Clear et a\: Validation of annual increments in otoliths of Thunnus maccoyii 



35 



Number 



of 

 samples 



2 Number of 



1 translucent zones 



after Sr mark 



Age at recapture 



(estimated from increment counts 



on wfiole otoliths) 



Figure 7 



The number of strontium-marked otoliths used to validate age estimates that were made by 

 counting mcrements on whole otoliths. The number of translucent zones observed after the stron- 

 tium mark is shown for each age class. For all otoliths analyzed, the number of translucent zones 

 counted after the Sr mark equalled the number that were expected from the period at liberty after 

 fish were tagged and injected with SrCl.,. 



position of translucent zones and for locating the stron- 

 tium mark, either on a section (on the Robinson detector), 

 or from the whole, etched otolith (with EDS). 



Early in the experiment we found that the Sr marks 

 in fish tagged and injected when they were 90 cm 

 PL or larger were consistently fainter than in smaller 

 fish. The concentration of Sr in the Sr marks of large 

 fish was also significantly lower than in smaller fish 

 caused, possibly, by loss of some of the solution during 

 injection. To overcome this, dosages for large fish were 

 increased in 1993 (Table 2), after which the bands 

 were markedly more intense and easier to detect. 



There was no apparent correlation between the time 

 at liberty (i.e. time between Sr injection and recap- 

 ture) and the intensity of the Sr marks; the longest 

 time at liberty for a fish from which otoliths were 

 analyzed for Sr was 1638 days. There was also no 

 correlation between the intensity of the Sr marks and 

 the delay between otolith recovery and analysis. Unlike 



tetracycline, which is photosensitive, the strontium 

 mark did not fade after exposure to light. 



Validation of annual increment formation from Sr marks 



The 59 fish in which Sr marks were located ranged 

 from 45 to 102 cm FL at the time of release, which 

 corresponds to estimated ages of 1 to 4 (Gunn et al.'^). 

 Times at liberty ranged fi-om 8 to 1638 days. The 

 oldest recaptured fish was estimated to be 6-i- years 

 old; it had been at liberty for 1242 days (over 3 years). 



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

 J. Johnstone. 1995. Age and growth of southern bluefin tuna. 

 Paper SBFWS/95/8. First scientific meeting of the Commission for 

 the Conservation of Southern Bluefin Tuna (CCSBT), Shimizu, 

 Japan, 37 p. Commonwealth Scientific and Industnal Research 

 Organization (CSIRO) Marine Research, GPO Box 1.538 Hobart, 

 7001 Australia. 



