Kamimura, Tadao, and Misao Honma 



Distribution of yellowfin in the longline 

 fishing ground in the Pacific Ocean, 

 especially on the regional variation 

 of t h e density in each size group. 

 ^Conference Paper I - Z] 



Yellowfin catch data for the years 1954- 

 59 were classified by area, season, and fish 

 size category. In general, fish smaller than 

 120 cm. were located in east longitudes, whereas 

 those larger than 140 cm. were in west longitudes. 

 Artificial causes of this pattern (effect of fishing, 

 gear selectivity, and land mass effect) were dis- 

 counted, and the distribution instead hypothe- 

 sized as caused by a west-to-east migration of 

 fish. 



Kikawa, Shoji 



Studies on the spawning activity of the 

 Pacific tunas Parathunnus mebachi and 

 Neothunnus macropterus by the gonad 

 index examination. /Conference Paper 

 VII - 87 



This paper gives a general review of the 

 spawning of bigeye and yellowfin in the Pacific 

 Ocean. The "gonad index" is used to represent 

 degree of sexual maturity, and is defined as 

 follows: 



G.I. = W /L 3 X 10 4 , where W is the 

 o o -> 



weight of a pair of ovaries in grams, and L. is 



the cube of the fork length of the fish in cm. 

 Frequency distributions of gonad indices were 

 used to evaluate group maturity of fish indiffer- 

 ent areas. 



The frequency distribution of bigeye gon- 

 ad indices in a spawning area enables the sepa- 

 ration of spawning from nonspawning fish; the 

 former may be distinguished by having a mode 

 around 3. 5 to 5. 5 and a very wide range. The 

 nonspawning group has a mode centering between 

 0. 6 and 1. 5. The gonad index of 3. 1 was selected 

 to separate the two groups. In the case of the 

 yellowfin, the two groups were not as clearly 

 distinguishable. An arbitrary index of 2.1 was 

 chosen to separate spawning from nonspawning 

 groups. 



In order to delineate spawning localities 

 of deep-swimming bigeye and yellowfin, the 

 monthly mean gonad index in unit areas was cal- 

 culated. It was assumed that spawning occurs 

 in the vicinity of areas showing high gonad 

 indices. 



In bigeye tuna, the basin of the Equatorial 

 Countercurrent is a principal center of spawning 

 in the western Pacific. It is also a good bigeye 

 fishing ground. There are indications of pro- 

 longed spawning throughout this area, with a 

 probable peak during the summer. In the eastern 

 equatorial Pacific (110° W. to 150° W.), spawning 

 is likely to occur between January and June in 

 the area just south of the Equator. There are 

 indications of spawning in the South Pacific near 

 the Tuamotu Islands between 10° S. and 20° S., 

 but very few indications elsewhere in the Pacific. 



For yellowfin, it has been reported that 

 spawning occurs nearly throughout the year in 

 the equatorial Pacific (8° S. to 10° N. , 120° W. 

 to 180°), with peak activity between March and 

 July. The present study indicates that through- 

 out the areas between the Equator and 12° N. , 

 peak spawning probably occurs between July and 

 September. In the eastern area (100° W.to 

 150° W.) spawning occur s largely between April 

 and June in the area between the Equator and 

 10° S. Spawning in Hawaiian waters has been 

 reported to extend from about mid-May to the 

 end of October. Another spawning area in the 

 North Pacific is found in waters from Luzon 

 Island to southern Japan, where the season is 

 between April and June. In the South Pacific 

 it is highly probable that spawning occurs in the 

 Coral Sea and its adjacent waters and in the 

 vicinity of the Tuamotu Islands (10° S. to 25° S. , 

 130° W. to 150° W. ) betweenOctober and March, 

 It has been reported that spawning of yellowfin 

 along the coasts of New Caledonia is likely to be 

 between October and March, with a probable 

 maximum during the summer. 



In general, it appears that yellowfin 

 spawning occurs over a broad area, but is cen- 

 tered in the area of the South Equatorial Current. 

 The spawning season of yellowfin seems to vary 

 withlatitude. It is extended in equatorial water s, 

 but confined to the early summer in waters off 

 southern Japan, and to the summer months in 

 the Coral Sea and the Tuamotu Islands area. 

 The spawning seasons in these latitudes coincide 

 with the periods of best fishing. 



Near-spawning bigeye (over 100 cm. in 

 length) comprise under 40 percent of thetotal 

 catch throughout the year in the western equa- 

 torial Pacific. The percentage composition in- 

 creases from west to east, reaching as high as 

 90 percent during the peak spawning season in 

 the eastern area. This marked increase of 

 mature fish from west to east indicates that the 

 spawning potential of bigeye is highest in the 

 eastern Pacific. 



27 



