fish under 100 cm. long make up the 

 overwhelming proportion is an outstanding 

 point that they have in common. Furthermore, 

 when we take into consideration the fact that, 

 despite differences of season ajid fishing 

 method, the fishing grounds for the two are 

 formed in the same sea areas, it is probably 

 correct to think that a considerable portion of 

 the northward-moving albacore are a legacy 

 from the southward-moving fish. 



The tendency for pole-and-line fishing 

 vessels to concentrate on certain particularly 

 productive grounds is very strongly marked. 

 Consequently, the tendency for the peculiarities 

 of the catch of certain grounds to extend an 

 Influence over the whole is thought to be es- 

 pecially strong in the case of the pole-and-line 

 catch. Also, as was remarked earlier, the 

 upper limit of the size of fish that can be taken 

 in large numbers by pole-and-line fishing is 

 thought to be in the neighborhood of 100 cm. A/ 



We cannot deny that such peculiarities of 

 the pole-and-line fishing method may be a 

 cause of the difference in the size composition 

 of the two groups. On the other hand, if we 

 consider that around April and May large alba- 

 core of 90-120 cm. break off from the northward- 

 moving group and cross the Subtropical Conver- 

 gence to the southward (Suda 1954b), it also 

 cannot be denied that the cause of the extra- 

 ordinary scarcity of this large size-group in 

 the catch of northward-moving albacore may be 

 a decrease in the numbers of the group itself.^/ 

 Furthermore, with regard to the point that the 

 size composition of the northward-moving alba- 

 core shows highly irregular changes from year 

 to year, we might also consider the fact that 

 the pole-and-line fishing grounds are formed 

 within the limits of a comparatively snnall sea 

 area. However, if we take into consideration 

 the point that the size connposition of the 

 southward-moving group captured within the 

 same area is quite stabilized, it is believed to 

 be important to think of this phenonaenon in re- 

 lation to seasonal changes in the population. 



8/ 



— There may be a possibility that large 



size-groups are included among the northward- 

 moving albacore but that they are not captured. 



9/ 



— Fish of the 90-120 cm. size range are 



also seen among the albacore taken by long- 

 line during the period of northward movement, 

 however, they are far fewer than during the 

 period of southward movement, and indeed 

 vestigial. 



Finally, it cannot be denied that in the 

 differences of size composition seenbetweenthe 

 northward- and southward-moving albacore there 

 may be a factor of seasonal changes in the pop- 

 ulation itself, in addition to such causes as the 

 peculiarities of the fishing method. 



In conclusion, the author wishes to express 

 his thanks to all of the staff nnembers of the 

 Nankai Regional Fisheries Research Laboratory, 

 who gathered the data, to Miss Michiko Momota, 

 who made the drawings, and to Director Naka- 

 mura and Section Chief Yabe, for their constant 

 guidance. 



Bigeye Studies. I. Size Composition of the 



Bigeye on the North Pacific Fishing Grounds 



(and Especially on the Alternate -year 



Cycle in the Size Composition)* 



By 



Tadao Kannimura and Misao Honma 

 Nankai Regional Fisheries Research Laboratory 



/English title and summary/ 



Biology of the Big-Eyed Tuna, Parathunnus 



mebachi (Kishinouye) - I. Length Frequency 



of the Big-Eyed Tuna Caught in the North Pacific 



with Special Reference to Biennial Frequency 



The present paper deals with analysis of 



length frequency of the big-eyed tuna, Parathunnus 



mebachi (Kishinouye), in the light of various 



hypotheses on the basis of data obtained from 



length determination conducted at several landing 



places and from information furnished by tuna 



fishing boats. Number of the samples used for 



the study were caught in several parts of the 



North Pacific, extending north of latitude 26 N. , 



o / o 



and between longitudes 130 E. and 165 W. in 



five successive seasons from October to March 



during a period of 1948 through 1953 (Table 1). 



For determination of t h e length, the distance 



from the tip of the snout to the nnedian part of 



the caudal fork was used. 



♦Contribution No. 46 of the Nankai Regional 

 Fisheries Research Laboratory, published in 

 1953 in Contributions of the Nankai Regional 

 Fisheries Research Laboratory, No. 1. 



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