Experimental Comparison among Four Types Tori-Line Designs in the Western North Pacific
The most seabird bycatch in tuna longline fisheries occurs by the mechanism which is the seabird attacks the bait thrown to the sea before the bait sink to the deep area. Tori-line was developed by the Japanese fisherman to reduce the seabird bycatch. This mitigation concept is avoiding the seabird approach to the vessels with the streamer, and sinking the bait to the deep area in which the seabird cannot attack the bait. Two tori-lines have been used mainly in the area of North Pacific. One is the WCPFC long streamer tori-line with the dangling long streamer and the other is the light streamer tori-line with the short streamer. We have conducted the research of tori-line in North Pacific and suggested that the light streamer is effective for reducing the bycatch (Yokota et al. 2007a, 2007b and 2008). Although these results showed the effectiveness has not been significantly difference between both tori-lines, the sample number was comparatively small and the research period was limited during April and July because the data were collected by a few research vessels. In this paper, we compared the effectiveness of these tori-lines using 20 offshore commercial longliners to answer the problem. Moreover, the effectiveness of two new tori-lines designs was examined to develop more effective design for reducing the seabird bycatch. One is the hybrid streamer tori-line which is used in the South Africa EEZ (Melvin et al. 2010) and south east of Brazil (Mancini et al. 2010). The other design is the modified light streamer tori-line, which we develop in this study.
Melvin et al. (2010) have observed the attack behaviour to the bait in the Japanese tuna longline fish bout in South Africa EEZ, and they showed there were two categories of seabird attacks: primary and secondary attack. Primary attack is an attempt by a seabird to take the bait from a hook. On the other hand, secondary attack is other bird attack the primary bird as the bait is brought to the surface. In South Africa EEZ the diving birds such as white-chinned petrel attack the bait and cause the bycatch of albatross by the secondary attack. This study also conducted the detailed observation of the seabird attack to understand the effect of secondary attack of the diving seabird.