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



381 



a 24-h period. Surface-oriented behavior was defined 

 as the behavior of fish that remained less than 10 m 

 below the surface for periods greater than 10 minutes. 

 Deep-diving behavior was defined as the behavior offish 

 that whose dives exceeded 500 m in depth. The quan- 

 titative characteristics describing the different types of 

 behavioral events were derived from the AT records for 

 the five fish. 



The AT data sets were separated into periods of 

 nighttime and daytime by the times of nautical twilight. 

 Nighttime was classified as the period between the time 

 of the first evening record after nautical twilight until 

 the time of the last morning record before nautical twi- 

 light. The individual data sets for night and day were 

 used in evaluations of diel differences in behavior and 

 in habitat use. 



The depth data obtained from the archival tags were 

 analyzed in conjunction with ambient ocean tempera- 

 tures, as measured by a calibrated Sea-Bird SBE 39 

 time-temperature-depth probe (Sea-Bird Electronics, 

 Seattle, WA) deployed on 9 April 2004 to about 500 m 

 depth near the TAO mooring at which the fish were 

 tagged and released. For ambient temperatures at 

 depths greater than 500 m we utilized the temperatures 

 measured by a CTD (conductivity-depth-temperature) 

 probe deployed off the National Oceanic and Atmospher- 

 ic Association (NOAA) ship Ka'imimoana by the TAO 

 Project Office (NOAA/Pacific Marine Environmental 

 Laboratory) on 15 April 2004 to 1000 m depth near the 

 same TAO mooring. These two temperature-depth pro- 

 files to depths of about 500 m, obtained from hydrocasts 

 conducted six days apart, were virtually identical. 



Results 



Behavior patterns 



Evaluations of the complete AT depth records, which 

 ranged from 9.3 to 10.1 days, for the five skipjack tuna 



(Table 1; tag numbers: 4881, 4885, 4908, 4930, 6740) 

 indicated that all five fish remained associated with 

 the drifting vessel, following separation from the TAO 

 mooring, until the vessel departed the area at 0940 h 

 on 11 April 2004. Within 48 h of the time of separation, 

 all five fish demonstrated distinct behavior unassociated 

 with a floating object, including repetitive bounce diving 

 during the day, and for each of the six subsequent days 

 of the AT depth records. The variability in the daytime 

 unassociated behavior patterns for each of the five fish 

 during the approximate 8 days of AT depth records, fol- 

 lowing separation from the vessel, indicated that all five 

 fish did not remain together. 



The depth records for two representative fish (Fig. 1) 

 showed similarities for both day and night for the first 

 48-h period, including a change in behavior when the 

 vessel departed. The following day, both fish showed 

 deeper average swimming depths, and the day after 

 that both fish began to show repetitive bounce-diving 

 behavior which continued for the remaining 7 days. The 

 first day in which bounce diving occurred for a skipjack 

 tuna (tag no. 4930), included the deepest dive, to a 

 depth of 596 m, recorded for any of the five fish. Dur- 

 ing the last 3 days of depth records, in addition to the 

 distinct diel differences seen for both fish, the average 

 depth after a dive for the fish with tag 4930 (mean=47.1 

 m, 95% confidence interval (CI) = 1.1) was significantly 

 deeper (ANOVA: F=3.9, P<0.05) than for the fish with 

 tag 4885 (mean=20.3 m, 95% CI=3.7). These depth dif- 

 ferences and behavior patterns indicate that these two 

 similar-size skipjack tuna did not remain together for 

 long after their separation from the vessel. 



Evaluations of the depth records for the five skipjack 

 tuna carrying ATs in the equatorial EPO resulted in 

 the discrimination of four distinct behaviors: 1) be- 

 havior associated with floating objects (abbreviated 

 to "associated behavior" in this article; 2) behavior 

 unassociated with floating objects (abbreviated to "un- 

 associated behavior", 3) deep diving behavior, and 4) 

 surface-oriented behavior. For the five archival tag data 



