The adjustment of fishing depth is achieved by 

 slackening the main line while setting the gear as well 

 as by adjusting the length of the float line. With the 

 float line of 20-40 m and the slackening rate of the 

 main line of 20-50%, the adjusted depth of hooks 

 appears to range from 60 to 150 m (Katsuo-Maguro 

 Nenkan, 1969). 



Recently, much effort has been directed at develop- 

 ing labor-saving devices and devices for improving 

 fishing efficiency in the Japanese longline fishery. This 

 effort has led to the gradual improvement of devices 

 within the actual fishery and the development of es- 

 sentially different methods which may represent break- 

 throughs in fishing technology. The former is repre- 

 sented by the introduction of two similar devices of 

 longlining, the reel-type and the tub-type gear with 

 improvement in related equipment such as the gear- 

 transporting conveyor and guide stand system, and 

 the latter, for example, by the development of the so- 

 called wire fishing or automatic snapping (Katsuo- 

 Maguro Nenkan, 1969). 



5.12 Boats 



Most Japanese tuna longline vessels which have 

 recently been built are in the 240-ton class. Table 5 

 gives the specification of larger vessels in the 340-ton 



class. Recently, many of these vessels have been 

 equipped with refrigerators capable of preserving the 

 catch at very low temperatures of 40° or -45° C. 



5.2 Fishing Area 

 See 2.1, 2.21, 2.22, 2.3. 



5.3 Fishing Season 

 See 2.1, 2.21, 2.22, 2.3. 



5.4 Fishing Operations and Results 

 5.41 Effort and intensity 



Fishing effort for billfishes can practically be ex- 

 pressed by the fleet size of the tuna longline fishery or, 

 in more detail, by the number of hooks used. For the 

 shortbill spearfish, however, this has never been es- 

 timated. To assess the fishing effort exerted on a given 

 species of fish, information is first required on the 

 catches of that species and its inherent geographical 

 pattern of density in addition to the crude total effort. 

 In this sense, the improvement of data format from 

 commercial vessels for preparing the catch statistics 



Table 5. — Specification of 340-ton class tuna longline vessels. 



52 



