464 



Fishery Bulletin 100(3) 



(Holland et al., 1990b; Holts and Bedford, 1990; Block et 

 al., 1992a). Black niarlin dive after release but usually 

 return to normal depths soon after (Pepperell and Davis, 

 1999). High, sustained, swimming speeds for up to the 

 first six hours of blue marlin tracks were attributed to 

 increased ram ventilation to reduce anaerobic debt (Block 

 et al., 1992b). Undoubtedly there is trauma associated 

 with capture and tagging, but pole-and-line capture is 

 fast, and SBT are not as tired as they would be if caught 

 on a rod and line. In most of the SBT tracks, swimming 

 speed was much slower in the half hour after release 

 than for the remainder of the track. Possibly this slow 

 swimming relates to a recovery phase. However, it more 

 likely reflects the nature of pole-and-line fishing and the 

 association of the chummed school with the boat. After 

 catching and releasing the fish, chumming is stopped. 

 However, the school usually remains with the boat for a 

 short period. The strong schooling behavior of SBT ap- 

 pears to quickly override trauma-induced behavior, such 

 as remaining at depth, that might keep tagged fish away 

 from the school. 



Association of the tracked fish with schools is probably 

 the most encouraging sign to the field worker that the 

 tuna is behaving normally, or at least that its movements 

 are representative of the school as a whole. This associa- 

 tion was marked in our study. However, on the occasions 

 when the tracked fish appeared to be alone, it was not clear 

 whether this isolation was due to the normal breakup and 

 dispersal of the schools aggregated by the chumming op- 

 eration or to the tagging and tracking procedure. We were 

 concerned that "herding" (the vessel following the fish in- 

 fluencing the direction of its movements) over a period of 

 time might separate a fish from its school. However, it is 

 also not clear whether it is possible to detect if the tracked 

 fish is solitary or with a school. Depending on the skill and 

 knowledge of the obsei-ver, schools can usually be detected 

 in good weather if there is surface or subsurface activ- 

 ity. At night or in rough seas, the usual way of detecting 

 schools is by echo-sounder, but this works only if the school 

 is under the tracking vessel. The school must therefore be 

 large and extend well behind the tracked fish (which is 

 nominally tracked at a distance of 400 m). Often "solitary" 

 tracked tuna displayed the repeated and predictable be- 

 haviors seen in other SBT tracks and generally accepted 

 as criteria of "normal" behavior, the most striking of which 

 are the predawn and postdusk dives. 



Three methods of tag attachment were used in our 

 study — five tags were attached externally behind the sec- 

 ond dorsal fin. ten were placed internally in the stomach, 

 and one tag was accidentally struck by a free-swimming 

 tuna and was attached by the force of the strike. The 

 effects of attaching tags are not currently quantifiable 

 because of the diversity of tuna behaviors. However, the 

 tracks of SBT with stomach tags were more dynamic 

 than the tracks of tuna with tags attached externally. 

 Tunas with stomach tags seemed more responsive to 

 their environment: their behavior included fast, sustained 

 swimming, large translocations, and these tuna were 

 frequently observed with other tuna attracted by chum- 

 ming operations. Externally attached tags presumably 



created physical disturbances, such as drag, turbulence, 

 and attachment wounds to which the tuna had to adjust. 

 These are more likely to cause short-term disturbance 

 in behavior than swallowed tags. However, stomach tags 

 had unpredictable, and often short, retention times, which 

 counteracted their advantages. 



Acknowledgments 



We thank Barry Bruce, Jeff Cordell, Jessica Farley, Ran- 

 dall Gray, Lindsay MacDonald, and Matt Sherlock for 

 helping with tracking operations. We very much appreci- 

 ate the efforts of the captains of the tracking vessels, Stan 

 Lukin, Rex Hall, Stuart Richey, and their crews in the field 

 program. The filtering program was originally developed 

 by Miroslav Ryba and subsequently modified for tempera- 

 ture filtering by Andrew Betlehem. We thank John Gunn 

 and Kim Holland for their reviews of the manuscript and 

 Vivian Mawson for editing. This research was part of a 

 joint Australian Commonwealth Scientific and Industrial 

 Research Organisation/Japanese National Research Insti- 

 tute for Far Seas Fisheries recruitment monitoring project 

 funded by the Australian Fisheries Management Author- 

 ity, Japan Marine Fishery Resources Research Center and 

 Japan Fisheries Agency. 



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