58 



Abstract— Southern bluefin tuna( SBT) 

 ^Tluinnus maccoyit) growth rates are 

 estimated from tag-return data associ- 

 ated with two time periods, the 1960s 

 and 1980s. The traditional von Ber- 

 talanffy growth model (VBG) and a 

 two-phase VBG model were fitted to 

 the data by maximum likelihood. The 

 traditional VBG model did not provide 

 an adequate representation of growth 

 in SBT, and the two-phase VBG yielded 

 a significantly better fit. The results 

 indicated that significant change oc- 

 curs in the pattern of growth in rela- 

 tion to a VBG curve during the juvenile 

 stages of the SBT life cycle, which may 

 be related to the transition from a 

 tightly schooling fish that spends sub- 

 stantial time in near and surface shore 

 waters to one that is found primarily 

 in more offshore and deeper waters. 

 The results suggest that more complex 

 growth models should be considered for 

 other tunas and for other species that 

 show a marked change in habitat use 

 with age. The likelihood surface for the 

 two-phase VBG model was found to be 

 bimodal and some implications of this 

 are investigated. 



Significant and substantial differ- 

 ences were found in the growth for 

 fish spawned in the 1960s and in the 

 1980s, such that after age four there 

 is a difference of about one year in the 

 expected age of a fish of similar length 

 which persists over the size range for 

 which meaningful recapture data are 

 available. This difference may be a 

 density-dependent response as a con- 

 sequence of the marked reduction in 

 the SBT population. Given the key role 

 that estimates of growth have in most 

 stock assessments, the results indicate 

 that there is a need both for the regu- 

 lar monitoring of growth rates and for 

 provisions for changes in gi'owth over 

 time (possibly related to changes in 

 abundance) in the stock assessment 

 models used for SBT and other species. 



Estimating long-term growth-rate changes of 

 southern bluefin tuna (Thunnus maccoyii) 

 from two periods of tag-return data 



William S. Hearn 



CSIRO Marine Research 



Private Bag No. 5 



Wembley, Western Australia 6020 



Australia 



E-mail address, bill.hearnigimanne.csiro au 



Thomas Polacheck 



CSIRO Marine Research 

 GPO Box 1538 

 Hobart, Tasmania 7001 

 Australia 



Manuscript accepted 28 May 2002. 

 Fish. Bull 101:58-74(2003). 



Estimating growth rates has been 

 a major focus of fisheries research 

 throughout the twentieth century, and 

 a large body of literature exists on the 

 topic (e.g. Lee, 1912; Ford, 1933; Wal- 

 ford, 1946; Manzer and Taylor, 1947; 

 Allen, 1966; Yukinawa, 1970; Pitcher 

 and MacDonald, 1973; Kimura, 1980; 

 Fournier et al., 1990). This literature 

 reflects, at least in part, the funda- 

 mental importance of information on 

 growth rates in stock assessments 

 and the subsequent provision of man- 

 agement advice for commercially har- 

 vested fish populations. For example, 

 growth information is required for 

 yield-per-recruit analyses and for the 

 estimation of spawning stock biomass 

 in the estimation of stock-recruitment 

 relationships. In addition, for a number 

 of species, estimates of growth rates 

 have been the primary or only source 

 of information that can be used to esti- 

 mate the age of individual fish and the 

 age distribution of commercial catches 

 (particularly for tropical species and 

 for tunas and billfish). Such informa- 

 tion on age is a critical component 

 required in the analyses and models 

 used to assess and manage these fish 

 stocks (Bayliff 1991; Clay, 1991; Caton. 

 1991; Wild, 1994; Wild and Hampton, 

 1994; Polacheck etal.'). 



Almost all the work on modeling 

 growth has centered on modeling 

 growth rate as a continuous, smooth, 

 monotonically decreasing function of 



age, and the von Bertalanffy (1938) 

 growth (VBG) equation, and its ex- 

 tensions, have been the most common 

 approach used. In addition, the growth 

 process has frequently been modeled 

 as static. Temporal variations in aver- 

 age growth for fish of the same size, 

 or age, (due, for example, to changes 

 in the physical environment or popu- 

 lation density) are often ignored or 

 considered to be relatively minor (with 

 some notable exceptions — e.g. Le Cren, 

 1958; Southward, 1967; de Veen, 1976; 

 Toresen, 1990; Ross and Nelson, 1992; 

 Kaeriyama, 1996; Sinclair and Swain, 

 1996). 



For the large pelagic tunas and 

 billfishes, the von Bertalanffy growth 

 equation and extensions has been the 

 standard used for modeling growth 

 (Bayliff, 1980). For a variety of tuna 

 species, numerous growth studies have 

 been conducted, and generally a rea- 

 sonable range of parameter values has 

 been estimated (e.g. see the sets of pa- 

 rameter values estimates for the eight 

 scrombrid species in Bayliff, 1980). 



' Polacheck, T, A. Preece, A. Bctlchem, and 

 N. Klaer 1998. Treatment of data and 

 model uncertainties in the assessment of 

 southern bluefin tuna stocks. In Fish- 

 ery stock assessment models (F Funk et 

 al", eds.), p. 613-637. Alaska Sea Grant 

 College Program Report AK-SG-98-01. 

 Univ. Alaska, PO. Box 755040, Fairbanks, 

 AK 99775-5040. 



