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Fishery Bulletin 88(2). 1990 



applications, it is necessary that the data be analyzed 

 as a series of separate release sets rather than as a 

 pooled data set, and corresponding shedding-rate esti- 

 mates are thus required; (2) there are now reliable 

 recovery data for 10,416 double-tagged southern blue- 

 fin available for analysis (compared with the 1511 re- 

 coveries considered by Kirkwood 1981); and (3) models 

 are now available that utilize exact periods at liberty 

 rather than data aggregated by time period. Provided 

 accurate recapture times are available (as is the case 

 here), these new models are preferable because of their 

 greater ability to deal with low recovery numbers 

 towards the end of an experiment. 



Tagging data and tagging methods 



The tagging data used in this analysis consist of all 

 records received by 31 March 1987 of southern bluefin 

 that were originally double-tagged and that had accu- 

 rate recapture dates. Recovery dates were considered 

 accurate if at least the month of recapture was known 

 with certainty. In total, there were 10,416 recoveries 

 that met this criterion. Of these, there were 671 recov- 

 eries for which the month of recapture was known but 

 the exact day was uncertain; however in almost all of 

 these cases, the uncertainty was estimated to be no 

 more than +5 days. 



The data were grouped into eight double-tagging 

 experiments: (1) NSW releases 1963-70 (CSIRO*); 



'Tagged by staff of the Commonwealth Scientific and Industrial 

 Research Organization (CSIRO). 



(2) NSW releases 1963-70 (fishermen contracted by 

 CSIRO); (3) SA releases 1964-69 (CSIRO); (4) SA 

 releases 1977 (CSIRO); (5) WA releases 1963-67 

 (CSIRO); (6) WA releases 1970-78 (WA Dep. Fish.); 

 (7) WA 1983 (CSIRO); and (8) SA 1984 (CSIRO). This 

 classification of the data was made to ensure that, 

 within experiments, the geographical area, fish size, 

 tagging and fishing methods, and tagging personnel 

 were as similar as possible. 



Except for experiment 2, the primary method used 

 to catch fish for tagging was commercial bait and pole; 

 in experiment 2, trolling was used. In all experiments, 

 fork lengths of the fish selected for tagging were mea- 

 sured before tagging. For the first six experiments, this 

 was done by placing the fish on a measuring board. In 

 experiments 7 and 8, the fish were placed in a specially 

 designed vinyl cradle supported by a metal frame and 

 their fork lengths measured using gr-aduations marked 

 on the cradle. While the fish were restrained on the 

 measuring board or in the cradle, two numbered tuna 

 tags of a standard type (Williams 1982) were inserted 

 forward into the musculature at an angle of about 45°, 

 1-5 cm below either side of the posterior insertion of 

 the second dorsal fin. Ideally, the tag barb anchored 

 behind the second dorsal fin ray extensions or the 

 neural spines, and in experiments 7 and 8 greater effort 

 was made to achieve this than in earlier experiments. 



A summary of the fishing method, tagging method, 

 and the numbers of tuna released and recovered is 

 given for each experiment in Table 1. For ease of pre- 

 sentation, the numbers of recoveries in each experi- 

 ment are grouped by period at liberty in Table 2; as 

 noted above, exact times at liberty for each recovery 

 were used in the subsequent analyses. 



