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Fishery Bulletin 101(3) 



It is unlikely that the tags on the two tunas (nos. 10 

 and 23), which stayed at the seamount longest, were shed 

 and lay on the bottom. The reasons supporting their being 

 attached to living tunas are as follows. First, the two tags 

 were not recorded with equal frequency during all times of 

 the day as might be expected of a tag lying at one location 

 within the range of the monitors. The tags were usually de- 

 tected for a few hours and then absent for a similar period. 

 This pattern of detection is consistent with the tunas mov- 

 ing within the range of the monitor and later outside its 

 range. Second, the two tags were jointly detected after long 

 periods of absence or ceased being detected simultaneously 

 after long periods of presence. This reception pattern is 

 consistent with the two tunas moving in and out of the de- 

 tection range of the monitors within the same school. Third, 

 one tuna (no. 23) was detected by the monitor on the south 

 side of the seamount, but not on the north side during one 

 day; the same tuna was detected by the northern monitor, 

 but not the southern monitor on the next day. This pattern 

 of detection was consistent with the tuna swimming over 

 the northern region of the seamount on the first day and 

 over the southern region on the second day. 



The yellowfin tunas were present at the seamount at 

 all seasons of the year Five of the tunas tagged during 

 August and September 1998 (nos. 7, 8, 9, 16, and 17) emi- 

 grated during early fall as the water temperature began 

 to decrease (Fig. 2A). However, three individuals (nos. 10, 

 21, and 23) remained at the seamount from January 1999 

 to April 1999 when the temperature dropped to 18°C. Two 

 (nos. 10 and 23) remained present when the subsurface 

 water temperature descended to 16°C during the following 

 winter of 2000 (Fig. 2B). 



The yellowfin tunas remained at the seamount at all 

 times of the day. This is evident from a 24-h record of the 

 arrivals of 10 tunas during a 15-d period from 16 to 30 

 September 1998 (Fig. 3). The tunas were present more 

 often during daytime, from 06:00 to 18:00 hours, during 

 the first 12 days. Notice the clustering of the different 

 symbols in Figure 3, each indicating a specific tuna, in 

 separate columns during the period from 06:00 to 18:00 

 hours. However, the amount of time spent at the sea- 

 mount became more evenly distributed between daytime 

 and nighttime by 28 September Note the even dispersion 

 of the symbols over the 24-h period during the last three 

 days of the 15-day period. There was little variation evi- 

 dent in the frequency of arrivals at different times of the 

 day when the arrivals were summed over the entire study 

 (Fig. 4). The percentage of arrivals during each hour of 

 the day (see crosshatched polygon) differed little from an 

 even distribution of arrivals (4.2''// /h) throughout the day 

 (see dashed circle). 



We determined the frequency of various lengths of 

 stays at the north (Fig. 5A) and south sites (Fig. 5B) at 

 the Espiritu Santo Seamount. A stay for a particular tuna 

 was defined as the period of detections with no separation 

 intervals greater than 15 min. Let us say that tuna 1 was 

 detected at 08:00, 08:14, 08:28, and 09:00 hours. The dura- 

 tion of the stay of tuna 1 would be 28 min. The second detec- 

 tion followed the first by 14 min (<15 min), and the third 

 followed the second also by 14 min (also < 15 min). However, 



the fourth detection followed the third by 32 min (>15 min) 

 and was therefore not pooled into a single duration. This 

 stay would then be placed in the 15:00-29:59 min time 

 class in Figure 5. Twenty-seven percent of the detections 

 at ESN and 33% of those at ESS were separated by greater 

 than 15 min and were thus considered single detections 

 and included in the 00:00-h class. Fifty-three percent of the 

 visits to ESN and 37% of the visits to ESS were between 

 00:01 and 14:59 min. Twenty-one percent of the visits to 

 ESN and 20% of the visits to ESS were between 15:00 and 

 59:59 min. The majority of visits were less than 1 hour in 

 duration and only a few exceeded an hour. 



The intervals spent away from the seamount were simi- 

 larly short. Sixty percent of all absences at ESN were less 

 than 1 h (Fig. 6) as were 65% of the absences from ESS. 

 Ninety percent of the absences from both sites were less 

 than 5 hours. Only 0.1% of the visits exceeded 23 hours. 

 There appeared to be no favored period of absence as 

 indicated by the smooth slope of both curves in the log- 

 survivor plot. Only 72 periods of absence at ESN and 114 

 periods at ESS exceeded a day. Of these longer periods, 

 42% of the absences from ESN (Fig. 7A) and 46% of the 

 absences from ESS (Fig. 7B) were for two days. Only 7 % 

 of the absences from ESN and 4% of the absences from 

 ESS were between 10 and 19 days. Only 2% of periods of 

 absence from ESN exceeding a day were greater than 100 

 days (Fig. 7A). 



Discussion 



We found that yellowfin tuna remained at the seamount 

 for periods ranging from a few days to greater than a year. 

 Fifty percent of 458 yellowfin tuna tagged with dart tags 

 at the Cross Seamount off Hawaii were recaptured at that 

 seamount within 15 days of tag application (Holland et 

 al., 1999). This "half-life" of tuna residence was short, sug- 

 gesting that the seamount served as a landmark to guide 

 migration and not as a destination for feeding. 



Thirty-eight yellowfin tuna were tagged with ultrasonic 

 beacons at two buoys off the western coast of Oahu and 

 monitored over a 13-month period by automated "listen- 

 ing" monitors (Klimley and Holloway, 1999). These moni- 

 tors (VR20) possessed a more sensitive receiver than our 

 monitors (VROl and VR02). The former had a maximum 

 range of 1.1 km. The maximum published range of our 

 monitors was 0.5 km. Twenty-seven of the tuna returned 

 to the buoys a mean of 4.2 visits per tuna. The mean dura- 

 tion of each visit was only 40.1 min and the mean period of 

 absence was 17.2 days. Seventy-three percent of the tuna 

 tagged on the same day returned together The tunas often 

 arrived at the same time of the day and returned only to the 

 buoy at which they were tagged. This allegiance of tunas 

 to one school, their predilection for returning to the site of 

 tagging, and the precise timing of their visits are consistent 

 with the theory that the species has migratory pathways 

 consisting of way-points that are visited with regularity. 

 That the tuna spent little time at the FAD suggests that 

 the buoys are not feeding destinations, but rather land- 

 marks used in migration. 



