SIZE VARIATION OF CENTRAL AND WESTERN 



PACLFIC YELLOWFIN TUNA 



By 



Edwin S. Iversen 

 Fishery Research Biologist 

 Pacific Oceanic Fishery Investigations 

 Honolulu, T. H. 



Theyellowfin tuna, Ne othunnus 

 macropterus (Temminck and Schlegel), is an 

 importaint commercial species in many areas of 

 the Pacific, and it is of interest to know whether 

 the stocks differ biologically from one area to 

 another. Since sonne aspects of the life history 

 of a fish can be inferred from the sizes of the 

 fish caught by a standard type of gear, we have 

 herein assembled a number of size frequencies 

 classified by area, sex, and season, and have 

 examined them for evidence of growth and 

 migration. 



Moore (1951) studied the age and growth 

 of Hawaiian yellowfin by the size-frequency 

 method. Using data collected at the Honolulu 

 market in 1948 and 1949, he demonstrated 

 rapid growth amounting to about 60 pounds per 

 year. Additional Hawaiian market data for the 

 years 1950-53 considered herein support Moore 's 

 findings on the growth of the fish and also provide 

 evidence of a migration of the yellowfin in this 

 area. In addition, yellowfin size-frequency data 

 from along the Pacific Equator are examined for 

 evidence of growth and nnigration. 



Appreciation is expressed to the members 

 of our field parties who have collected measure- 

 ments of the tunas . Sonnany persons have assist- 

 ed by gathering data and offering suggestions 

 that no attempt will be made to name and thank 

 them individually. Mr. T. Nakata prepared the 

 figures in this report. 



SOURCES OF DATA 



All yellowfin considered in this report 

 were caught by long line (flagline) gear. In 

 Hawaii, where there is a commercial longline 

 fishery, described in detail by June (1950) and 

 Otsu (1954), the landings at the market were 

 sampled as reported by Moore (1951). In addi- 

 tion the Pacific Oceanic Fishery Investigations 

 (POFI) has carried out exploratory fishing oper- 

 ationsi./ in the central Pacific using longline 

 fishing gear^' described by Niska(1953). One 

 sample is available fronn a commercial venture 

 in Samoa (Van Campen 1954). Additional size- 

 frequency data from the western Pacific are 

 available from Japanese nnother ship expeditions . 



These fishing expeditions were reviewed in de- 

 tail by Shimada (1951a and b), Ego and Otsu 

 (1952), Van Campen (1952), and Murphy and 

 Otsu (1954). The general areas from which 

 these various samples were obtained are shown 

 in figure 1. 



APPLICABILITY OF THE SIZE- 

 FREQUENCY METHOD 



Most of the conclusions drawn in this 

 report are dependent on the positions and 

 changes in positions of size-frequency modes. 

 There are a number of biologicEil characteris- 

 tics of the yellowfin other than those we wish 

 to describe that may affect the size distribu- 

 tions and modal positions. 



Samples of a schooling fish may not be 

 representative of the stock present in any area 

 if the samples are small in number and/or 

 poorly distributed. Murphy and Elliott (1954) 

 have studied variability in longline catches of 

 yellowfin in the central Pacific, and they re- 

 port that "there is considerable evidence that 

 yellowfin tuna are not randomly distributed in 

 space but rather are aggregated. " Schaefer 

 (1948), referring to the difficulty involved in 

 using the modal progression nnethod of growth 

 analysis, says "it is desirable that the total 

 sample be composed of subsamples drawn by 

 either representative or random methods from 

 each of a large number of schools selected 



1/ 



— Measurements of the catches taken 



aboard the POFI research vessels John R. 

 Manning and Hugh M. Smith and the cooper- 

 ating commercial vessel North American 

 have been used in this study. 



2/ 



— In general the fishing gear of the 



Japanese, Hawaiian, and POFI vessels is 

 made of cotton and has 4 to 6 branch lines per 

 basket. However, POFI has experinnentally 

 varied the number of hooks per basket from 6 

 to 21. Comparison of samples between this 

 experimental gear and the standard 6-hook 

 gear indicates that the type of gear had no 

 appreciable effect on the size of the fish caught. 



