460 



Fishery Bulletin 100(3) 



10 10 20 



Percent of observed deptfis 



30 



Figure 10 



Distribution of observed depths (5-m inter- 

 vals) by day (open bars) and night (shaded 

 bars) for all tracked southern bluefin tuna. 



at night (n=607) and probably accounts for much of the 

 difference in distributions. 



Discussion 



Horizontal movements 



The average swimming speeds of SBT over entire tracks of 

 0.5-1.4 body lengths/s were similar to the range of mean 

 speeds (0.6-1.0 body lengths/s) for Atlantic bluefin tuna 

 (Lutcavage et al., 2000) and 1.02-1.34 body lengths/s for 

 Pacific bluefin tuna (Marcinek et al., 2001). This is within 

 the range of minimum speeds (0.5-2.0 body lengths/s) 

 required to maintain hydrostatic equilibrium in scom- 

 brids (Magnuson and Weininger, 1978). A much higher 

 sustained swimming speed of 2.5 m/s was recorded for 

 tuna 6, which traveled at this speed for 18 hours, virtually 

 in a straight line, from Rocky Island to the shelf break. 

 The vertical movements of this fish were minimized by 

 it orienting itself with the thermocline interface or the 

 surface. And horizontal movements were not apparent. A 

 speed of 2.5 m/s or 2.6 body lengths/s is probably a real- 

 istic sustained swimming speed for SBT. Although tunas 

 are obviously capable of high bursts of speed, sustained 

 swimming speeds seem to be very much lower. Yellowfin 

 tuna have been tracked at speeds of 1.2-3.5 body lengths/ 

 s, and bigeye tuna at 1.4 body lengths/s (Holland et al., 

 1990a, Block et al., 1997). The sustained swimming speeds 

 of Atlantic bluefin tuna, determined from sequential aerial 

 photographs, were 0.8-1.6 body lengths/s (NOAA^). Brill 

 (1996) considered that the specialized anatomy, physiol- 

 ogy, and biochemistry of tuna evolved, not for high burst 

 and sustained swimming speeds as proposed by Dickson 

 (1995), but for rapid growth, digestion, and recovery from 

 exhausting activity. 



The horizontal movements and distribution of SBT 

 suggest strong associations with regions of topographical 

 variation or higher temperatures (or both). The SBT were 

 in the warmer waters of the NW region of the study area 

 in 1994, and the tracked fish also remained within areas 

 of warmer water In 1992 and 1993, all the tuna that trav- 

 eled some distance swam to waters of similar or higher 

 temperatures. Lutcavage et al. (2000) found that tracked 

 North Atlantic bluefin tuna also traveled on the warm side 

 of surface fronts and that ofTshore movements were gener- 

 ally associated with offshore warming of deep basins. 



The temperatures measured during tracking sometimes 

 did not match up with satellite sea-surface temperatures, 

 owing to the inability of sea surface temperature (SST) 

 algorithms to correct for the variability in atmospheric 

 water vapour, cloud, and aerosols, etc. However, the rela- 

 tive temperature differences considered here appear to be 

 robust. Within these temperature limitations, the tuna 

 were initially located on lumps or near islands. Migration 

 to new areas was often interrupted by brief stays at lumps 

 on the way. The shelf break was clearly the destination of 

 tuna 6. It was tracked near Rocky Island for three hours, 

 before it left abruptly and moved in a straight line to 



'" NOAA (National Oceanic and Atmospheric Administration). 

 1975. A study of the applications of remote sensing techniques 

 for detection and enumeration of giant bluefin tuna. Southeast 

 Fish. Cent, contrib. no. 437 (MARMAP no. 108). 48 p. National 

 Space Technology Laboratories, Bay Saint Louis, Mississippi 

 39520. 



