72 



Fishery Bulletin 101(1) 



between the 1960s and 1980s. For smaller sizes, the diver- 

 gence is substantially less (e.g. for fish 140 cm or less the 

 divergence is less than three months). In terms of using 

 the growth rate data to estimate ages from lengths, these 

 results indicate that for the older reproducing fish the re- 

 sults will be highly sensitive to assumptions about L^2- 



The results from these tagging studies clearly show that 

 growth rates for SBT hatched in the 1980s had increased 

 in relation to those cohorts hatched in the 1960s. The in- 

 crease in growth rates is substantial, so that a fish, on av- 

 erage, would have been expected to take four years to grow 

 from 55 cm to 111 cm in the 1960s, but only three years 

 to do so in the 1980s. In other words, after age 4 there is a 

 difference of about one year in the expected age of a fish of 

 similar length, and this difference persisted over the size 

 range for which meaningful recapture data were available. 

 The change in growth and its magnitude are consistent 

 with the analyses in Leigh and Hearn (2000) of the modes 

 in length-frequency distributions of juvenile fish captured 

 in the Australian surface fishery. The underlying causes of 

 the change in SBT growth rates are unknown. They could 

 be associated with changes in environmental conditions, 

 population size, or a combination of the two. The change in 

 SBT growth rates between the 1960s and 1980s is associ- 

 ated with very substantial declines in both the adult and 

 juvenile components of the SBT stock (Polacheck et al.'; 

 Anonymous^). 



There is an increasing number of examples in which 

 growth rates have been reported to be inversely corre- 

 lated to fish population numbers because of intraspecific 

 competition. For example, Le Cren (1958) documented an 

 increase in the growth rate of perch after a planned reduc- 

 tion of a lake population. In a converse case, Kaeriyama 

 (1996) reported a decline in the growth rate of Japanese 

 chum salmon following a many-fold increase in its popula- 

 tion size because of a most successful hatchery enhance- 

 ment scheme. Other accounts are published in Southward 

 (1967), de Veen (1976), Toresen (1990), Ross and Nelson 

 (1992), and Sinclair and Swain (1996). However, the re- 

 ports are mainly on species for which direct aging data are 

 reliable and regularly collected over a lengthy period, or 

 the fish are hatchery reared. 



The hypothesis that the increase in SBT growth rates 

 was the result of the marked reduction in SBT papulation 

 size would seem plausible, given the similar associations 

 that have been found in a number of fisheries phenom- 

 ena. As discussed in Leigh and Hearn (2000), changes in 

 juvenile SBT growth rates based on analyses of length- 

 frequency data are also consistent with the change having 

 a density-dependent component. In this regard, it is worth 

 noting that preliminary analyses of tag return data from 

 the 1990s indicate that growth rates in the 1990s were 

 similar to those in the 1980s (Polacheck and Preece"^). 

 Thus, these preliminary results are also consistent with 

 the change in growth being a density-dependent response 



as both juvenile and adult SBT abundances remained at 

 low levels during this period (e.g. Anonymous^; Polacheck 

 and Preece'°). Large uncertainty exists about possible 

 recovery of the SBT stock in the near future (e.g. Anony- 

 mous^), but continued monitoring of SBT gi'owth may 

 provide one indicator of stock recovery. 



To simplify our investigation we did not consider sea- 

 sonal growth. We avoided possible bias, due to seasonal 

 growth, by analyzing data only from fish with times at 

 liberty more than or equal to 270 days. This restriction 

 provided an efficient mechanism to focus on the long-term 

 growth process and was effective because the resultant 

 sets were large. Large numbers of recaptured fish with 

 reliable information and times at liberty more than 9 

 months seem rare for other tunas, in which case the added 

 complication of accounting for possible seasonable growth 

 would be required to ensure the robustness of the results. 



The analyses in this paper represent the first docu- 

 mented examples of substantial temporal changes in 

 growth rates that persisted for an extended portion of the 

 life span in a large pelagic tuna resource. For tuna stocks 

 in general, estimates of growth rates play a major role in 

 stock assessments and in the subsequent management 

 advice derived from these assessments. 



Acknowledgments 



We thank the many crew and scientific staff who par- 

 ticipated in the 1959-84 SBT tagging operations. We are 

 especially grateful for Australian and Japanese fishermen 

 who returned tags with information on recapture length. 

 The 1983-84 tagging program was financially supported 

 by an Australian Government grant. 



Literature cited 



Akaike, H. 



1974. A new look at the statistical model identification. 

 Institute of Electrical and Electronic Engineers Transac- 

 tions on Automatic Control, AC-19, p. 716-723. IEEE 

 Control Systems Society, New York, NY. 

 Allen, K. R. 



1966. A method of fitting growth curves of the von Berta- 

 lanfl'y type to observed data. J. Fish. Res. Board Can. 23: 

 163-179. 

 Bayliff.W. H 



1980. Synopsis of biological data on eight species of scorn- 

 brids. Inter-Am. Trop. Tuna Comm., Spec. Rep. 2 (W. H. 

 Bayliff, ed. ), 530 p. lATTC, San Diego. CA. 



1988. Growth of skipjack, Katsuwonus pelamis. and yellow- 

 fin, Thiinniis alhaccires. tunas in the eastern Pacific Ocean, 

 as estimated from tagging data. Inter-Am. Trop. Tuna 

 Comm. Bull. 19(41:311-385. 



■' Anonymous. 1998. Report of the 1998 Scientific Committee 

 meeting 3-6 August 1998, Tokyo, Japan. lAvailable from the 

 Commission for the ('onservalion of Southern Hluefin Tuna, PO 

 Box 37, Deakin West, ACT 2600, Australia.] 



'" Polacheck, T., and A. Preece. 1998. Preliminary comparisons 

 of the growth rates of southern bluefin tuna in the 1990s with 

 tho.sc in the 1960s and 1980s. Tenth SBT recruitment moni- 

 toring workshop, 14-17"' September 1998, Hobart. Australia. 

 RMWS/9H/5, 11 p. lAvailable from CSIRO and the Commis- 

 sion for the Conservation of Southern Bluefin Tuna, P.O. Box 

 37, Deakin West, ACT 2600, Australia.] 



