25 



Abstract.-From 1990 to 1996, during 

 a large-scale tag-and-release program 

 in the Great Australian Bight, 20,204 

 southern bluefin tuna (SBT), Thiinnus 

 maccoyii. were injected with strontium 

 chloride (SrCl,^). The objectives of the 

 marking experiment were to examine 

 the efficacy of SrClo as an otolith marker 

 and to determine the periodicity of 

 increment formation in SBT otoliths. 

 Nine-himdred and sixty-one Sr-injected 

 fish were recaptured and 616 otoliths 

 were sampled from these: the high level 

 of sampling success was attributable to 

 a major liaison effort throughout the 

 multinational SBT fishery. The same tag 

 return rates for fish that were tagged 

 and injected and for fish that were 

 tagged only, indicated that the injection of 

 strontium did not affect the survival rate 

 of tagged fish. Strontium marks were 

 detected with a Robinson detector or an 

 energy dispersive spectrometer ( EDS ) ( or 

 with both) linked to a scanning electron 

 microscope; 59 of the 67 otoliths from 

 injected fish examined had discernible 

 marks. The intensity of the strontium 

 mark and the dosage rates were linked; 

 a dosage of 100 mg Sr/kg fish weight 

 is recommended to ensure easy identi 

 fication of the strontium mark. Using the 

 strontium marks, we established that in 

 SBT with 1 to 6 increments in their 

 otoliths, one increment is laid down 

 per year at liberty. In the 59 marked 

 fish that were examined, there was 

 100'7f agreement between the ex- 

 pected and observed number of incre- 

 ments after marking. These results, and 

 the data fi-om two supplementary tag 

 returns ft-om unmarked fish recaptured 

 after long times at liberty, provide un- 

 ambiguous evidence that increments on 

 the otoliths of SBT are formed annually, to 

 at least the age of 13 years. In addition, a 

 recent study that used bomb-radiocarbon 

 levels to estimate ages of older SBT has 

 provided strong evidence that annual 

 increments are deposited in the sagittal 

 otoliths of SBT throughout life. 



Direct validation of annual increments in 

 the otoliths of juvenile southern bluefin tuna, 

 Thunnus maccoyii, by means of a 

 large-scale mark-recapture experiment 

 with strontium chloride 



Naomi P. Clear 

 John S. Gunn 

 Anthony J. Rees 



CSIRO Marine Research 



GPO Box 1538 Hobart, Tasmania, 7001, Australia 



Email address (for N P Gear) Naomi ClearaTnanneairoau 



Manuscript accepted 11 January 1999. 

 Fish. Bull. 98:25-40 (2000). 



Southern bluefin tuna (SBT), Thu?2- 

 nus maccoyii Castelnau, 1872, is a 

 large, long-lived, migratory, pelagic 

 fish with a circumglobal distribution 

 between 30°S and 50°S (Caton, 

 1991). Its only known spawning 

 ground is in the Indian Ocean south 

 of Java, between 7°S and 20°S (Caton, 

 1991). Since it was first exploited 

 in the 1950s the stock has declined 

 dramatically to between 16% and 

 25% of its initial level (SainsburyM. 

 The fishery is currently managed by 

 individual transferable quotas (ITQ) 

 and the total allowable catch (TAG) 

 is assessed each year. 



Vutual population analysis (VPA) 

 has been the main method of as- 

 sessing the condition of SBT stock 

 since 1980 (Murphy and Majkowski, 

 1981). The age structue of the pop- 

 ulation, a major input to VPA, has been 

 estimated by converting lengths and 

 weights to ages, using growth curves 

 derived fi-om tagging data (Hampton, 

 1991; Polacheck et al.^). lb reduce the 

 unmeasurable uncertainties that the 

 estimates introduce into VPA assess- 

 ments, a validated direct method for 

 determining age was required. 



In 1992 we began a study to de- 

 velop reUable techniques for deter- 

 mining ages of SBT. Validation of as- 

 signed ages is critical in age estimation 



studies (Beamish and McFarlane, 

 1983; Smith, 1992; Secor et al., 1995); 

 therefore a large-scale mark-recapture 

 experiment was initiated to provide 

 the basis for vahdating the age 

 estimates. From the validation study 

 we aimed to confirm the periodicity 

 of the zones that were counted on the 

 hard parts collected from SBT. The 

 overall objective of these two studies 

 was to develop a validated length-at- 

 age key for the entire size range of the 

 SBT population. We present details 

 of the mark-recapture experiment 

 and our evidence that increments in 

 otoliths are formed annually. 



1 Sainsbury, K. 1993. What is happening 

 to the southern bluefin stock? In W. White- 

 law and V. Mawson (eds.), Proceedings 

 of the inaugural southern bluefish tuna 

 science-industry-management workshop. Port 

 Lincoln, Australia, p. 5-19. Commonwealth 

 Scientific and Industrial PJesearch Organi- 

 zation (CSIRO I Marine Research, GPO Box 

 15,38 Hobart, 7001 Australia. 



2 Polacheck, T, K. Sainsbury. and N. Klaer. 

 1995. Assessment of the status of the 

 southern bluefin tuna stock using virtual 

 population analysis— 1995. Paper SBFWS/ 

 95/17. First scientific meeting of the Com- 

 mission for the Conservation of Southern 

 Bluefin Tuna (CCSBT). Shimizu, Japan, 70 

 p. Commonwealth Scientific and Industrial 

 Research Organization (CSIRO) Marine 

 Research, GPO Box 1538, Hobart, Tasmania, 

 7001, Australia. 



