Figure 1.— Geographical locations of skipjack tuna fisheries in the Pacific Ocean. The dark shaded areas denote surface fisheries; the 

 the light shaded areas denote areas where skipjack tuna have been taken by the Japanese tuna longUne fishery. The broken lines 

 indicate the northern and southern limits of the longUne fishery. 



adequate catch data from mid-oceanic areas using other 

 means, the longline data can be helpful in providing 

 clues to the distribution of skipjack tuna on an ocean- 

 wide basis. The extensive use of longlines geographically 

 (Fig. 1) and in all months of the year makes the data 

 especially useful for this purpose. 



Certain characteristics of the longline gear affect the 

 catch and catch data, and these should be cited so that 

 conclusions drawn from the data are viewed in the 

 proper perspective. First, the catches of skipjack tuna 

 by this gear are extremely small. In the years 1962-66, 

 for example, the catch of skipjack tuna in the Japanese 

 commercial tuna longline fishery averaged 1,180 metric 

 tons per year, as compared with an average of 130,000 

 metric tons caught by the pole-and-line fishery. The 

 smallness of the catch reflects the inefficiency of the 

 gear for taking this species, but, because of the wide 

 areal coverage by this gear, these data should be useful 

 for determining areas of high or low relative abundance. 



Second, longlines tend to catch the large fish, al- 

 though there is evidence that shows this gear to be 

 capable of catching smaller ones also. Miyake (1968) 

 examined the lengths of skipjack tuna caught by longline 

 and by pole and line from the Japanese mother ship 

 Tenyo Mam off the Solomon Islands and found that fish 

 taken by longline were sig^nificantly larger than those 

 taken by pole and Une. Length-composition data for 

 longUne-caught skipjack tuna from other areas in the 

 Pacific also show this tendency toward larger fish (Fig. 

 2), but in the U.S. Trust Territory of the Pacific 



Islands, for example, the longUne catches were com- 

 posed mainly of fish less than 65 cm, the same group 

 that makes up the bulk of the pole-and-hne catch. 

 Longline catches of fish less than 55 cm (3.5 kg) are not 

 uncommon, and the smallest skipjack tuna recorded by 

 scientists of the National Marine Fisheries Service, Ho- 

 nolulu Laboratory measured only 35 cm (Murphy and 

 Shomura 1955). 



Third, although the Japanese Government issued re- 

 gulations in 1963 requiring all tuna fishing vessels to 

 submit copies of their fishing logbooks after each trip, it 

 may be that not all longline-caught skipjack tuna were 

 recorded, since this species is of minor economical im- 

 portance to the longline fishery and is taken incidentally 

 to other tunas. Failure to do so by these longliners is 

 suspected from the low proportion (expressed as per- 

 centages) of skipjack tuna recorded compared to those 

 landed (Table 1). In Table 1, the number of skipjack 

 tuna recorded was converted to weight, using 6.88 kg as 

 the mean weight of skipjack tuna taken on longlines. The 

 mean weight was obtained by converting the mean fish 

 length from four areas of the Pacific Ocean (Fig. 2, panel B), 

 using the length to weight relationship, Logio W = -8.342 -t- 

 3.368 Logio L, developed by the Honolulu Laboratory (the 

 length to weight relationship is based on 1,298 skipjack tuna 

 caught in Hawaiian waters, including both males and fe- 

 males measuring from 32.7 to 87.7 cm). In early years, 

 1962-64, the proportion of skipjack tuna recorded was ex- 

 tremely low, averaging 21.3%, but in later years, 1965-67, it 

 rose to an average of 60.5%. 



