506 



Fishery Bulletin 88(3). 1990 



offset in this species by the allometric growth of the 

 swim bladder, thereby precluding the need to increase 

 hydrodynamic lift through higher swimming speeds. 

 However, the sustained swimming speeds observed 

 during these tracks (average of 1.24 m/second) are con- 

 siderably higher than the 0.5 to 0.6 m/second predicted 

 by Magnuson (1978) for fish of this size. Obviously, field 

 observations of larger fish would be useful in further 

 testing the Magnuson model. 



Periods of possible fly-glide behavior were observed 

 in sections of tracks of yeliowfin and bigeye tunas. 

 These oscillations, which in one instance (YF8506) were 

 exhibited for the entire daylight portion of a straight 

 azimuth track, have been hypothesized to result in a 

 saving of energy required for locomotion between two 

 points (Weihs 1973, Magnuson 1978). As predicted by 

 this model, most gliding phases lasted longer than the 

 ascending phases, and the descending and ascending 

 legs were connected by abrupt angles, which maximize 

 energy transfer from the gliding to the flying phase. 

 Thus, using the "tly" angle (fl) of 9.55 and "glide" angle 

 (a) of 6.06 obtained from YF8506, and a swim/glide 

 drag ratio (k) of 1.2 (Magnuson 1978), the equation: 



Energy saving = 1 



tan a 



sin ft + (tan a  cos ft) 



1 + 



1 sin p\ 

 k sin a 



results in an energy saving of 9.4% compared with level 

 swimming over the same distance. And, where T is the 

 increased time to travel the same distance using a 

 fly/glide strategy as opposed to level swimming, using 

 the equation 



sin o -I- sin ft 

 sin {a + P) 



indicates that this strategy results in only a 0.9% in- 

 crease in time to travel the same distance. 



The extremely regular, large, upward excursions 

 made by all the off-FAD bigeye tuna during daylight 

 hours may represent behavioral thermoregulation. At 

 the low (14-17°C) ambient temperatures adopted by 

 these fish, their core temperatures possibly drop below 

 some threshold level which requires movement into 

 warmer water to regain optimum body temperatures. 

 If this is the case, it would suggest that there exists 

 a strong motivation for inhabiting the deep cold layers 

 observed during these tracks. A possible motivation 

 would be the opportunity to feed on deepwater fish, 

 squid, and crustaceans which the bigeye tuna then 

 follow into shallower depths at night when these prey 



organisms migrate toward the surface. Monitoring the 

 core muscle temperatures of bigeye tuna would indicate 

 if, in fact, the large upward excursions are a form of 

 behavioral thermoregulation which, when combined 

 with physiological thermoconservation, allows these 

 fish to exploit an otherwise unreachable resource. 



Acknowledgments 



This work was supported by the University of Hawaii 

 Sea Grant College Program (Ultrasonic Telemetry of 

 Horizontal and Vertical Movements of Pelagic Fish 

 Associated with FADs project, MR/R-25) under Institu- 

 tional Grant No. NA85AA-D-SG082 from the NOAA 

 Office of Sea Grant, Department of Commerce; the 

 National Marine Fisheries Service, NOAA; State of 

 Hawaii Department of Planning and Economic Devel- 

 opment; and the Federation of Japan Tuna Fisher- 

 men's Cooperative Association. The help of Lance 

 Asagi, Robert Bourke, Zig Ching, Scott Ferguson, Jeff 

 Koch, and Ruben Yost is also gratefully acknowledged. 



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