448 



Abstract— The vertical and horizontal 

 movements of southern bluefin tuna 

 (SBT). Thunnus maccoyii, in the Great 

 Australian Bight were investigated 

 by ultrasonic telemetry. Between 1992 

 and 1994, sixteen tuna were tracked 

 for up to 49 h with depth or combined 

 temperature-depth transmitting tags. 

 The average swimming speeds (mea- 

 sured over the ground) over entire 

 tracks ranged from 0.5 to 1.4 m/s or 

 0.5 to 1.4 body lengths/s. The highest 

 sustained swimming speed recorded 

 was 2.5 m/s for 18 hours. Horizontal 

 movements were often associated with 

 topographical features such as lumps, 

 reefs, islands and the shelf break. 

 They spent long periods of time at the 

 surface during the day (nearly 30%), 

 which would facilitate abundance esti- 

 mation by aerial survey. At night, they 

 tended to remam just below the surface, 

 but many remained in the upper 10 m 

 throughout the night. SBT were often 

 observed at the thermocline interface 

 or at the surface while travelling. A 

 characteristic feature of many tracks 

 was sudden dives before dawn and 

 after sunset during twilight, followed 

 by a gradual return to their original 

 depth. It is suggested that this is a 

 behavior evolved to locate the scatter- 

 ing layer and its associated prey when 

 SBT are in waters of sufficient depth. 

 SBT maintained a difference between 

 stomach and ambient temperature of 

 up to 9°C. 



Vertical and horizontal movements of 

 southern bluefin tuna iThunnus maccoyii) 

 in the Great Australian Bight 

 observed with ultrasonic telemetry 



Tim L. O. Davis 

 Clive A. Stanley 



CSIRO Division of Marine Research 



Castray Esplanade 



Hobart. Tasmania, Australia 7000 



E mail address (for T L O Davis) tl davis®csiro au 



Manuscript accepted 17 May 2001. 

 Fish. Bull. 100:448-465 (2002). 



Southern bluefin tuna, Thunnus mac- 

 coyii, spawn in the northeast Indian 

 Ocean south of the Sunda Islands from 

 August to May (Farley and Davis, 1998). 

 Young-of-the-year move down the west 

 coast of Australia and first appear in the 

 Great Australian Bight as 1-year olds. 

 They aggregate in the Bight during the 

 Austral summer, disperse to the east 

 or west within latitudes of 30-40°S 

 in autumn, and return to the Bight in 

 spring. Juveniles between 1 and 4 years 

 old return each summer to the Bight 

 where they form extensive surface 

 schools. 



To provide a fishery-independent in- 

 dex of recruitment, line transect aerial 

 surveys have been flown in the Bight 

 each summer since 1990 to estimate 

 the relative abundance of juveniles 

 visible in the top 5 m (Chen et al.^). 

 The largest source of variance in these 

 estimates is thought to be environmen- 

 tal factors that influence both surfac- 

 ing behavior and aerial detection. To 

 investigate these problems, we used 

 ultrasonic telemetry to provide the 

 first information on surfacing behavior 

 and short-term horizontal and verti- 

 cal movement patterns that might 

 influence sightings from the air. More 

 comprehensive information on tuna 

 surfacing for the whole of the three- 

 month survey period is expected from 

 an archival tagging program begun in 

 1992 (Gunn et al.^). Because these data 

 are dependent upon the recapture of 

 archival-tagged fish, this information 

 will not be available for some time. 



Because of Carey's pioneering work 

 on Atlantic bluefin tuna (Carey and 

 Lawson, 1973), ultrasonic telemetry 



has been used to study many tuna 

 species, including Atlantic (Lutcav- 

 age et al., 2000) and Pacific bluefin 

 tuna (Marcinek et al., 2001), yellowfin 

 tuna (Carey and Olsen, 1982; Cayre 

 and Chabanne, 1986; Yonemori, 1982; 

 Holland et al, 1990a; Cayre, 1991; 

 Cayre and Marsac, 1993; Block et al., 

 1997; Brill et al., 1999; Marsac et al.^), 

 skipjack tuna (Yuen, 1970; Dizon et al., 

 1978; Levenez, 1982; Cayre and Cha- 

 banne, 1986; Cayre, 1991), bigeye tuna 

 (Holland et al, 1990a, 1992; Holland 

 and Sibert, 1994), and albacore (Laurs 

 et al., 1977). A number of small (35-46 

 cm) southern bluefin tuna (SBT) have 

 been tracked in the Indian Ocean off 



' Chen, S., A. Cowling, and T Polacheck. 

 1995. Data analysis of the aerial surveys 

 (1991-1995) for juvenile southern bluefin 

 tuna in the Great Austrahan Bight. 1995 

 Southern Bluefin Tuna Recruitment Moni- 

 toring Workshop Report RMWS/95/6, 57 p. 

 CSIRO Marine Laboratories, Castray Es- 

 planade, Hobart, Tasmania, Australia 7000. 



2 Gunn, J., T. Davis, T, Polacheck, A. Betle- 

 hem, and M. Sherlock. 1995. The appli- 

 cation of archival tags to study SBT migra- 

 tion, behaviour and physiology. Progress 

 Report — 1994—95. Southern bluefin tuna 

 recruitment monitoring and tagging pro- 

 gram workshop, 7-10 August 1995, Ho- 

 bart, Australia. Rep. RMWS/95/8, 17 p. 

 CSIRO Marine Laboratories, Castray Espla- 

 nade, Hobart, Tasmania, Australia 7000. 



3 Marsac, F., P. Cayre, and F. Conand. 1995. 

 Analysis of small scale movements of yel- 

 lowfin tuna around FADs using sonic tag- 

 ging. Expert Consultation on Indian Ocean 

 tunas. 6th session. Colombo, Sri Lanka, 

 25-29 September 1995. Rep. TWS/95/2/ 

 10, 20 p. ORSTROM, Seychelles Fish- 

 ing Authority, BP 570, Victoria, Mahe, 

 Seychelles. 



