Davis and Stanley: Movements of Thunnus maccoyii in the Great Australian Bight 



459 



Q 60 



Local time (hours) 



Figure 9 



(A) Track of southern bluefin tuna 14 on 31 January 1994. (B) Track of southern bluefin 

 tuna 16 on 1 February 1994. See Figure 6 for legend details. 



proportion of their time (nearly 30"^) in the upper 5 m. 

 At night they tend to remain somewhat deeper, although 

 they are present at the surface for part of the night. Tunas 

 4 and 12 were not surface-oriented during the day, but 

 ranged evenly within the mixed layer. Both these fish were 

 associated with large schools throughout the track and they 

 exhibited rapid vertical oscillations, which we assumed 

 spanned the vertical boundaries of the school. Tuna 6 also 

 spent little time at the surface. It was associated with a 

 small school of tuna and spent most of its time at or below 

 the thermocline while travelling at 2.5 m/s towards the 

 shelf edge. Tuna 14 also spent much of its time at the ther- 

 mocline boundary while traveling during the day. 



Swimming speed 



The average swimming speeds of SBT over the entire 

 tracks were in the range of 0.5-1.4 m/s or 0.5-1.4 body 

 lengths/s. These speeds are based on the movements of the 

 tracking vessel and therefore are considered conservative 

 because they do not incorporate the horizontal or vertical 

 meandering of the tracked fish or water currents. There 

 were small differences in the mean speeds between day 

 (0.90 m/s) and night (0.94 m/s) for all fish combined. The 

 frequency distribution of these speeds differed markedly 

 between day and night (Fig. 11). However, the number of 

 observations during the day (n=1119) far exceeded those 



