36 



Fishery Bulletin 98(1 ) 



We counted six translucent zones on the otolith and 

 the Sr mark occurred between the third and fourth 

 zones (Fig. 7). 



For all otoliths examined, there was agreement 

 between the number of increments observed after 

 the strontium mark and the number of increments 

 expected, calculated from time at liberty. Thus, the 

 annual periodicity in formation of increments 2 to 6 

 was validated for the otoliths analyzed. Because we 

 were unable to tag young-of-the-year fish, in which the 

 first translucent zone on the sagitta had yet to form, 

 we could not determine when this translucent zone is 

 laid down and when the formation of the first increment 

 is completed. However, studies of daily microincrements 

 (Itoh and Tsuji, 1996; Rees et al.^) have calculated 

 that the approximate size at age 1 is 50 cm. We found 

 otoliths of 50-cm fish had one increment. 



Of the otolith increments counted, the first translu- 

 cent zone was typically the most difficult to measure. 

 The beginning of the first translucent zone occurred 

 between 2.2 and 3.2 mm fi-om the primordium along 

 the postrostral axis, the most commonly used axis for 

 analysis. Rees et al.^ found microincrements in this 

 region to be narrower than those deposited earlier, in- 

 dicating a period of slow growth of the fish. 



Additional validation of annual increment formation 

 from tagged fish at liberty for extended periods 



During the course of our experiment, two fish tagged 

 by CSIRO in the 1980s were recaptured and their 

 otoliths sampled. From lengths at first release of 45 

 cm and 82 cm FL, the fish had grown to 163 cm and 

 162 cm after being at liberty for 9 years, 7 months, and 

 10 years, 8 months, respectively. From the age-length 

 key developed by Gunn et al.** we calculated that the 

 45-cm fish tagged in 1983 was one year old when 

 tagged, whereas the 82-cm fish tagged in 1984 was 

 two years old. The ages at recapture of these two fish 

 were estimated fi-om transverse sections through the 

 primordium of the sagittal otoliths. Eleven increments 

 (opaque and translucent zones) were counted on the 

 otoliths from the fish released as a one-year-old and 

 caught 9.58 years later; 13 increments were counted 

 in the fish released as a two-year-old and recaptured 

 10.75 years later. 



' Rees,A.J.,J.S.Gunn,andN. P.Clear. 1996. Age determination 

 of juvenile southern bluefin tuna, Thitnnus maccoyii, based on 

 scanning electron microscopy of otolith microincrements. In 

 J. Gunn, N. Clear, T. Carter, J. Farley, A. Rees. and C. Stanley, 

 Appendix 1 : The direct estimation of age in .southern bluefin tuna. 

 Second scientific meeting of the Commissionfor Con.servation of 

 Southern Bluefin Tuna (CCSBT). Hobart, Australia. 26 August-,'5 

 September 1996, 22 p. Commonwealth Scientific and Industrial 

 Re.search Organisation (C^SIRO) Marine Research, GPO Box 1,5.38, 

 Hobart, Tasmania, 7001 Australia. 



Discussion 



Validation 



This study demonstrated that, in the sagittae of SBT, 

 the second through sixth increments, are deposited 

 annually. This validation is independent of when the 

 marked fish were tagged or recaptured. Because daily 

 age estimates have been used to demonstrate that 

 the first major increment in the sagitta forms in the 

 first year of life (Rees et al.^), the armual formation 

 of translucent zones appears to hold for the first six 

 increments in SBT sagittae — corresponding to fish up 

 to approximately 133 cm fork length. 



The close agreement between increment counts on 

 otoliths and the sum of age-at-tagging and time-at- 

 liberty for two fish tagged in the 1980s and recaptured 

 in the 1990s indicated that increment formation 

 continues to be annual in fish up to at least 13 years 

 old. Further evidence that increments in SBT sagittae 

 are formed annually throughout life has been provided 

 by a recent comparison of increment counts with age 

 estimates derived fi-om levels of bomb-radiocarbon in 

 the early growth zones of sagittae (Kalish et al. 1996). 

 This study reports close agreement between the two 

 methods of estimating age for fish up to 34 years old. 



Three sources of data — those fi-om our marking ex- 

 periment, the increment counts for two fish at liberty for 

 over a decade, and the bomb radiocarbon data — provide 

 strong evidence that seasonal changes in growth are 

 expressed as clearly identifiable annual increments in 

 the sagittal otoliths of SBT. These increments can be 

 used to estimate the age of individual fish at any point 

 in their lifespan. 



Prior to our studies, Yukinawa (1970, using scales) 

 and Thorogood (1987, using otohths) used marginal 

 increment analyses to demonstrate the annual check or 

 translucent band deposition in fish they considered to be 

 between 2 and 4 years old. Their results differ from ours 

 only in the identity of year classes; their two- to four- 

 year-olds correspond to our one- to three-year-olds. The 

 difference in scale readings derives from Hynd's (1965) 

 observation of two "checks" on the scales of new recruits 

 (approximately 50 cm FL) to the Western Australian 

 fishery. Interpretation of otolith microincrements (Itoh 

 and Tsuji, 1996; Rees et al.^) indicates that these 

 fish are only one year old. Unequivocal validation of 

 these estimates is not possible at this stage because 

 samples from prerecruits were not available to either 

 Hynd or Yukinawa and we were not able to tag and 

 mark prerecruit fish. In a number of other Thiinnus 

 species however, 50-60 cm fish were found to be 

 aroimd one year old (Uchiyama and Struhsaker, 1981; 

 Wild, 1986; Foreman, 1996) and our counts of otolith 

 microincrements and the data based on their inter- 



