Schaefer et aL: Vertical movement patterns of Katsuwonus pelamis in the eastern Pacific 



385 



300 m and 250 m, where it remained for about 12 min- 

 utes before returning to depths within the mixed layer. 

 The peritoneal cavity temperature and delta T were 

 28.7°C and 3.8°C, respectively, at the beginning of the 

 dive, 26.1°C and 18.4°C at the bottom of the dive, and 

 21.8°C and 9.8''C before the quick ascent at 12:38 h from 

 271 m to above the thermocline. 



Surface-oriented behavior 



For unassociated behavior, the mean number of sur- 

 face-oriented events per day ranged from 5 to 23 (grand 

 mean = 9.4, 95% CI = 2.4). The greatest number and 

 longest duration of surface-oriented events occurred 

 between 01:00 and 12:00 h (Fig. 6, A and B). The dura- 

 tion of events ranged from 10 to 214 min (mean=48.8 

 min, 95% CU6.2 min) (Fig. 6 C). 



Vertical habitat use 



The vertical habitat use by the five skipjack tuna, for 

 unassociated behavior, is presented as composite dis- 

 tributions by night and day along with the thermal 

 profile in Figure 7. The vertical habitat-use distributions 

 indicated that the fish remained above the depth of the 

 thermocline (44 m) during the night 98.6% of the time 

 but spent 37.7% of their time below the thermocline 

 during the day. 



Discussion 



The results obtained in our study are useful for evaluat- 

 ing vertical movement patterns and habitat use for skip- 

 jack tuna on temporal scales previously undocumented. 

 Knowledge about skipjack tuna movements, behavior, 

 and habitat use, when tuna are associated and unassoci- 

 ated with floating objects, specifically in oceanic regions 

 where large-scale industrial purse-seine fisheries oper- 

 ate, are important for understanding the ecology of this 

 species, assessing catchability of the species for inclu- 

 sion in stock assessments, and for evaluating potential 

 modifications in fishing techniques for reduction of the 

 bycatches of nontarget species. 



In the present study, the vertical movement patterns 

 of skipjack tuna when associated with floating objects 

 were similar to those reported by Schaefer and Fuller 

 (2005) for both skipjack and bigeye (Thunnus obesus) 

 tunas associated with floating objects in the equatorial 

 EPO. In both studies, the behavior of skipjack tuna 

 was characterized by swimming depths predominantly 

 shallower than the depth of the thermocline, throughout 

 the day and night, and average nighttime depths were 

 slightly deeper than those during the day. A plausible 

 explanation for the greater average depths at night 

 than during the day in these studies is the observed 

 nighttime vertical distribution of the DSL and the for- 

 aging behavior of these tunas while associated with 

 the moored buoys and the drifting vessel in this study 

 area (Schaefer and Fuller, 2005). Ultrasonic telemetry 



^ I I I r 



"I— I— I I I I I I r I I I I I 1 I I 

 0:00 4:00 8:00 12:00 16:00 20:00 

 Hour 



10 14 18 22 26 30 34 38 42 46 50 

 Number of dives 



250 350 450 



Deptfi (m) 



Figure 4 



Summary of frequency data for repetitive 

 bounce diving behavior not associated with 

 floating objects for five skipjack tuna (Kat- 

 suwonus pelamis) over all dives. (A) Time 

 the first dive of the day occurred. (B) Total 

 number of dives for each day. (C) Depth of 

 all dives. 



