SPAWNING OF YELLOWFIN TUNA IN HAWAIIAN WATERS 



By Fred C. June, Fishery Research Biologist 



The rapid growth of the Pacific tuna fisheries 

 during the past several decades has focused atten- 

 tion on the biology of the various species that 

 contribute to this industry. Studies are being 

 made by the Pacific Oceanic Fishery Investiga- 

 tions, of the United States Fish and Wildlife 

 Service, in those phases of the life history and 

 habits of tunas which are basic to an interpreta- 

 tion of patterns of occurrence, movements, and 

 dynamics of population numbers. Pertinent to 

 such interpretation is the knowledge of the place, 

 time, and extent of spawning. 



Although records of spawning localities of tunas 

 in the Pacific, based on collections of larvae and 

 juveniles and the examination of gonads, have been 

 furnished by various investigators (Marr 1948; 

 Schaefer and Marr 1948; Wade 1950; Hatai, et 

 al. 1941), knowledge of the extent of spawning 

 in time for any particular locality and of the de- 

 velopmental changes that take place in the gonads 

 concomitant with spawning is notably frag- 

 mentary. 



The present paper deals with the spawning of 

 the yellowfin tuna (Neothimnus macropte?*us) in 

 Hawaiian waters, with particular reference to 

 the reproductive process. Ova-diameter measure- 

 ments have been used to demonstrate the develop- 

 mental changes that take place in the gonads dur- 

 ing the breeding season. 1 



Acknowledgment is made to Tamio Otsu, 

 Richard Shomura, and Kenji Ego for their as- 

 sistance in collecting data and making many of 



1 The use of ova-diameter measurements to Indicate the growth 

 that takes place In the ovaries prior to spawning was introduced 

 by Thompson (1915) in his life-history study of the Pacific 

 halibut. Clark (1925, 1929, and 1934) later applied the technique 

 to spawning investigations of the jack smelt {Atherinops cali- 

 fornieneis) , the grunlon (Leureathes tenuis), and the California 

 sardine (Sardinops caerulea). Other investigators also have used 

 the method in spawning studies of several other marine species 

 (Schaefer 1936 ; Olsen and Merriman 1946). 



239637—53 



the ova measurements used in this paper, and to 

 Tamotsu Nakata for preparing the various illus- 

 trations presented here. The author is also grate- 

 ful to O. E. Sette and Dr. M. B. Schaefer for the 

 time and advice they so freely gave him, and to 

 Dr. A. L. Tester, University of Hawaii, for the 

 many valuable suggestions offered him during 

 the preparation of the paper. 



COLLECTION AND TREATMENT OF 

 OVARY SAMPLES 



Because of the difficulty of determining and 

 describing the relative maturity of the gonads 

 from their external appearance, some objective 

 measure of the growth of these organs that is 

 associated with the breeding season was sought. 

 The testes were not suitable because no quanti- 

 tative measure of their sexual products could be 

 found that would provide reasonably accurate 

 estimates of their relative stages of development, 

 While a relation was found between the size 

 (weight) of the testes from individual fish and 

 proximity of the spawning period in Hawaiian 

 waters, variation in size of the testes among the 

 fish examined was so great that no precise de- 

 lineation of their growth changes could be ob- 

 tained. Consequently, only the ovaries of fe- 

 male yellowfin tuna were used. 



The material used in this study was obtained 

 from yellowfin-tuna landings made in waters 

 adjacent to the Hawaiian Islands by the com- 

 mercial longline fishery. The ovaries were col- 

 lected at the Kyodo Fishing Co., Ltd., of Hono- 

 lulu, where the catches were weighed and sold at 

 auction. Observations were made on 112 fish, 

 or approximately 4 percent of the total number 

 of yellowfin tuna landed at Honolulu between 

 January and December 1950. The distribution, 

 by dates, is given in table 1. 



47 



