Davis and Stanley: Movemenis of Thunnus maccoyli in the Great Australian Bight 



449 



Western Australia (Fishery Agency of Japan''"''). 

 All SBT in the current study were released and 

 tracked in shelf waters, except for one fish that 

 moved from the shelf into slope waters. 



Methods 



Tracking experiments on SBT were carried 

 out in the Great Australian Bight over three 

 years. In 1992, the tracking system was tested 

 on a short cruise late in the fishing season and 

 close to Port Lincoln. In 1993, the strategy was 

 to obtain extended tracks of up to 48 hours to 

 investigate day and night patterns in depth dis- 

 tribution and horizontal movement within the 

 Great Australian Bight. In 1994, we planned 

 short tracks to maximize the number of individ- 

 ual fish observed during daytime, so that their 

 behavior could be linked to sightings by aerial 

 survey carried out at the same time. 



All SBT were caught by pole and line, except 

 for tuna no. 7 (referred to simply as "tuna 7") 

 which was caught on light game fishing tackle, 

 and tuna 9, which swallowed the tag while 

 free-swimming. The tuna were landed on a 

 wet, plastic-covered, foam mat. The head was 

 covered with a wet cloth, length of fish was 

 measured, the tag attached or inserted, and the 

 fish was released back into the school. 



V16 (16 mm diameter) sonic tags (VEMCO, 

 Halifax, Nova Scotia, Canada) of various fre- 

 quencies (50-69 kHz), equipped with pressure 

 sensors, were deployed on two fish in April 1992 

 and four fish in January and March 1993 (Table 

 1 ). Five of these tags were attached externally, 

 behind the second dorsal fin of the tuna, by us- 

 ing the attachment technique described in Hol- 

 land et al. (1985). The sixth was placed in the 

 stomach with an oesophageal catheter. 



V22 (22 mm diameter) tags transmitting 

 on 40 kHz were used on ten tuna tracked in 

 January and February 1994. These tags, which 

 have stronger signals and lower transmission 

 frequencies, have a much larger detection range 

 than V16 tags. They also have the capacity for 



* Fishery Agency of Japan. 1990. Report of 1988 

 research cruise of the RA' Shoyo-Marii. Distribution 

 of juvenile southern bluefin tuna off west coast of 

 Australia, December 1988-January 1989. Rep. Res. 

 Dep., Fish. Agency Jpn. 63:1-125. Pelagic Fish 

 Resource Division, 5-7-1 Orido. Shimizu, Shizuoka 

 424, Japan. 



5 Fishery Agency of Japan. 1992. Report of 1989 

 research cruise of the R/V Shoyo-Maru. Distribu- 

 tion of juvenile southern bluefin tuna off west coast 

 of Australia, September-December 1989. Rep. Res. 

 Dep., Fish. Agency Jpn. 64:1-166. Pelagic Fish 

 Resource Division, 5-7-1 Orido, Shimizu, Shizuoka 

 424, Japan. 



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