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Fishery Bulletin 100(3) 



Reference depths 



Predawn and postdusk dives 



Reference depths — the depths to which tuna return after 

 regular excursions above or below them — are not well 

 defined in SBT in the Bight. Bounce dives from the sur- 

 face occur in tracks 1,3, and 11, and brief excursions above 

 and below the bottom of the mixed layer could be seen in 

 parts of tracks 4, 6, and 14. None of these showed the 

 movements characteristic of bigeye tuna, which use the 

 15-16° isotherm as a reference point and make regular 

 excursions to the surface (Holland et al., 1990a). These 

 excursions are thought to be a way of regaining optimum 

 body temperature after foraging in cooler deep water 

 (Holland et al., 1990a; Holland et al., 1992). Bluefin tuna 

 have a more advanced vascular anatomy for physiological 

 thermoregulation than bigeye tuna (Holland et al., 1992); 

 therefore one would expect SBT to display less pronounced 

 behavioral thermoregulation than bigeye tuna under the 

 same thermal regime. Unfortunately, the temperature 

 range of the shallow waters of the Bight was also not large 

 enough to induce such a response. 



The interface of the surface mixed layer and the ther- 

 mocline is a reference point used by many tuna species. 

 Yellowfin tuna often remain within this narrow depth 

 band, especially when travelling in a straight line (Carey 

 and Olsen, 1982; Holland et al., 1990a). Traveling fish 

 have been observed to alternate abruptly between the 

 thermocline interface and the surface, which might as- 

 sist in straight-line orientation (Holland et al., 1990a). 

 Tracks 6 and 14 provide the clearest examples of use of 

 the thermocline interface during sustained straight-line 

 migration. Tuna 6 remained at a depth of about 20 m 

 while staying in the area. However, once it started swim- 

 ming toward the shelf break, it maintained a relatively 

 constant depth within or just above the thermocline. 

 While this behavior is apparently a characteristic of 

 yellowfin tuna travelling alone (Carey and Olsen, 1982), 

 our tracks were for SBT travelling in schools. Tunas 7 

 and 8 remained at the surface during fastest straight- 

 line swimming, suggesting that both depth zones might 

 be used during straight-line swimming. Although it 

 appears that the thermocline interface provides some 

 advantage to sustained straight-line movement, it is not 

 clear whether the advantage is in orientation: possibly 

 this temperature range balances heat build-up from sus- 

 tained swimming. 



In contrast to returning to a reference depth, some 

 tracked fish oscillated rapidly within a depth band. This 

 behavior was observed in parts of tracks 4, 7, 10, and 12. 

 This "frenzied" activity might be ascribed to disturbed 

 behavior caused by the tagging or tracking (or both). How- 

 ever, this behavior might reflect individuals traversing the 

 vertical boundaries of the school (Carey and Olsen, 1982). 

 When this behavior was observed in our study, the tracked 

 fish appeared to be part of a school. However, there were 

 many times when the fish was in a school but rapid verti- 

 cal oscillations were not apparent. Possibly the schools 

 did not extend over sufficient depth range to make this 

 behavior obvious. 



A feature of many of the tracks of SBT was the deep dive 

 made just before dawn as the sky began to lighten, and 

 just after sunset before all light was gone. In these dives, 

 the tuna descended rapidly, stayed a short time at the 

 bottom of the dive, and then usually rose gradually to its 

 original depth. These signature dives were observed in 

 tracks 1, 3, 4, 6, 7, 11, 12, 13, and 14 and possibly occurred 

 in other cases obscured by repeated dives over the twilight 

 period. Archival tag data showed clear signature dives in 

 the Bight and more varied behavior off the shelf (Gunn et 

 al.-). They also showed that in the central Indian Ocean, 

 SBT dive at dawn, remain at great depth throughout the 

 day feeding on squid, and return to the surface at night. 

 The dawn dives we tracked might correspond to the first 

 part of this movement, but the shelf waters of the Bight 

 are too shallow to have a significant scattering layer, and 

 therefore usually do not allow for the detection of prey. 

 Perhaps this dive at dawn is a fixed behavior that has 

 evolved to locate the scattering layer when SBT are in 

 waters of sufficient depth. 



If tuna rely largely on sight to locate prey, twilight 

 would be the first opportunity (dawn), and last opportu- 

 nity (dusk), in which to find the scattering layer as it rises 

 during the night and descends during the day. Dawn and 

 dusk dives have been observed in eastern spotted dolphin 

 in the eastern tropical Pacific Ocean (Holland*'). During 

 the day, these dolphin remained in the shallow mixed lay- 

 er (10-15 m). However, at late dusk they dived to depths 

 of about 65 m. These were followed by further dives, each 

 one a little shallower than the last, until diving was again 

 largely confined to the mixed layer. Just before dawn the 

 pattern was reversed. A line drawn along the bottom of 

 the dives from dusk to dawn would describe a bell shape. 

 The dolphins" behavior was thought to be in response to 

 the vertical movement of the scattering layer. Dawn and 

 dusk dives have since been noted in Atlantic bluefin tuna 

 tracked in shelf waters off New England (Lutcavage et al., 

 2000) and are apparent in tracks of Pacific bluefin tuna 

 in the eastern Pacific (Marcinek et al., 2001). Block et al. 

 (1997) reported predawn dives in yellowfin tuna tracked 

 in the California Bight. Their vertical track plots sug- 

 gested that postdusk dives may also occur Similar verti- 

 cal behavior has been apparent in the track of one skip- 

 jack (Cayre and Chabanne, 1986) and one yellowfin tuna 

 (Carey and Olsen, 1982). However, the absence of similar 

 dives in other studies suggests that the timing of the dives 

 was coincidental. 



Stomach temperature 



Muscle and stomach temperatures of Atlantic bluefin tuna 

 were monitored in several tracking experiments (Carey 

 and Lawson, 1973; Carey et al., 1984). Both were shown 



" Holland. K. N. 1996. Personal commun. University of Ha- 

 waii, PO Box 1346, Coconut Island, Kaneohe, Hawaii 96744. 



