OCEANOGRAPHIC CONDITIONS AND THE BLACK TUNA FISHERY A translation I Marine Bioiogicai Laboratory I. I E K .A. it "V 0 1952 WOODS HOLE, MASS. SPECIAL SCIENTIFIC REPORT: FISHERIES Na78 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 utilization practices and as guides for administrative or legislative action. It is issued in limi- ted quantities for the official use of Federal, State or cooperating Agencies and in processed form for economy and to avoid delay in publicationo Washington, D«C. July, 1952 United States Department of the Interior, Oscar L„ Chapman, Secretary- Fish and Wildlife Service, Albert M. Day, Director ON THE RELATED BEIWESN THE lUWA FISHERY AND OCEANOGRAPHIC CCWDIIIONS By Takeshi Kawana Report of Fisheries Investigations /Suisan Chosa Hokoku/ Volume 31 Hokkaido Fisheries Experiment Station, Published March 19 3U Translated from the Japanese language by Wilvan G. Van Canpen Pacific Oceanic Fishery Investigations Special Scientific Report: Fisheries No. 78 mSHINGTON s JULY 19^2 lo Observations Concerning the Fisning Situation Eo History The "tuna" referred to in this paper is the black tuna, Ihunnus ori en talis (Temminck & Schlegel): The drift-net fisnery for this species began when the llsh were taken mixed in vdth the_ catch of rakudazame / Isurus nasus J (called nezumizame at Tokyo, moka in the Sanriku area, katozame in Hokkaido, and nezumizame in the following paragraphs) » It is in a flourishing condition at present at Kushiro, Urakawa, Muroran, and Akkeshi. The long-line fishery operates in the waters between Erimosaki and Shiriyasaki and at times near Shikotan Island, Trolling and harpooning are sometimes carried on in conjunction with these fisherieso Setniet fishing is chiefly carried on in Funkawan, where it has the longest history, but in recent years it has also been established at Etorofu Island. The fisheries of each region will be discussed below. Drift-net Fishery Kushiro (1) In 1905 the Shiranuka Fishing Guild made a trial of long-line fishing for nezumizame and caught one tunao In the same year one Taumi, basing his operations at Akkeshi- To kotan, fished a Niigata-type drift net off Daikoku-jima and made good catches of tuna and nezumizame , In I9O6 at Kushiro one Ikeda began fishing on June 20 and caught one large tuna. He was joined by a number of otners until the boats fishing totaled 17o In the following year, 1907, there were 38 vessels fishing; these were all sampans, but in I9IU two powered vessels entered the fisheryo Good results were obtained, and the following year, 19159 saw the establishment of the Kushiro Tuna Drift Net Guild, In 1920 this organization was ex- panded and became the Kushiro Powered Fishing Boat Guild, In 192^ this Guild became the owner and operator of a fish and vegetable market, and the number of vessels fishing reached U3, forming the basis of today's prosperity. Fishing was good the next year, 1926, and from 192? on the number of vessels participating rose above 200 and the value of the catch exceeded a million yen, Urakawa The year I905 saw the beginning of the drift-net fishery for nezumizame, and the number of tuna included in tne catch reached quite a figure, Ihe number of t\ma taken was particularly high in 192^, and the following year specialized drift-net fishing for tuna began. After 1928 the number of vessels participating rose ?bove 30 and the value of the catch exceeded 100,000 yen. Murcran Washiau was the earliest., fish being taken 'onere in drift note around 1905» Just as at Urakawaj nezumizame were the r-iain object of the fi;h3ry„ Akkeshi Four drift-net boats began fishing in 1931s and ij entered the fishery in 193^ » Long-line Fisnery The beginnings of this fishery were the tests of shark and tuna long- line fishing conducted by this Station from 1910 to 191u on tiie Kitami coast, in Funkawan, and between Ezashi and Kuto"„ On the Pacific coast in 1913 the Kushiro Branch Station ran some trials off Akkeshio In 1925 the Murorazi Braich Station supplied long lines to fishing boats for ejqjerimental fishingo In the same year one Matsuda of Hakodate City fiaaed successiully in the waters near Esan, and in 1926 'Iteciinician Yokoyajna and Assistant Technician Katsugi of this Station established this fishery firnily by tiieir experi- mental long-line fishmgo Tha vessels are based mainly at Hakodate and occasionally put in at Urakawa and Kushiro o Some long-line boats also trolls In 19ii7 a boat from Vfekayama fished successfuliyo Boats from Oioa Prefecture also engage in harpoon fishing, and in 1953 there were UO harpoon boats from Chiba Prefecture based at Urakawa, where they operated successfuliyo Set Nets The set nets of Punkawan are the oldest^ large catches having been made there at Kayabe-gun over 30 years agOoC-/ Tuna are also taken inci- dental to the catch of other species on the Japan Sea coast in the Suttsu, Yoichi, and Mashike areaso '^ salmon iiet nets for many yearsj, but because of the lack of adequate trans- portation facilities no attention was given to therao iilth tl:ie development of refrigerated boats these fish have become profitable^ and since 1917 tuna set nets have been establishedo At first they were used only in Utasutsuwemj but after 192p they also began fishing at Tannemoe, Naibowan, and on x-he east coast. They v.ere depressed by the rise of the Kushiro tuna fishery and also the number of fish migrating into the grounds fell off until at present set nets are operated only on the Utasutsu grounds. bo Fisning Seasons, Fishing Groxinds, Size of Fish Drift-net Fishery The drift-net boats of Urakawa and Muroran make their tlrst catches in the coastal waters of HitaKa in the middle or latter part of June, while the boats based at Kushiro begin their season off Srimosaki in the latter part of Jiine„ In both cases the fishing is done 10 to 30 miles off shore o In the middle of July the whole area from Hitaka to Kushiro enters its peak season „ Some years in August the fishery retreats northward to the region of Etorofu I„ The main season in the waters off Kushiro is from the middle of July to the latter part of October and the grounds are about 20 miles off shore (along the fringes of the third branch of the warm current) and about $0 miles off shore (on the edges of the second branch of the warm current). Table 1 shows the first day of fishing, the peak of the season, and the date fishing ends at Kushiro „ We look next at the seasons for the various sizes of fish. In table 2 we have followed Mr. Kinosuke Kimura in taking as the median period the 10-day period which corresponds to the statistical mathematical average and then taking twice the breadth to be the peak season^ Further, the number of largej medium, and small fish taken in each 10-day period on a 10-year average is shown in figure 1„ According to this graph, the 10-day period at which half of the catch of large tuna had been taken was earlier than the mean period, while for the medium and small fish they more or less coincided. In neither case is the distribution of the catch very un symmetrical. Table 3 shows the median period of the fishing season for each size of fish. As far as the sizes of tima taken in Hokkaido" are concerned, those from Etorofu Island are the largest, some of them attaining weights of 330 to 6^0 pounds, Ihe next largest are the fish taken in the set nets of Funkawan in the autumn, many of these weighing around 330 pounds o At Kushiro, as is clearly shown in tables 2 and 3 and in figure 1, large fish predominate at the beginning of the season, with the average weight commonly over 250 poundso Thereafter medium and small tuna appear, and by the middle of August the number of small fish exceeds the number of large oneso In October with the strengthening of the cold currents the small tuna are gradually driven out and the average size of the fish increase So Table U shows the average weights of fish for each month. Long-line Fishery The first long-line catches are made in August, later than those of the drift-net fishery. The fishing grounds are chiefly in the area from off Eriraosaki to off Shiriyasaki and farther out to sea than the drift-net fishing grounds. The waters east of Srimosaki provide no good fishing except in the autumn when the tuna are returning southvfard. Good fishing is sometimes found to the southeast of Shikotan Island, The peak season is from September to November, and the last catches are made off Shiriyasaki in December, Si Si J2 O 0 •■ o o ^ ■P "H (N -P -P CO AJ 6 CO CO CO o CO i> o o to a -P CO "H -P -P •H CQ t>s^ m m s ^1 ^ CO in U ■H U Ui -H -H 1 o Ch "H, O tM tH 1 -p cv xi e 1 (0 fl. t^C/D -H t>, »> u ii W 0 o (y ^ oH .H -p CJ •H B a X! 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The summer tuna are first taken in June, sometimes earlier than the drift-net fishery in Hitaka waters. At first large fish are taken, and small fish latero The peak of the summer season is in July, The autumn tuna begin in October and reach their peak in Novembero The first catches were made as followss 1928, Mori June 17, Oshamanbe June 27; 1929, Mori June 29, Oshamanbe July 12j 1930, Mori July 1, Oshamanbe July 11; 1931, Oshamanbe July 10 o Amount of Catch Table $ shows the catch at KushirOo In order to indicate clearly the quality of the fishing in each year, the average catch per cruise per boat is shown in table 6„ Table 7 gives the catch of the Etorofu Island set nets» Co Characteristics of the Fishery in the Past 10 Years 1922o— Kushiro had a poor year. In July there was good fishing for large tuna, but in August and September the fishing became pooro In September there was good fishing for small t\ma of about 25 pounds weight on the coasto The appearance of the greatest catch of large tuna in July is abnormal o 1923o— A poor year at Kushiro, Small tuna were scarce. The Etorofu Island set nets had a poor year, I92U,— Kushiro had a poor year. Large tuna were predominant, Ihere were absolutely no medium fish of 83 to 125 pounds. The Etorofu Island set nets made better catches than they had in the previous year, 1925»— Kushiro had a good year. There were many small tuna and the average size of the fish was ^mall. The set nets at Etorofu Island had good fishing, 19260 — 'The Kushiro drift nets had good fishing, Ihere were few small fish and the average size was large. The schools were sparse east of Akkeshio Long-line fishing was good near Ezan, Hitaka, and Iburi, In November the set nets in Funkawan had good fishing, and at the same time large schools came into the Tsugaru Strait, The Etorofu Island set nets had poor fishing, 1927, — The Kushiro drif-*- nets fared poorly, Ihe schools moved north to the Shikotan and Etorofu areas and the grounds were widely scattered, Mediiwi and small tuna were rather abundant. The long -line grounds ex- tended to the waters of Hitaka and Erimo, but the fishing was very poor. Quite a few fish were taken by trollingo In July the set nets of Funkajran had large catches o The Etorofu Island set nets had their best fishing in 10 years. 4^ CO o XI u r>, -P f-i » 1 1 1 1 1 1 ON CM lr^ tr\ O c^ CJN t> O CO CD O 3 1 1 1 1 i 1 ON CO - NO -* Oi •< r^ O o o o». »v •v "". •\ 9\ *. •\ ^1 a* rH CM rH rH C\i r-i r-i rH 0) 0 W Q CD ^ 0) F> a nO 91 • CM •o 3 C vO q; >rv to C\J CM m r^ -~t C~- Oi r-i -<}• r-i 00 cr> o u ^^^ (D rH ON -4- m l> vD O t^ t> r-i O r-i c^ !> o CO 3 m CO •H •^ ^ •^ »> #v v\ 9: CN ■^ •\ 1^ »\ »* ^ ^ o. o P > CV tf^ vf > nO - cv -p m vO CM ^ rH ipi -t r\; Oi CM vO NO r^ ^ ON UA ICN. c^ •p 01 CO [> u -=1! 0 X! 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S-i o 3 -P O o -p CO •-3 •-3 e cv o ta o o CO iH O t C\i CV o o o cv •s}- O O (T- cv C\i o CO cv cv CO CXj 00 rH C^ CO cv cv cv o c^ -4- cv cv (js c^ cv 0^ \0 CO O !> cv fV o cv o cv c^ o c^ r-J !> O ^ CO f^ rH to «^ rH CV c- o o cv CO Oj CO CV cv E- -4- - o cv o o o m rr^ t^ r^ CV cv CO cv cv o CV d sO CV -. 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In November the set nets in ^unkawan had good fishing, and at the same time there were dense schools in the Tsugaru Strait o The set nets at Etorofu Island had good fishings 1929 0— The Kushiro drift-net grounds had their best fishing in the 10-year period„ Ihe groimds did not extend east of Akkeshi and were close in to the coast. There were few small tuna and the average size of the fish was large o Long-line fishing was good on the grounds west of Erimosaki » The catch of the set nets in Funkawan was small, and the set nets of Etorofu Island had rather poor fishings I93O0— The Kushiro drift-net grounds extended over a wide area from Erimosaki to the waters off Otchishisaki, and the fishing was moderately good, with no concentrated groimdso There were many small tima and the average size of the fish was small. The long-line grounds had extremely poor fishingo Ihe set nets in Funkawan made good catches of small tuna in Augusto The Etorofu Island set nets had poor fishing„ 1933.0=- The Kushiro drift-net grounds were not as close to shore as in the previous year and their northern end was at Shikotan Island, Ihe large tuna were unusually large in size, there were few medium fish, and small fish were very numerous. The average size of fish was the smallest in the 10-year period. The long-line grounds were good off Erimosaki in August and off Shikotan Island in September and October, The set nets of Funkawan had good autumn fishing. The Etorofu Island set nets had poor fishingo 1932o™Big catches were made at Kushiro in the latter part of July, but from August on the fishing was very poor. Medium tuna were very scarce, Urakawa had the best fishing in years, and small tima were remarkably abundant. The long-line fishing was poor, II, Observations Concerning Habits a. Migrations In ascertaining the migrational movements of fishes there is the direct method of tagging an-' then recapturing the fish under considera- tion, and there is the indirect method of hypothesizing these movements from the shifts in the seasons and grounds, Ihe latter is liable to be confused with changes in environmental conditions, but in the case of large tuna, which are difficult to tag, this deductive method must be used. 11 With regard to the tagging of tvma, Teclinician Yokoyama of this Station has devised the method of using a regvilar trolling line rigged so that when a fish strikes the tag, the hook and a part of the leader will break off. Table 8 shows the results of the tagging of small tuna from the set nets of Yoichi and Furuhira in August and September 1933, by Assistant Technician Nakashima of this Station, using tags attached to the caudal pedunclso From these facts it appears that although the fish make small localized migrations in search of food during the summer season, they move southward with the falling water temperatures in the autumn. It should be borne in mind, however, in this example that the fish were released at just about the northern limit of their area of migration „ Since taggings of small tuna are scheduled for the Pacific Coast in the future it is to be expected that migrations of this sort will gradually become clarified. However, in the present paper we will expound our hypothesis concerning the migrations of the tuna in the Pacific coastal waters of Hokkaido based on obser^/ation of the shifts in the fishing seasons and the fishing groundso The place in Hokkaido where the earliest catches of tuna are made is in the set nets of Kayabe-gun, following the northern part of Aomori Prefecture, Therefore these schools must be fish which have been drawn from the Iwate area by the Tsugaru Current and have moved north through the waters off Shiriyasakio The theory is also held that they migrate from the Japan Sea through the Tsugaru Strait, but the truth or falsity of this waits upon future investigations^ Next the Kushiro and Urakawa drift- net boats make their first catches in the latter part of June from 33 miles south of Erimosaki to off Hitaka. These schools, through the third branch of the warm current described in a later section, move north farther off shore than the previously mentioned fish. In some years these schools move on from the Kushiro area north to the Etorofu area. Part of them must turn left from the coastal waters of Hitaka to enter Funkawano Furthermore, some of the fish off Kushiro move north along the second branch of the warm current. The above are the deduced migra- tions of the so-called ascending tuna, east of Erimosaki the large tuna migrating comparatively close to the coast while the small fish are farther off shore. From the latter part of September to October the fish gradually begin to move south. Their route follows the outer edge of the coastal stream of the Chishima cold current to the waters off Erimosaki, from where some of the fish head for the Funkawan and Shiri- yasaki areas while others head for the Kamaishi area. There appear to be some fish which move southward off shore with the retreat of the second branch of the warm current off Kushiro, The above is what may be called the grand migration, however, along the route at Shiriyasaki, Erimosaki, and off Kushiro there are localized migrations covering considerable periods of time. These are food-seeking migrations which 12 m 0) -p o CD ■O U ® :3 ® m +J ^^ CO P. (D CO -p o~\ rH O Ph 0) 0) (0 (D u u O -P CI) In M o CNi c •?1<1 Tl TJ 1) I> 0) a; 0 CO !^ !h I^ CO 01 (D > tH p> o o Q) j-< n CO CO c •H QD CO Eh 1 tXJ ■H CO Eh C CO S-{ to Q) - aj CO T3 o o o o o o (D O c CO -p to o vO vO 0) to to &■ >j CO CO rH 73 s cv cv CT^ U 0 o CO E -P C 0) o o CO »-P C CV I CV iH I CO e ■^'^ CO o CO CO a; e CO I •H to ;3 CO to M I CO X! O 0) CO >-• CO V< CO e "H B U •H JS X! O O CO •rH e CO ! tH ^ C X3 X! O 3 O .3 CO <+H CO u e 'H B &4 M •H ] X! I •rH to •H JSI O X! O CO W to 0) CO X! O O I CO "^ K! !h g ^ 3 3 CO J •H ^ O CO in CO •rH to -H to o x: X CO I to f ^H H (D -H H X! C X! ■ O 3 O CO <4H CO e -H E U x! u d 13 unquestionably change with the changes in the movements of sardines and squid dependent on the changes in oceanographic conditions o The increasing size of the fish as one goes farther north means that one must take into consideration the fact that the larger tuna are, as set forth in a later section, more adaptable to water temperatures than the medium and small fish, together with the fact that the larger the fish the greater is its speed of raigrationo b. Favorable Tenperatures Time of first catch and water tenperatures If we present the surface water temperatures measured by survey vessels at the time of the first catches, we find that on July 2, 1926, 30 to ^0 miles south of Erimosaki, when a school of large t\ma was sighted, it was 12o2°C to 13.6°C, and on July 11, 1927, 20 miles south of Urakawa the Sanyo Maru, which had sighted a school of large tuna, found a temperature of 13.5°C. Similar records taken by fishing vessels are lloO°C for the place where the first catch was made on June 20, 1927, 60 miles southwest of Erimosaki (off Shiriyasaki), l6oO C where the first catch was made on June 27, 1928, 3$ miles southwest of Erimosaki, and lii„$°C where the first catch was made on June 27, 1929, 30 miles southwest of Erimosaki o Thus the water temperatures on the grounds where the first catches are made, which in normal years are about 30 miles south of Erimosaki, are about lii°Co The first catches are made at higher temperatures in warmer years and at lower ones in colder years, but a zone of water of over 12°C should be sought » Peak season and water temperatures There have been few years in which the fishing grounds off Kushiro have shown water temperatures higher than 20oO°C, and the lowest recorded is about 13.0°C, like the temperatures of 1926o Generally the range is llioO°C to l8,0°Co Ten miles south of Hitokappu on Etorofu Island on August 2U, 1927, the SanyC Maru sighted a large school of tuna in water of 9<.9°C, and on September 2k of the same year kO miles south of Hitokappu a large school of tuna was sighted in water of 12o5°Co Since 1927 was a warm year, in normal years the fish must move northward in the Chishima area through zones of water of 10oO°C or colder o Aid of the season and water temperatures Although there are no observations of water tenperatures at the end of the season, there must be times when good fishing is found at ten5)eratures of around 10.0 Co 11; At the time when they are moving northward through Hokkaido waters the tuna schools are seen in the waters of the warm-current system. However, they gradually become accustomed to cold water and many of them pursue schools of sardines even into zones of low-ten5)erature, turbid water (water color below 6, transparency less than 5 meters), so it is difficult to infer the area of migration from water temperature c Furthermore, because the water temperatures at which the fish occur vary from year to year and from area to area, it is hard to detennine accurately what is the most favorable temperature. In the foregoing only the surface tempera- ture has been considered. It is, nevertheless, an error to think that the vertical movements of the tuna take place only between the surface and a depth of 20 meters, for in the waters east of Erimosaki they go far deeper, down to 100 meters, as is presumed from the fact that they are hooked on octopus long lines. Even in the waters east of Erimosaki, where the temperatures at the deeper levels are low, the fish must descend to depths greater than iiO meters judging by the length of the long-line branches, Ihere must be cases in which the water temperatures are 5oO°iG or lower, and from this it can be seen that the adaptability of the tuna to ten^^erature is surprisingly great, Figure 2 shows the average surface temperature and the catch by 10-day periods along a line 10 to 50 miles south from Kushiro, Co Inhabiting Zones of Mixed Water Tuna are abundant in areas where different water masses are mixedo Examples s Near Shiriyasaki, Tsugaru warm current / coastal stream of cold current Near Erimosaki, third branch of the warm current / Tsugaru warm current / coastal stream of cold current Coastal grounds off Kushiro, third branch of warm current / coastal stream of cold current Kushiro off-shore grounds, second branch of warm current / coastal stream of cold current Near Shikotan Island, second branch of warm current / Okhotsk current / coastal stream of cold current lliis may be because the fish are attracted by the abundant production of such natural foods of tuna as sardines and squid. HI, Observations Concerning Oceanographic Conditions ao Pattern of Cold Currents -^n the Pacific Coast of Hokkaido" (Summer) The following is an account of the Chishima cold current as observed by this Station within a radius of 200 miles from shore. The current runs south along the east coast of the Chishima chain to about 30 miles south of Noshappu where it is cut by water of the warm-current system into two brancheso One branch runs from about 10 miles south of Kushiro 15 to about 10 miles off Erimosaki and then further divides into two branches, one of which goes to the Kinkazan area while the other runs from the Hitaka coast to Ezansaki and Shiriyasaki, This is tentatively called the coastal stream. The latter passes 30 to 50 miles south of Kushiro, reaches a point about UO miles south of Erimosaki, and then heads for the Kinkazan area, Ihis is tentatively designated the second current, and it sometimes divides into two branches. Number 3 moves south farther off shore than the second current, and flows down to the southwestward about 160 miles south of Kushiroo This is tentatively designated the third current, Ihis is the pattern of the Chishima cold current. b. Pattern of Warm Currents on the Pacific Coast of Hokkaido (Summer) In May and June the Kuroshio warm current reaches the waters off Cho"shi and flows by in ein approximately easterly direction. In July it is affected by the southerly seasonal winds and develops a branch which reaches into the waters off Kinkazan, In August this zone reaches to roughly the vicinity of kO N where it is split by the third stream of the cold current, one branch turning eastward while the other continues north. The former is tentatively termed the first branch of the warm current. The northbound branch is bisected by the second stream of the cold current off Erimosaki, and one stream turns toward the off-shore waters about 100 miles south of Kushiro while the other approaches Erimosaki and turns toward the waters off Kushiro. The former is tenta- tively designated the second branch and the latter the third branch. The third branch mingles with the Tsugaru warm current o Ihe above, if one looks from the Kushiro shore, are arranged from the coast toward the open sea in the order of the coastal stream of the Chishiraa cold current, the third branch of the warm current, the second stream of the cold current, the second branch of the warm current, the third stream of the cold current, and the first branch of the warm cur- rent. Hiis arrangement is shown in figure 3o c. Characteristics of the Oceanographic Conditions During the Past 10 Years 1922, — ^A, year of low tenperatures in the off-shore waters. September was the season of highest temperatures. According to the regular observations it was a year of high temperatxires on the coast in July, August, and September, 1923, — ^No transverse observations were taken and conditions off shore were not clear. According to the regular observations temperatures were low in July, August, and September, but they were high from October on, 16 192lio — Temperatures were low in July with the waters off Kushiro at 10 oO C or less 5 but from the latter part of the month they rose abruptly, and in the early part of August the third branch of the warm current showed l8oO°C to 21oO°C„ In the latter part of August the second stream of the cold current strengthened and the temperatures went down to l5oO°C or below, but in the early part of September the water temperatures recoveredo 1925„— Temperatures were 16„0°C or higher from July on and reached their highest in August, It was a year of high temperatureso At Kushiro the 20-mile station showed the lowest and the 30-mile station the highest readings, 19260— The surface water temperatures were unusually lowo August showed the maximumo In contrast to this, the middle and lower levels had high temperatures, that is, the vertical distribution of the warm current was thick. The range of the bottom layer of cold water, with temperatures of 2,0°C and lower, was small. At Kushiro the second stream of the Chishima cold current approached to the 20-mile station, and its range extended to the UO-mile station. The third branch of the warm current did not extend east of Kushiro, The water color was unfavorable and the transparency was notably low, 1927o— This was a yesir of high surface water tenperatures, the maximum being reached in August, and the third branch of the warm current was vigorous, (In August the coastal stream of the cold current was not present on the line south of Kushiro,) The cold bottom water was strong, water of temperatures of 2,0°C or less having a wide range. Consequently the vertical distribution of the warm current was thin, 1928, —This was a year of high surface temperatures. The peak was in September and the period of falling temperatures was somewhat delayed. The range of the cold bottom water with temperatures of 2,0°C and lower was not great, 1929 o— This was a year of low surface temperatures. The range of cold bottom water of Z,fJ°C and lower temperatures was extremely small. Accordingly the vertical distribution of the warm current was thick. The third branch of the warm current flourished off Erimosaki, but it was blocked off Kushiro by the cold current (the coastal stream and the second stream did not separate) and did not extend farther east, 1930, —The surface water temperatures were normal. The lower level was markedly cold and the rar^e of water with temperatures of 2,0°G or less was large. Consequently the vertical distribution of the warm current was thin. On the Erimosaki line the difference in temperature between the surface and kO meters was 11,7°C, the greatest in the past 6 years. The value on the Kushiro line was the fourth greatest in the past 8 years. 17 1931o — This was a Tear of rather low surface tenperaturesc IfLthin 100 utiles both currents had three branches apiece » Water ten^eratures at the kO to 100-B»Bter levBls showed oarkedly low readings, laeaning that toe vertical distribution of the warn current was extremely thin, 1932, — IMs was a year of generally low water teii5>eratureso Ihe coastal streaa of the cold current heading southward from Erimosaki was straag, and the third branch of the warm current did not pass north of the Erimo station line. The Erimo line had lower temperatures in AiJgust than in July, later temperatures on the Eriiao line were lower than those on the Krishiro line (July - Novaaber). 17. Oceanographic Conditions and the Fishing Situation a. Vertical Differential in later Tenmeratures and Fishing Conditions The results of observaticns aade in August, 10 to 50 miles south of Xushiro, have been takan. In conparing the ten5)erature differentials for the s-irfaoe and levels from 10 to 2X meters, the yearly changes are greatest as between the surface and the 50-meter level, so this will be taken as representative., If the total landings are taken for the amount of the catch, the figure is likely to be affected by the number of boats engaged in the fishery, while if we take the average catch per boat it is likely to be controlled by the nuncer of cruises made or by temporary changes in the nuoi)er of boats putting in to the port, and in either case the figures will be lacking in accuracy = !Ilierefore we have taken the average catch per cruise = Since in the case of the Kushiro boats one cr-i.ise is generally equivalent to one night, this is probably the most reasonable = ^e result is that in years whan the veartical differential in water tesperature is .qwaTi the fishing is good, and when the differential is great the fishing is poor, Ihe phenoB»enon of a small vertical temperature differential off tushiro means either that the wara current is not strong, so that its force does not extend all the way to the E.ijriles, or that the cold cur- rent is net strong, so that it does not reach to west of Kushiro, and thus the ccmtact of the two cijrrents is in the waters near Kushiro = In that case the center of the tuna schools is within the operating radius of the boats based at Kushiro and consequently they find good filling. Biis theory is also confirmed by the fact that when the vertical differential in water temperatures at the Hoshappusaki line and at the BriBOsaki line is cospared with the amount of the landings at Kushiro, the correlation is not dosee !!hese facts are shown by table 9 and figure lie Ihe coefficient of correlation is r = -O^^l^ 18 -t3 c M (D -P 0} P. e a: ■p o> i J3 U -P (D t> J3 ID O iH 3 -P ■^^ c ^ O Co ^T Oi XXj O -^ tn ss <£ 1 i - : o ^^ u o 1 1 ci o sO lO. Oi c^ O d fH o c o <-H iH t-i —1 rH rH rH at- I e tw if\ ? 1 vO iH Oi o sO sO -4- C~- rH O 3 f, 1 o 1 d o 0 3 c o 3 to C O E o £> t> Oi CO UA ■vr vO ir\ tr- s;-! e -^ > r-i rH rH !— 1 r— 1 !-i 1 — I 1 — i rH M r^ tajc o h ■a O cv »ri C^ s f^ o E> o CC a -o .c -^' P Ov CO --*■ >f\ -. 3^ . >. A (E sn .M >-i o <-i Oi r-i rH Oi —1 r-i iH 1 p. o s o a. 1 <; a P. 1 vD 1 ttf OQ to O. iH -P u -rt a: a O^ CO - a u Q. D Oi C\i --? 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