabit warm waters of constant temperature, I believe it is logical to search for their spawning grounds in areas providing these condltioris, namely, in the Celebes Sea, the Sulu Sea, etc. There are, however, localities where small skipjack are caught throughout the year such as in the Senkaku Islands off Miyako, Then there are some areas to which skipjack do not mi- grate except for certain periods. What these facts suggest is as yet questionable, I believe the necessity exists for studying whether skipjack and tuna remain in specified localities at any certain period during their development, if coastal schools differ from migratorj schools, and if a single large group migrates in the warm waters of the vast Pacific. In order to do this, however, it is important to study by. some means the age of fish caught in the various areas. One method is by examining the rings appearing on scales and vertebrae, I feel that the selection of Palau as a principal research point is ideal as skipjack and tuna can be caught there throughout the year and it is near their presumed spawning grounds, Mr. Takeda, did you visit the Philippines and Borneo areas? Takeda : I was informed of this meeting by Mr. Marukawa, but now that I am here, I find I have verj' little to contribute to the discussion. Because I specialize in fishing, I have experimented with fishing techniques but not in biology. With the recent increase in the price of skipjack and tuna livers, the entrails of these fish are being retained. Therefore, I believe the collection of reproductive or- gans has been simplified. In 1930, 1 went to the Sulu Sea between November and December to fish for tuna. Skipjack, however, are abundant in this season. In Japanese waters, the fishing vessel is generally directed to a sighted school of fish, but in the Sulu Sea, skipjack approach the boat. At first, small carangids gather around the buoy tied to the longllne and these gather around the stem of the ship after following the longllne when it is hauled In. The skipjack strike for the carangids as the vessel proceeds to take in the line. These skipjack, weighing about one kan (8.27 lbs.) were brought back to Japan and their ovaries measured over 1.2 ahaku (1.19 ft.) In length [t J . These are presently preserved at the Kochl Prefectural Fisheries Ex- perimental Station. Thereafter, I went to Borneo at the request of Mr, Marukawa to experiment in the artificial fertilization of skipjack. When I arrived there, how- ever, I discovered that the ovaries of skipjack were not as large as described. Ovaries about 5 to 6 sun (6 to 7.2 inches) can be found almost all year round, but I never found many which were ready to extrude mature eggs. Dried skipjack sticks are made of female and male skipjack. It is very difficult to distinguish between males and females by observation. We once thought that fish with large bellies were females, but were surprised to find them males. As a consequence, it is Impos- sible to conduct experiments on artificial fertilization haphazardly aboard skipjack vessels. Actually, we never did conduct this experiment because we couldn't dif- ferentiate between a male and female skipjack. I believe that research would gain through study of the egg, Marukawa: The simplest way to tell between a male and female skipjack is to observe the shape of the tip of the head. Fish with pointed lii^when observed from over- head are males and those with rounded lips are females. I don't think that there are any other methods for distinguishing between the two sexes. When fish are observed from posteriorly, those iriiich become narrow from the eyes forward and have sharply pointed lips are males j females are those with somewhat rounded lips. Previously, vThen I was aboard the fishing vessel Fukuchi Itaru Noo 5 from Yaizu, I had the crew arrange skipjack on deck. Those fish with pointed snouts were placed on the starboard side of the bridge and those with rounded snouts were placed on the port side„ I declared those fish on the starboard to be males and those on port to be females, and had them opened. There was practically no error in my selection and the fishermen were amazed. I believe that this is a simple method for distinguishing between male and female skipjack. Takeda ; It certainly is convenient to learn of such a method. Amonf tunas, the most commonly known is the bluefin tuna which visits our waters; relatively less is known df the yellowfin and big-eyed tuna. I have heard that in Kiay and June, the waters between Formosa and Okinawa become black vsith sperm emitted by blue fin tuna. I was in Shikoku for a long time and I observed fishermen catching skipjack and tuna with well-developed egfs between lyo and Tosa during the same time of the year. In waters adjacent to the Nicobar Islands off the Malay Peninsula, about 70% of the tuna carry mature eggs between November and March. I was told that the large spawning sizes were caught on long lines. This method limits the propagation of tuna and skipjack so that fishing was restricted to pole and line fishing. I concluded this period to be the spawning -season. Skipjack weighing from 200 momme (1.6$ lbs.) to 250 momme (2»06 lbs.) are caught near Japan between May and June, Small skipjack such as these can sometimes be caught in Borneo. The smallest fish range from 60 to 70 momme (.5 to .58 lbs.). I believe it is possible to catch even smaller sizes if fishing methods were changed and if fishing were done parti- cularly for this purpose. Kanai; How many dried skipjack sticks can be obtained from a large skipjack carry- ing mature eggs? Takeda: Four sticks can be obtained from the larger fish, Kanai; Not long ago, I saw what I thought to be a dolphin between the Philippines and Borneo. However, according to the ship's captain, this fish was a large skip- jack from which eight instead of four sticks could have been obtained. Takeda: The large fish are tuna from which eight dried sticks called "renga-bushi" rTN:''brick-stickrfj can be made. ?Vhat you mistooh for a dolphin was probably a Wia. The largest skipjack weigh about /^ kan (33.08 lbs.); those in Borneo, two kan (16.54 lbs.). Kawakamii A three and a half kan (28.94 lb.) skipjack was once caught at Saipan. Marukawa : Mr. Kawahara, won't you say something? Kawahara: We frequently found skipjack with eggs between March and May. However, we did not report on the size of efgsor fish. It seems that skipjack carry matured eggs regardless of body size. Eggs were never extruded when pressure was applied to the belly of skipjack immediately after capture. We did find, however, ovaries which emitted eggs through light pressure after these were removed from the fish. We occasionally caught young skipjack up to about 3 sun (3.6 inches) in length. We tried to find juveniles measuring 1 sun (1.2 inches) in length and usually examined the stomachs of skipjack, tuna, and other fish in search of these young skipjack. Fish containing many small specimens were found, but we couldn't tt^ll whether these young fish were skipjack. If I recall rightly, I believe it was in November, 1931, that I nas requested by Mr. Takuo Mimura of the Fisheries College to forward him a large number of skipjack eggs sp that he could study these. At that time I iras also interested in this subject so I collected the eggs, but I was unable to send Mr. Mimura any of my samples which .were left behind when I left the Experimental Station in May of 1932 to return to Japan, From my investigations of the waters around Palau, I be- lieve that the spawning season of skipjack is in i.arch, April and fc'- peared to be of the same shape, there was a narked variation in development. TTils indicates that the spawning of skipjack is continuous and that there nay be peak spawnings which are not necessarily confined to any definite period. Keitaro Uehlda and Yaichl Tosa both state that skipjack do not spawn near the surface but at considerable depths. They say that tbs eggs probably float to the surface after deposition. Although there is no accurate bsisls for these statements, there are some points which nust be taken into consideration. The spawning grounds of tuna are definitely not localized but are widespread. The blue fin tuna seems to spawn around July in Tbyama Bay in the Japan Sea as schools of adult fish with mature reproductive organs appear in this area. This, however, does not happen every year* Marukawax The tunas in Formosa carry extremely large sggs in May. Okada; The study of the spawning grounds of skipjack is a major problem comparable to the previous study on eels by Schmidt, It apnears that the tunas of the Atlantic have been fairly well studied while research concerning those of the Pacific has been neglected. In referring to Mr. Amemlya' s statement that flying fish spawn continuously, I feel that this phenomena occurs in localities where there are slight changes in water temperature. For example, the six-pointed sponge which is found at a depth of about 250 fathoms carries eggs throughout the year. The change in watar temperature at such a depth is very slight during the year. i The same is true of anlnals which inhabl'-; waters of high constant temperature. The kawanina. a hot springs form, always carries egps when found in hot springs, but those at the periphery of the springs have definite egg-carrying periods. The fact that the skipjack and tuna spawn over a long period even though they may have an occasional prolific period is probably related to changes in water temperature. Walford states that tunas spawn at 20- 100 fathoms and that following spawning, the eggs float in the middle stratum of the sea. There may be some tuna which spawn at the surface, as mentioned by Mr. Marukawa, but I believe that in search- ing for eggs it is necessary to investigate the middle or lower strata. I also believe that it is necessary in connection with a study on spawning habits that the. food of large fish other than those which feed on young skipjack should be studied. That tunas frequent deep waters is suggested by the fact that they eat Splrula. a gastropod. Hatai: I wonder whether the fact that eggs are found throughout the year can be interpreted to mean that the egg is carried in the ovary for a long time. Also, If an oil layer envelops the egg they should float immediately aftor spawning. What do you think of this? Amemiya: Such interpretations are possible, but they are still unconfirmed. If one desires to seek small skipjack, I believe ha should look for other large fish which prey upon the smaller ones. As Dr. Klshinouye uentloned, it is best to re- cover Juveniles from natural collectors. Hatal; However, in order to study this problem, wouldn't we have to restrict our- selves to Juveniles; Amemiya t It would be a good idea to collect Juveniles by means of a jack-lamp. Marukawa; The use of a Jack-lamp may be a good idea. The Hakuo W.-iru is collecting small fish in the South Seas by means of a Jack-lamp. Amemiya; It Is said that the skin of skipjack becomes coarse after spawning. Marukawa; It is said that those which have spawned become reddish in color. Amemiya; There are skipjack which follow those ^hich have been hooked. On one occasion, a swordfish extruding eggs was caught. Takeda; It seems that other fish accompany tl-e albacore when it is caught. Hatal; f.5r, Takagi, have you anything to say? Takaglt I am definitely not acquainted with the subject of skipjack and tuna snawn- Ing, I am, however. In support of this new program. On the basis of information presented by individuals having practical experience, it seems that mature skipjack are not difficult to catch and that artificial fertilization is a great possibility, Embryological research on skipjack through selection and training of qualified per- sonnel will not only be a great contribution to genetics, but I believe that it will also be basic to the expansion of the fisher/. Therefore, I hope that this project will be" Initiaterl immediately, whatever the cost. Hatal; In regard to the question of steps to be taken on the study of skipjack spawning grounds, I will read today's proceedings carefully and will draw un a plan for your comments at the next meeting. We have heard a great deql on the subject matter, but only a verv small part is clear so that the problem remains Itti practically unsolved o It is art extremely complex subject which I want to see solved by the Japanese, This cannot possibly be done fnrough the efforts of one man. I wish to mention here again that I shall rely upon your future cooperation. Since it is late, I suggest we close this discussion for the present. Vnarik you all for your informative contributions. 11 According to reports, the Pacific Society has contributed a certain amount of survey expense funds to the Japanese Society for the Pro- motion of Scientific Research in order to make a study of the spawn- ing grounds of skipjack and tuna, and research on this important and difficult problem is being carried on by the Tropical Biology Experi- ment Station at Palau. Several years ago, Mr. Keizo Takahashi of the Experiment Station was already conducting research on skipjack spawning grounds. As a result of conducting artificial fertilization on mature skipjack eggs which he collected from Palau waters, he was able to report the verification of the assumption that the skipjack spawning groiind is located somewhere in that area. The Celebes Sea linking the Philippines, Borneo, and Celebes was considered the most promising area of investigation in the solving of this problem. Recently I heard that Dr. Masamitsu Oshima, who was to be dispatched to that area, was prevented by several circumstances from going into the Outer South Seas area (TNt Celebes Sea area). After arriving in Palau last April, he sailed southward from Palau aboard the Zuiho Uaru and conducted a detailed investigation of the sea around Helen Reef, Tobi Island, and l&lmahera Island. The Pan-Pacific Scientific Conference held in Canada the year before last, assumed that the solution to this problem could be found generally in the South Seas area. Since then, the fact that investigation and study of the matter have been encouraged steadily in Japan has provoked great interest among our circle, which has some connection with the matter, lie must not be indifferent. The work of discovering the skipjack and tuna spawning grounds, which is the most difficult problem of world fishery science, cannot be accomplished overnight. Ifith all due respect for the efforts of these researchers, we must do everything in our power to supply data relative to this subject. At present, data obtained from tuna surveys are forwarded from the scene of operations to the Tropical Biology Experiment Station as the occasion arises and are being used to aid in their research. I hope that this material will aid in furthering the investigation and research and that the research will be completed in the near future. The day of completion will mark a new era in world fishery science. In our survey of tuna fishing grounds, the catch is immediately eviscei-ated and thrown into the fish hold. Thus we have many opportunities for observing the roe of the female fish. From our past experience it seems that we have come in contact with a number of tuna with considerably matured roe in various areas around the isleuads. 12 It would perhaps be hasty to oonolude that the spawning ground of the migratory fishes such as skipjack and tuna, whose range of movement is wide, is necessarily to be found wherever there are mature fish with mature eggs, but we can surely think that the spawning grounds are not far from the place these mature fish inhabit. If this is so, then we can conclude that the spawning grounds are not necessarily limited to the Sulu Sea or the Celebes Sea, but that they can also be found in the seas around the South Sea Islands. Pvu-thermore, young (immature) tuna are sometimes caught on the tuna longlines during tuna fishing ground surveys. From past ex- perience (though the experiences are few) these young tuna are seldom caught in deep water; they are generally caught near an island (land). Since these young tuna, whose migratory power is comparatively weaker than that of the mature truia, are caught near islands, we can assxjme that the spawning ground of their species is found near islands. It is recorded, also, that a few young yellowfin (immature yellowfin) were caught by tuna longlines during the survey conducted by the Zuiho Maru. In observing the above record, we cannot disregard the fact that the young tuna were all caught in waters close to islands. It is regrettable that the length measurements of the tuna were (anitted in 9 to 20. Hereafter, we will do our best to take accurate body measure- ments of these immature fish eoid to collect material regarding this phase* If this material is of any use as reference, it would be an unexpected good fortune, I hope that you researchers will give me your frank opinion regarding this manuscript. (June 10, 1941) 13 Date Pooition of Body Body Probable No ui FlBhing Length Weight Age Roraark Catch Ground (cm) (Kan) 19° 04' N 1 5/28/40 U5C 48' E 55 0„890 .2-3 Near Agrihan 2 (I N 56 0c950 2-3 Island In the 3 n ft 59 1,020 2-3 Marianas U II It 51 0o730 2 5 N It 50 43.5' M 69 1.700 3 6 12/U/40 I690 40 • E 0° 12« N 82 2,800 4 Near Jaluit 7 5/u/a 1310 U' E 50 lo200 2-3 Walgeo I., New 3 n It 10 11» N 50 1«200 2-3 Guinea 9 5/i5/a 1310 50 « E ^m> 1,500 ^« Near Helen Reef 10 H It »- 1,200 a»w and lobl Island 11 It It eam< 1.200 ^m- 12 It n .« lo200 13 n If »m 1,000 _. U It •t •m 1,000 .« 15 It It ■m'» 1,000 «»<» 16 It It • • 1,600 .. 17 It It «• 1,700 •_ 18 It i» flr« 1,900 .. 19 It It »«> 1,400 .. 20 N It •« 1,500 — ° I 14 1 Kan : 3„27 lbs, 75319 :;;>