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Fishery Bulletin 101(1) 



to one that is found primarily in more offshore and deeper 

 waters. In this regard, recent information from archival 

 tags indicates that SBT between 80 and 90 cm (about 

 two to three years old) commonly migrate during winter 

 months to offshore oceanic waters in the Indian Ocean and 

 the Tasman Sea and begin to feed at substantial depth 

 (Gunn and Block, 2001). In contrast, catches and samples 

 off Albany, Western Australia, show that many SBT less 

 than 70 cm stay in nearshore Australian waters during 

 winter (Hynd, 1965; Murphy"^; and release data analyzed 

 in this study). Thus, the growth changes estimated to be 

 near L* = 80 cm may correspond to a marked change in 

 the SBT behavior during these winter months. 



The von Bertalanffy growth equation and its modifica- 

 tions have been the standard for modeling tuna growth. 

 The life history dynamics for most tuna species (e.g. north 

 Pacific bluefin, albacore, bigeve, and yellowfin tuna) have 

 a bimodal component analogous to that of SBT. Thus, 

 juveniles are frequently found in densely packed surface 

 schools, whereas at larger sizes individuals are rarely 

 found near the surface and appear not to occur in densely 



" Murphy, G.I. 1979. Southern bluefin tuna. Au.st. CSIRO Div. 

 Fish. Oceanogr. Fishery Situation Report 1, 10 p. (Available 

 from CSIRO Marine Research, CPO Hox 1538 Hobart 7000, 

 Australia. I 



packed schools (although there is little direct information 

 on schooling for these larger fish). Moreover, mature tuna 

 expend considerable energy in the spawning process, and 

 in some cases swim thousands of kilometers and incur 

 considerable weight losses during spawning (Warashina 

 and Hisada, 1970). Bayliff et al. (1991) also found that 

 growth models with a rate discontinuity at a certain size 

 provided a better fit to Pacific northern bluefin tuna tag- 

 return data than a simple continuous growth model. The 

 extent to which this may be a general phenomenon in tuna 

 or other fish species with marked changes in habitat use 

 with age is not clear However, the results from our study 

 and those of Bayliff et al. ( 1991 ) suggest that a growth rate 

 with a discontinuity at a certain size may be more common 

 than existing modeling of growth may indicate. Complex 

 growth models, which deviated from a simple continuous 

 growth curve, have generally not been considered, and 

 the available data, in many cases, may not have sufficient 

 power to be able to statistically identify more complex 

 growth processes if they exist. 



Although the complex two-stage growth model used in 

 our study clearly provided a substantial and significant 

 improvement in fit to the growth-increment data, the mod- 

 el itself presents problems in terms of the biological inter- 

 pretation of the parameter estimates for L*. The bimodal 

 nature ol' the likelihood function means that the size and 



