CONTRIBUTIONS TO THE BIOLOGY OF TUNAS FROM THE 

 WESTERN EQUATORIAL PACIFIC 



By Bell M. Shimada, Fishery Research Biolosist 



Kesearcli into the biolojiy of Pacific tunas has 

 advanced rajiidly in recent years, yet much re- 

 mains unknown about the life history and habits 

 of tuna species inhabiting waters of the former 

 Man(hited Islands now known as the Pacific Trust 

 Territories, in the western equatorial Pacific 

 Ocean. In prewar years, some scientific studies 

 were conducted by the Japanese, but these were 

 limited in scope and directed primarily towards 

 exploitation of the extensive tuna resources to be 

 found near their island possessions. 



With the opening of the Trust Territories on 

 May 11, 1950, to Japanese mothership-type tuna- 

 fisliing operations, an opportunity was given the 

 Pacific Oceanic Fishery Investigations of the 

 United States Fish and AVildlife Service to gather 

 important data on tunas of this region by send- 

 ing a scientific and technical observer along with 

 the first mothership expedition to leave Japan. 

 I was subsequently detailed aboard the mother- 

 ship Tenyo Maru No. 2 and accompanied the expe- 

 dition from June 12 to September 14. 1950. Dur- 

 ing this assignment my principal duties were to 

 observe Japanese methods of fishing and process- 

 ing tuna, and to collect morphometric data on 

 various tuna species for use by the Pacific Oceanic 

 Fisliery Investigations in current studies on Pa- 

 cific tuna populations. Some information was 

 obtained also on other biological aspects of tunas. 

 These incidental observations on the spawning of 

 yellowfin and big-eyed tuna, on the occurrence of 

 juvenile oceanic skipjack, and on the capture of 

 adult bluefin tuna in the area covered by the expe- 

 dition are summarized in this report. 



Tliese studies were made possible througii the 

 cooperation of the High Commissioner for the 

 Trust Territoi-ies of the Pacific Islands and the 

 Natural Resources Section, General Headquar- 

 ters, Supreme Commander for the Allied Powers. 

 The assistance rendered by various membcis of 

 the Japanese Fishery Agency and the Taiyo Fisli- 

 ing Co., Ltd., aboard the mothership is also 

 acknowledged. 



953180°— 51 



COLLECTION OF DATA 



The expedition, consisting of a mothership and 

 25 iongline-fishing vessels, commenced its activi- 

 ties in the vicinity of 4°35' north latitude and 

 143°32' east longitude on June 17, 1950. As the 

 season progressed, the center of fishing gi-adually 

 shifted eastward at a rate of about 100 nautical 

 miles a week, the changes in position of the vessels 

 being dictated largely by the success of fishing in 

 any one area. The deployment of fishing vessels 

 in a north-and-south direction was bounded by 1° 

 and 9° north latitude, but in general fishing was 

 mostly between 1° and 5° north latitude, for it 

 was here that the best catches were made. Wlien 

 operations were terminated on September 5, 1950, 

 the mothership's position was 8° north latitude, 

 15r)°46' east longitude, whence it returned to 

 Japan. The easternmost limit reached by the 

 catcher boats was 160° east longitude. In all, the 

 expedition fished an area of approximately 305,- 

 000 square miles from wliich it took over 4,055 

 tons of tunas, spearfishes, sharks, and other fishes 

 (table 1). 



Table 1. — Total catch, by species, of Japanese tuna mother- 

 ship expedition, J une-Se.ptember 1950 



1 Statistics provided by the Japanese Fishery Agency and converted to 

 pounds using conversion factor of 8.27 lbs.=l kan, 



- Includes short-nosed marlin (Tetraplurus breriroslris). 



3 Includes barracuda (Sphyraena argeiUea), wahoo (Acanthocvbium solaTidri), 

 and doliihin {CoTuphaena liippurus). 



A few tunas wex'e caught by pole and line at tiie 

 surface, but gear employed chiefly was the long- 

 line. This type of gear was developed to a great 



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