382 



Fishery Bulletin 105(3) 



Time (local) 



i: 00 1 2 no i: 0(1 i: m i: on 12 no ' 1: on 1: no 1: on 1: 00 12 



12 00 12 00 12 00 12 00 12 00 12 00 12 00 12 00 12 00 12 00 12 00 12 



Figure 1 



Depth and temperature records for skipjack tuna iKatsuwonus pelamis) 

 with implanted archival tags after release. (A) Skipjack tuna (tag no. 

 4930) during April 10-20, 2004. (B) Skipjack tuna (tag no. 4885) during 

 April 9-20, 2004. Time periods shaded gray indicate nighttime. 



sets, behavior types 1, 2, 3, and 4 were evaluated for 

 each day at liberty, and the descriptive characteristics 

 presented. 



Behavior associated with floating objects 



The behaviors of the skipjack tuna released at the TAO 

 mooring were distinct and discernible from the behaviors 

 48 h after the tuna separated from the mooring and the 

 vessel. When the fish were associated with the mooring 

 and the vessel, they remained primarily between the 

 surface and 50 m during the night and day (Fig. 2A). 

 The mean swimming depths were 13.6 m (range: 1-57.4 

 m) at night and 10.8 m (range: 1-92.3 m) during the day. 

 Shortly before dusk, several dives to about 90 m were 

 recorded, which were followed by an apparent change in 



the vertical movements during the night. The greater 

 frequency of vertical movements observed during the 

 night (1900 h to 0700 h) within the mixed layer probably 

 represented foraging activity on prey organisms of the 

 deep-scattering layer (DSL). 



The delta T values, differences between the peritoneal 

 cavity temperatures and the ambient temperatures 

 (Fig. 2B), corresponding to the simultaneous depth 

 records (Fig. 2A), showed an average of 2.2°C (95% 

 CI=0.1°C, range:-0.4 to 14.1°C) during the day, and an 

 average of 1.8°C (957<5 CI=0.1°C, range: -0.9 to 12.9°C) 

 during the night. The depths of the mixed layer and 

 the thermocline (depth of the 20°C isotherm) (Fiedler, 

 1992) were approximately 15 and 44 m, respectively. 

 Whenever skipjack tunas made dives in excess of the 

 thermocline depth, the delta T values were observed 



