has been mentioned previously. It may be pertinent to 

 note here that when billfishes are caught by hook or 

 harpoon, they first make several leaps into the air and 

 then swim wildly in broad circles near the surface. 

 Tunas, on the other hand, try to escape in a vertical 

 direction by diving deep (Nakamura, 1949). 



4 POPULATION 

 4.1 Structure 



4.11 Sex ratio 



Sex ratio of the population as a whole is unknown; 

 but from the data of Williams (1967), Kume and 

 Joseph (1969b), and Merrett (1971), females usually 

 predominate in longline catches. The notable excep- 

 tion to this is on the spawning grounds, where males 

 tend to predominate (Kume and Joseph, 1969b). The 

 percentage of females tends to increase with size of 

 fish (Kume and Joseph, 1969b). There is a tendency 

 for spawning grounds to be dominated by larger fish 

 than in nonspawning areas. 



4.12 Age composition 



Since age determination of individual fish is not 

 possible at present, age composition of the population 

 has not been studied. See 3.43 on growth rate. 



4.13 Size composition 



Size composition varies greatly between stocks and 

 with seasons (see 3.52). 



Size at first capture by tuna longline gear is about 

 80 cm eye-fork length. 



Size at first maturity is between 140 and 160 cm 

 eye-fork length. 



Maximum size is probably about 290 cm eye-fork 

 length or 258.6 kg (570 pounds). 



See 1.31 for length-weight relationships. 



4.2 Abundance and Density (of Population) 



4.21 Average abundance 

 No data. 



4.22 Changes in abundance 



Relative abundance in terms of average CPUE 

 (catch per unit of effort) for major ocean areas is 

 shown in Table 5 for the years 1962-70. These data 

 show no apparent trend for the eastern, northern or 

 the Pacific Ocean at large, however, there is a general 

 decline noted for the South Pacific and a slight 

 decline in the Indian Ocean, the latter since 1966 

 (Fisheries Agency of Japan, Research Division, 1965- 

 72). 



Year-to-year changes in CPUE for the various 



Table 5. — Catch statistics by major ocean areas of Japanese longline fishery for striped marlin for 1962-1970 (Fisheries Agency 



of Japan, Research Division, 1965-72). 



'E = Effort in hooks X 10 s . 



! C = Catch X10 J . 



'CPUE = Catch/100 hooks. 



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