60 



Fishery Bulletin 101(1) 



CSIRO personnel during the 1960s (1959 to 1968) and 

 10,743 during the early 1980s (1980 to 1984). Of these fish, 

 1972 and 4280, respectively, were later recaptured. 



On recapture, fishermen recovered the tags and re- 

 corded the fish's length (if measured), location, and date. 

 The tags with the recorded information were returned to 

 the scientific staff at CSIRO, who then provided a reward. 

 Most of the recapture lengths were measured by fisher- 

 men or factory staff but about 31% were measured by 

 scientists. Those measured by scientists cannot be con- 

 sidered a representative sample. In particular, all of the 

 measurements for longer-term recaptured fish come from 

 fishermen aboard Japanese longline vessels. In addition to 

 length, longliners often reported the dressed weight and 

 sometimes the whole weight, or both, of recaptured fish. 

 In the 1960s Australian fishermen seldom reported any 

 weight measurements, but in the 1980s they commonly 

 reported the whole weight of recaptured fish. 



Data selection The tagging experiments were conducted 

 mainly within a narrow range of months at each site; 

 therefore returns within a few months would be most 

 strongly influenced by the seasonal differences found in 

 SBT growth (Hearn, 1986; Burgess et al., 1991; Leigh and 

 Hearn 2000). A nine-month period at liberty coincides 

 with a low frequency in the times at liberty for the experi- 

 ments; therefore we excluded data from analyses with 

 less than 270 days at liberty. We also excluded data for 

 which fish were tagged by fishermen, or when the recovery 

 length, year, or month were reported by the tag finder to 

 be unknown or uncertain. 



Previously reported weight-length relationships (Wara- 

 shina and Hisada, 1970; Hampton, 1986; Robins^) were 

 used to identify and screen out dubious recapture data. 

 The details of the screening procedures are documented 

 by Hearn" and Anonymous.-^ Longline recaptures were 

 excluded if the expected weight of a recaptured fish for its 

 reported length was either less than 2/3 of the reported 

 weight or greater than 1.5 times the reported weight. 

 Some of the major inconsistencies were thought to be due 

 to measuring the length of a fish without its tail or with- 

 out its head (Lucas^). For surface fish in the 1980s, a high 

 proportion of the weight-length data for recaptured fish 

 from four vessels was inconsistent with the weight-length 

 relationships noted above. All tag-return data from these 

 four vessels were excluded. Another 2.5% of the 1980 data 

 were excluded because of highly unlikely values for the ra- 

 tio of the reported weight to length of the recaptured fish. 



'" Robin.s, J. P. 1963. Synopsis of biolofjical data on .southern 

 bluefin tuna, Thiinnus ihvnnus maccoyn (Ca.stlcnaui 1872. 

 FAO Fisheries Report 6(2), p. 562-587. [Available from CSIRO 

 Marine Laboratories, GPO Box 1538, Hobart, Tasmania 7(X)1, 

 Australia.) 



" Hearn, W. S. 1982. Fish tagging: data processing, editing and 

 storage. In CSIRO data base for .southern bluefin tuna {Than- 

 nus maccoyii (Castlenau)) (J. Majkowski, ed.l. p. 8-9. CSIR(J 

 Marine Laboratories, Rep. 142. lAvailablc from CSIRO Marine 

 Laboratories, GPO Box 1538, Hobart, Tasmania 7001, Australia.] 



For the screening methods used, no assumption was 

 made about the underlying growth curve, and these meth- 

 ods were designed so that they would not induce a bias 

 into the results. The selection process yielded data sets 

 that were sufficiently large for valid analyses, being 730 

 and 1450 for the 1960s and 1980s data sets, respectively. 

 Note that for other tuna species the selection process used 

 in our study (particularly the deletion of recaptured fish 

 with short times-at-liberty) may cause problems because 

 of smaller data sets (e.g. skipjack and yellowfin tunas in 

 Bayliff, 1988). 



Experimental assumptions The use of the tag-return in- 

 crement data for estimating growth rates requires the fol- 

 lowing assumptions about the tagging protocols and data 

 collection procedures: 



1 Tagging does not retard growth. 



2 The tagged fish are uniquely and correctly recorded at 

 release and recapture. 



3 The lengths of fish are measured without bias at re- 

 lease and recapture. 



4 A wide range of fish sizes are represented, in recap- 

 tures at least. 



5 There are no significant size-selection processes for 

 fish within similar age ranges. 



With respect to tagging effects, Hampton (1986) and 

 Hearn (1986) have shown that there can be a significant 

 weight loss of 7-12% for tagged fish in the first month 

 after release. However, tagged fish recover this weight loss 

 within a year at liberty, and there is no apparent difference 

 between tagged and untagged fish after this time (Hearn, 

 1986). (There is little information available on weight loss 

 of tagged fish at liberty between one month and one year) 

 In terms of length, Hearn and Hampton' could not detect 

 a reduction of growth from growth increment residuals 

 in the tag-return data even within the first 30 days after 

 release. Limited data from the effect of handling and tag- 

 ging fish in commercial farm pens indicated no retarda- 

 tion in growth in length after 150 days. These farm fish 

 did show a loss in weight when first caged, but the weight 

 was regained over a period of a few months (Anonymous''); 

 therefore we do not think that tagging had any substan- 

 tial effect on the growth rate of tagged fished in our study. 

 With respect to the other assumptions, all fish were tagged 

 with uniquely numbered tags. During tagging operations, 

 tags were arranged in blocks of sequential numbers to 

 avoid confusion and the misrecording of tag numbers. 

 Return of the physical tag was required for fishermen to 

 obtain the reward, and the double tagging of almost all 

 fish since 1963 has allowed cross verification of tagging 

 numbers, which allows little scope for error in the record- 

 ing of tag numbers. Approximately 23*7^ of the length mea- 

 surements for the selected recaptured fish were measured 

 by scientists. Mainly due to the deletion of short-term 

 recaptured fish (i.e. < 270 days), this is less than that 

 for all data (31%). For the fishermen-measured lengths, 

 there was no reason to suspect any consistent bias, and 

 comparison of the residuals for fishermen- and scientist- 



