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



59 



Bayliff ( 1988) also investigated regional growth differenc- 

 es in Pacific skipjack and yellowfin tunas. Interpretation 

 as to whether any differences found are merely an artifact 

 of the data collection or procedures used or whether they 

 reflect real temporal or spatial difference has generally 

 not been possible because the basic data (e.g. tagging, hard 

 parts, length-frequency data), data collection procedures, 

 analytic approaches, and the areas and time periods from 

 which the data were collected have varied greatly among 

 studies. 



For southern bluefin tuna (SBT) (Thunnus maccoyii), 

 extensive juvenile tagging programs were conducted in 

 the 1960s and 1980s, and a large number of returns with 

 measured lengths were recovered. From both periods, 

 some returns were received after times at liberty in excess 

 of 10 years. These two sets of tagging experiments provide 

 the basis for the direct comparison of growth over a time 

 span of 30 years. Also, because of the large number of tags 

 returned in these studies, a more detailed examination of 

 the adequacy of the von Bertalanffy growth equation as a 

 model of the growth process is possible than with many 

 data sets. These tagging data (primarily those from the 

 1960s) have been used as a basis for a number of analyses 

 of growth rates (Murphy, 1977; Kirkwood. 1983; Hearn, 

 1986; Hampton, 1991; Lucas-). In the present paper, we 

 present results of the analyses of the growth increment 

 data from these two sets of tagging experiments. We ex- 

 amine these data both in terms of 1) whether SBT growth 

 differed between the tagging periods and 2) whether there 

 was a change in the growth process between adult and 

 juvenile SBT (i.e. whether a more complex model than the 

 simple von Bertalanffy equation is required to provide an 

 adequate description of SBT growth). 



The results presented here incorporate and build upon 

 the already cited published analyses of these tag-return 

 data, unpublished reports, and discussions of SBT growth 

 in scientific meetings on SBT (e.g. Hearn and Hampton'; 

 Hearn and Polacheck''; Anonymous^). 



- Lucas, C. 1974. Working paper on southern biuefin tuna pop- 

 ulation dynamics ICCAT ( Intenational Commission for the Con- 

 servation of Atlantic Tunas), SCRS/74/4. Collective Volume of 

 Scientific Papers, vol. HI, p. 110-124. [Available from CSIRO 

 Marine Laboratories, GPO Box 1538, Hobart, Tasmania 7001, 

 Australia.) 



3 Hearn, W. S. and J. Hampton. 1990. SBT growth change. 

 Ninth trilateral meeting on SBT, Hobart, Australia, September 

 1990, SBFWS/90/8. 19 p. (Available from CSIRO and the Com- 

 mission for Conservation of Southern Bluefin Tuna. P.O. Box 37, 

 Deakin West, ACT 2600, Australia.) 



■> Hearn, W. S., and T Polacheck. 1993. Estmiating SBT age- 

 at-length relations for the 1960s and 1980/90s. Twelfth trilat- 

 eral meeting on SBT, Hobart, Australia, October 1993. SBFWS/ 

 93/4, 21 p. [Available from CSIRO and the Commission for 

 Conservation of Southern Bluefin Tuna, P.O. Box 37, Deakin 

 West, ACT 2600, Australia.) 



■' Anonymous. 1994. Report of the southern bluefin tuna trilat- 

 eral workshop; Hobart, Australia, January/February 1994, 161 

 p. [Available from CSIRO and the Commission for the Conser- 

 vation of Southern Bluefin Tuna, P.O. Box 37, Deakin West, ACT 

 2600, Australia. [ 



Background: the SBT stock and fishery 



SBT is a highly-migratory species that begins to spawn at 

 about 10-12 years of age in waters south of Java during 

 the southern summer, mainly from September to April 

 (Farley and Davis, 1998). During the first year of life they 

 tend to be transported south by the tropical Leeuwin 

 Current to inshore waters between Perth and Esperance, 

 Western Australia. From ages 1 to 4 years, they appear 

 to mainly inhabit, at least in the summer months, the 

 waters off the Great Australian Bight, southern New 

 South Wales (NSW) and eastern Tasmania. Many move to 

 oceanic waters during the winter months and apparently 

 progressively so as they age. By five years of age almost all 

 have migrated to oceanic waters between 30° and 50°S at 

 all longitudes, but mostly in the Eastern Hemisphere. 



Substantial surface fisheries operated off the south 

 coast of Western Australia from 1969 to the mid 1980s, off 

 the south coast of NSW from 1963 to the early 1980s, and 

 off South Australia from 1964 to the present. Since 1959 

 a major Japanese longline fishery has operated in oceanic 

 waters between 30° and 50°S, mainly from the mid-Atlan- 

 tic and westwards to a few degrees west of New Zealand. 



Materials and methods 



Tagging programs 



Description Large numbers of tagged fish were released 

 by CSIRO staff in the period from 1959 to 1968 and again 

 in the period from 1980 to 1984. The releases from these 

 two periods are used in our present study. Most of the 

 tagged fish were initially caught with pole-and-Iine gear 

 with barbless hooks, although a relatively small number 

 were caught with troll lines. After a fish was hooked, it 

 was hauled aboard the vessel and placed on a measuring 

 board (in the 1960s) or a vinyl cradle (in the 1980s), and 

 its nose to caudal fork length was measured. The fish was 

 then tagged by an operator who inserted a 12-cm plastic 

 spaghetti dart tag into the fish about 4 cm to the rear of 

 the second dorsal fin on either side of the fish and rer- 

 eleased it into the water within about 30 seconds. After 

 1963 almost all fish were double tagged. The tag numbers 

 and length of each fish were recorded, together with loca- 

 tion and date of release. This information was later trans- 

 ferred to a computer database. 



Tagging operations in both the 1960s and 1980s were 

 concentrated in the nearshore, surface-water fisheries 

 bordering the central and western southern coast of Aus- 

 tralia and the southern coast of NSW. In the 1980s no tags 

 were released from the NSW coast area because this com- 

 ponent of the fishery had collapsed, and surface schools 

 of juvenile SBT could no longer be found (Caton, 1991). 

 The South Australian tagging took place in the Great 

 Australian Bight or in the adjacent shelf waters generally 

 between longitudes 128° and 136°E. Releases in Western 

 Australia occurred in the Albany (between longitudes 112° 

 and 119°E) and Esperance (between longitudes 119° and 

 125°E) areas. There were 33,309 juvenile SBT tagged by 



