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



39 



in commercial fisheries. Strontium chloride, on the 

 other hand, is regarded as safe for human consumption 

 because it is a salt with a low order of toxicity (Sax and 

 Lewis, 1987). It is even used in toothpaste by some 

 manufacturers (e.g. "Sensodyne"). Strontium chloride, 

 unlike fluorescent markers such as oxytetracycline, is 

 not photosensitive. Neither the marking solution nor 

 the marked otoliths need to be stored in the dark, and 

 the mark does not fade with exposure to light or with 

 time. In our study, strontium marks were as evident 

 in fish that had been at liberty for long periods as in 

 fish recaptured soon after release. 



In summary, strontium chloride injection has proved 

 to be a very successful way to mark the otoliths of 

 southern bluefin tuna: 959f of those marked and re- 

 captured in this study had detectable Sr marks in sec- 

 tioned otoliths. This high "success rate," the harmless 

 nature of SrCl, to both fish and humans, the capacity 

 of EDS to positively identify the strontium mark, the 

 insensitivity of the strontium mark to light, and the 

 longevity of the strontium mark indicate that it should 

 be seriously considered by those interested in large- 

 scale marking experiments on commercial fishes. 



Acknowledgments 



The study was part of a tag-recapture project run by 

 the Commonwealth Scientific and Industrial Research 

 Organisation (CSIRO) and the Japan Marine Fishery 

 Resource Research Centre (JAMARC). The efforts of 

 K. Williams, D. Waddington, W. Whitelaw, T. Carter, 

 and the Australian Fisheries Management Authority 

 observers ensured the collection of adequate numbers 

 of marked otoliths. We acknowledge the valuable 

 contribution of W. Hearn, C. Proctor, and K. Williams, 

 who initiated and implemented the 1990-91 study to 

 assess the feasibility of injecting strontium chloride to 

 mark juvenile SBT which was partly financed by an 

 Environmental Research Grant from the Australian 

 Government Department of Primary Industries and 

 Energy. Our thanks also go to Sandy Morison, Craig 

 Proctor, and other anonymous reviewers who gave 

 their time to assess this manuscript and to Vivienne 

 Mawson for her editorial comments. 



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