26 



Fishery Bulletin 98(1) 



Previous attempts to estimate SBT ages directly, 

 either did not attempt validation, or attempted it for 

 only a few age classes. Hynd (1965) used scales to 

 estimate ages of fish up to 80 cm fork length (FL) 

 but did not attempt to validate his age estimates. 

 Yukinawa (1970) counted up to eight rings on scales, 

 using marginal increment analysis, to show that the 

 rings form at the same time each year. Thorogood 

 (1987) used otoliths to estimate age in fish ft-om 

 42 to 167 cm FL and, using marginal increment 

 analysis, was able to show seasonal band formation 

 in what he called ages 2 to 4. Jenkins and Davis 

 (1990) examined microincrements in the otoliths of 

 SBT larvae between 3.5 and 12 mm standard length 

 (SL) collected ft-om the same cohort over consecutive 

 days. From these microincrements, they validated 

 daily increment formation and assigned approximate 

 ages of 7 to 18 days to their samples. 



In many age determination studies of other species 

 of tuna, tetracycline has been used in marking ex- 

 periments to validate daily increment formation in 

 wild and captive tunas: e.g. yellowfin tuna, Thunnus 

 albacares in the wild (Wild and Foreman, 1980; Wild 

 et al., 1995) and in captivity (Yamanaka'^); skipjack 

 tuna, Katsuwonus pel amis, in the wild (Wild et al., 

 1995); black skipjack tuna, Euthynnus lineatus, in 

 captivity (Wexler, 1993); and Atlantic bluefin tuna, 

 Thunnus thynnus, in the wild (Inter-Am. Trop. Tuna 

 Comm.-*). 



However, similar experiments using oxytetracycline 

 (OTC) as a marker in SBT in the 1980s were less 

 successful. In high proportion of OTC-injected fish, a 

 mark failed to show up in the otoliths (Gunn^). Given 

 this previous failure, and public health concerns over 

 the use of tetracycline (in the USA, the Federal Drug 

 Administration [US FDA] prohibits its use in wild 

 fisheries), we selected strontium chloride (SrCl2) as an 

 alternative marker. 



Strontium chloride is a nontoxic salt that occurs 

 naturally in sea water. It is a component of some foods 

 and is considered to be benign at the concentrations 

 used as a marking agent (Sax and Lewis, 1987). 

 Strontium is readily incorporated into the bloodstream 

 offish and, although not used previously on scombrids. 



^ Yamanaka. K. L. 1990. Age, growth and spawning of yellowfin 

 tuna in the southern Philippines. FAO. Indo-Pacif Tuna Dev. 

 Man. Prog. Working paper 90AVP/21. 87 p. 



" Inter-Am. Trop. Tuna Comni. 1982. Annual Rep. for 1981, .303 p. 



^Gunn.J. S. 1992. Progress report on .strontium chloride mark- 

 ing of SBT during 1990-92 CSIRO^JAMARC tagging programs. 

 Paper M\VS4A\T-3. Fourth workshop of the southern bluefin 

 tuna recruitment monitoring and tagging programs. Hobart, 

 Australia, 9 p. Commonwealth Scientific and Indu.strial Research 

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

 7001. Australia. 



it has been used successfiilly with other fish species 

 to mark vertebrae (by introduction into food or in 

 the surrounding water [Behrens et al., 1990]), and 

 otoliths (by immersion and injection [Brothers, 1990]). 

 Strontium is chemically and biologically similar to 

 calciuffi. Because calcium and strontium ions have 

 the same valency (2+) and a similar ionic radius (Ca, 

 0.099 nm; Sr, 0.113 nm), strontium readily substitutes 

 for calcium during deposition of calcium carbonate. 



The first two objectives of this study were 1) to 

 evaluate whether intramuscular injection of strontium 

 chloride resulted in effective and reliable marking 

 of otoliths, and 2) to determine if- strontium chloride 

 injections increased mortality and, hence, affected 

 recapture rates. 



If successful and benign marking was demonstrated, 

 we planned to use the strontium chloride marks to 

 verify the accuracy of direct aging techniques by 

 determining the periodicity of increment formation for 

 as many year classes of SBT as possible. 



Materials and methods 



Tagging and marking 



From 1990 to 1996, a total of 64,497 juvenile SBT in 

 the Great Australian Bight were tagged and released. 

 Of these, 20,204 tagged SBT were injected with SrCl2 

 (Table 1). All fish were double-tagged (in case of "tag 

 shedding"! (Williams, 1992): strontium-injected fish 

 were tagged with orange tags, fish that were not in- 

 jected were tagged with yellow tags. When both orange 

 and yellow tags were being deployed, an equal num- 

 ber of fish from targeted schools were chosen at ran- 

 dom for injecting or not injecting. The smallest fish 

 caught during the tagging program was 40 cm fork 

 length (FL); we did not tag and inject fish smaller 

 than this size because they were considered prere- 

 cruits, i.e. were not caught in the fishery. The lengths 

 of orange-tagged fish ranged ft-om 41 to 120 cm in FL. 

 The return rates of yellow-tagged and orange-tagged 

 fish were compared by using a chi-squared test to de- 

 termine if they were significantly different. 



The strontium chloride solution for injection was 

 prepared in the laboratory. A stock solution of 1 g 

 SrCl2.6H20/mL was made up by dissolving 1 kg of 

 analytical grade SrCl^.eH.jO crystals in 1 liter of 

 distilled water, resulting in a 0.21 g/mL solution of 

 Sr2+. The solution was buffered to pH 7.0 with KOH 

 and stored between 0°C and 4°C. 



For rapid injections into large numbers of fish, a 

 5-mL automatic vaccinator fitted with a 0.2-mm needle 

 was used. Flexible tubing connected the vaccinator to 

 a plastic storage container that was either worn as a 



