Hearn and Polachek; Long-term growth rate changes in Thunnus maccoyii 



69 



SBT growth rates in the 1960s are estimated to be less 

 than those in the 1980s up to 144 cm (Fig, 4), Comparison 

 of the 1960s and 1980s expected growth curves over time 

 for a 55-cm fish are presented in Figure 5. In making this 

 comparison, we assumed that a 55-cm fish is approximately 

 one year of age (Anonymous"') and that size at age one did 

 not change between the 1960s and 1980s, as supported by 

 length-frequency data from these two time periods (Leigh 

 and Hearn, 2000; Anonymous''). Thus, Figure 5 can also be 

 considered as an estimate of the expected length-at-age 

 curve. Figure 5 indicates that the overall expected growth 

 was significantly faster in the 1980s than in the 1960s, so 

 that a fish of 55 cm or age 1 would take approximately four 

 years in the 1960s to achieve the same length that would 

 have been achieved in three years in the 1980s. 



A feature of the best-fitted estimated growth param- 

 eters is that the expected growth curves intersect at -170 

 cm, so that after age 13 a fish from the 1960s is estimated 

 to be larger than a fish from the 1980s. This crossover is 

 driven primarily by the difference in the estimates of L .;. 

 The standard log-likelihood test indicates a low probabil- 

 ity, P=0.01, that L,^., for the 1960s and 1980s are the same. 

 However, the analyses of the bootstrap estimates of L ,., 

 indicate that the estimates are bimodal, reflecting the bi- 

 modal distribution of L*. Random sampling from the boot- 



strap distributions for L.2 showed that in G.V/c of cases 

 the 1960s L„,2 estimate was less than the 1980s estimate. 

 For a two-sided test at the 5'7c significant level, at least 

 2.5'7f (and at most 97.5%) of the bootstrap samples would 

 have been expected to have the 1960s L ., less than that 

 of the 1980s to justify the hypothesis that the two L^., are 

 equal. Thus, based on the bootstrap results, the hypothesis 

 of equality cannot be rejected. Most of the 6.1% of cases 

 are associated with the 1960s L,,., less than 180 cm, which 

 are in turn associated with the upper mode of L* in Figure 

 3A, i,e, near L* = 91 cm. It is worth noting that only three 

 recapture lengths were greater than 170 cm. There are, 

 therefore, very minimal data for estimating growth rates 

 beyond 170 cm and for precisely estimating L^.,. 



Discussion 



The results in this study indicate that the traditional VBG 

 model does not provide an adequate representation of 

 growth in SBT. There appears to be a significant change in 

 the pattern of growth in relation to a VBG curve during the 

 juvenile stages of the SBT life cycle. This, in turn, may be 

 related to the transition from a tightly schooling fish that 

 spends substantial time in near and surface shore waters 



