Pollard et al : Chemical marking of Pagrus auratus 



119 



Australia and New Zealand (Battaglene and Bell, 

 1991; O'Sullivan, 1992; Treadwell et al.M. Commer- 

 cial catch rates of snapper have been on the decline 

 in many areas (Paul, 1982; Gilbert, 1986), and one 

 means of increasing natural populations is by reseed- 

 ing the sea with hatchery-reared larvae or juveniles. 

 Pagrus auratus is one of the targets for this approach 

 in both the northern (Smith and Hataya, 1982; 

 Ishibasi, 1986; Matsuda, 1992) and southern (Smith 

 and Francis") hemispheres. Tagging techniques are 

 required to measure the success of such stock en- 

 hancement measures. Very large numbers of small ju- 

 veniles must be tagged in these studies, and isotope 

 tagging may be one of the more suitable techniques. 



Tagging techniques have been extensively used for 

 research on Pagrus auratus. Studies involving tag- 

 ging have been used to investigate migration 

 (Crossland, 1976; Jones, 1981; Sakamoto, 1984; Kato 

 et al., 1991), the identification of separate stocks 

 (Sanders, 1974; Moran, 1987), growth rates (Sand- 

 ers and Powell, 1979; Francis and Winstanley, 1989; 

 Tsukamoto et al., 1989; Kato et al., 1991), fishery 

 exploitation and resilience (Crossland, 1980), and the 

 validation of aging with oxytetracycline (Ferrell et 

 al., 1992; Francis et al., 1992). With the exception of 

 fluorochrome markers, such as oxj^etracycline, many 

 conventional tagging techniques are, however, un- 

 suitable for tagging small juveniles or are too en- 

 ergy intensive or expensive for the tagging of large 

 numbers offish. 



The objective of our study was to determine 

 whether juvenile snapper (Pagrus auratus) could be 

 reliably chemically tagged by the incorporation of 

 strontium into the dorsal spines from immersion in 

 strontium chloride. The specific aims were as follows: 

 1) to investigate the persistence of chemical tags in 

 the dorsal spines of juvenile snapper following im- 

 mersion of the fish in strontium chloride; 2) to de- 

 termine minimum strontium concentrations and 

 immersion periods required for effective strontium 

 tagging; 3) to determine the feasibility of batch tag- 

 ging with different levels of strontium; and 4) to com- 

 pare the levels of strontium in tagged snapper with 

 naturally occurring levels in wild-caught snapper 

 from different estuaries. These multiple controls were 

 required to ensure that natural variation did not con- 

 found the identification of treated fish. 



Methods 



Experimental work was carried out at the New South 

 Wales (NSW) Fisheries Brackish Water Fish Culture 

 Research Station (BWFCRS), Port Stephens. The 

 experimental fish were hatched on 13 November 1993 

 from wild parent snapper caught off Broughton Is- 

 land, NSW. Rearing procedures and larval charac- 

 teristics may be found in Fukuhara (1985), Lopez 

 ( 1986), Pankhurst et al. ( 1991 ). Battaglene and Tal- 

 bot (1992), and Kingsford and Atkinson (1994). Ex- 

 perimental fish were kept in 2000-litre tanks on a 

 constant flowthrough system of biofiltered oceanic 

 water for the first three weeks, followed by biofiltered 

 estuarine water from Salamander Bay, Port Stephens. 

 From 14 January 1994, 770 juvenile snapper were 

 reserved for the experiments. 



The strontium salt used was strontium chloride 

 (SrCl,/6H,,0), a relatively cheap, nontoxic and non- 

 radioactive chemical. Strontium is found naturally 

 in seawater at an elemental concentration of approxi- 

 mately 8.1 mg/L (Home, 1969). For simplicity the 

 amounts of strontium chloride added to the saltwa- 

 ter experimental tanks are expressed as multiples 

 of this ambient strontium concentration, as follows: 



1) Sr2* five times ambient = 0.125 g/L of SrCl2-6H20; 



2) Sr^-^ ten times ambient = 0.250 g/L of SrC^-eHgO; 

 and 3) Sr'-* forty times ambient = 1.000"g/L of 

 SrCl,,-6H20. Preliminary experiments (Pollard-^) 

 showed increased levels of dorsal spine strontium in 

 juvenile snapper that had been immersed in stron- 

 tium chloride solution. 



Experiment 1 : persistence of strontium 



The aim of this experiment was to determine whether 

 the strontium marks induced in the dorsal spines of 

 juvenile snapper were persistent over at least a 36- 

 day period. The experiment was initiated on 22 

 March 1994 when the fish were 129 days old and 68 

 mm mean fork length (SD ±6.7 mm). The experiment 

 consisted of Sr^* five times ambient, Sr^* ten times 

 ambient, and a control, each with four replicate 100- 

 litre tanks containing eight fish each. Two fish were 

 sampled from each tank at zero, 12, 24, and 36 days 

 following the termination of the five-day exposure 

 period. For comparisons between sample times, treat- 

 ments, and tanks we used a partially hierarchical 

 three-way analysis of variance. 



1 Treadwell, R., L. McKelvie, and G. B. Maguire. 1992. Poten- 

 tial for Australian Aquaculture. Research Report 92.2 of the 

 Aust. Bureau of Agricultural and Resource Economics, 

 Canberra, Australia, 81 p. 



^ Smith, P. J., and M. P. Francis. 1991. Snapper reseeding in 

 the Hauraki Gulf: scientific considerations. N.Z. MAF Fish- 

 eries Internal Report 172, Wellington, New Zealand, 22 p. 



^ Pollard, M. J. 1994. Chemical marking of juvenile snapper 

 {Pagrus auratus: Sparidae) by the incorporation of iron and 

 strontium in the spines, scales and otoliths. B.Sc. Hons. the- 

 sis, School of Biological Sciences, Univ. Sydney, Sydney, Aus- 

 tralia, 119 p. 



