192 



Fishery Bulletin 101(1) 



(HI (I : 4 ll(> OS M 



).() 0.2 4 0(1 OS I II 



20  



120 - 



00 - 



^— Time Al Depth 

 — Maximum Dive Depth 

 • •• Djil> Maximum Dcplh 



I ()() 



Cumulative frequency 



0.8 



Figure 3 



The time-at-depth, maximum dive depth, and daily maximum dive-depth cumulative frequency distributions for all peri- 

 ods combined: (A) the two loggerhead sea turtles combined; (B) the two olive ridley sea turtles combined. 



By comparison, the tinie-at-depth and maximum depth- 

 frequency distributions of the two olive ridley sea turtles 

 showed considerably deeper depth distribution (Fig. 3). 

 These sea turtles spent only about 20*7? of their time in the 

 top meter and about W^i of their time deeper than 100 m 

 (Fig. 3). Their daily maximum depth exceeded 150 m at 

 least once in 20% of the days (Fig. 3). Status messages 

 reported that daily dives of 200 m occurred — one dive 

 recorded at 254 m. 



Discussion 



The loggerhead dive-depth distributions indicated that 

 these animals tended to remain at shallower depths than 

 that of 100 m. If shallow longline sets were replaced with 

 deep longline sets, the incidental takes of loggerhead sea 

 turtles should he reduced substantially. Further, even 

 though olive ridley sea turtles dived deeper than logger- 

 head sea turtles, only about 10% of their time was spent 

 deeper than 100 m. Therefore, their incidental catches 

 should also be substantially reduced with the elimina- 



tion of shallow longline sets. However, when deep sets are 

 being made or retrieved or when current shear prevents 

 the gear from sinking to its expected depth, hooks will be 

 present in relatively shallow depths and could result in 

 incidental catches of turtles. 



Results to date in the fishery confirm the reduction in 

 incidental catches of turtles that can be achieved from the 

 elimination of shallow sets. Beginning in April 2001, shal- 

 low sets were prohibited in the Hawaii-based longline fish- 

 ery. Data from the onboard obsei-vers in the longline fleet, 

 which now comprise 2Q'^i of the fishing effort, showed that 

 no loggerhead and only two olive ridley sea turtles were 

 caught from April through December 2001. 



The relatively shallow dive-depth distribution for log- 

 gerhead sea turtles in the central North Pacific is consis- 

 tent with our understanding of their ecology; they forage 

 and migrate along convergent fronts where they encoun- 

 ter a shallow aggregation of forage (Polovina et al., 2000). 

 Although oceanic loggerhead sea turtles have a shallower 

 dive behavior than that of olive ridley sea turtles, they 

 appear to dive deeper in oceanic habitat than loggerhead 

 sea turtles in coastal habitat. P^or example, the dive dis- 



