Pollard et al.: Chemical marking of Pagrus auratus 



125 



incorporation. Their inability to identify strontium- 

 marked adults in following years was attributed en- 

 tirely to dilution caused by the growth of vertebrae, 

 rather than a process of leaching from the tissue. 

 Behrens-Yamada and Mulligan ( 1982 ) found that the 

 use of smaller vertebral core samples allowed the 

 identification of strontium-labelled adult salmon. The 

 limiting core diameter was related to the vertebral di- 

 ameter of the juvenile salmon at the time of treatment. 



Similarly, the growth of juvenile snapper spines 

 will cause a dilution of the strontium signal. The ratio 

 of marked to unmarked spine matrix will decrease 

 as the spine enlarges. Spine gi-owth occurs from the 

 base upwards (e.g. McFarlane and Beamish, 1987), 

 and the strontium mark may remain only in the up- 

 per regions as spine elongation and enlargement oc- 

 curs. Only the uppermost section of the spine, corre- 

 sponding to the length of the spine at the age of in- 

 corporation, should be sampled in large fish; this will 

 result in a proportional increase in the amount of 

 strontium in the sample. 



Papadopoulou et al. (1980) found that a range of 

 chemical constituents in the otoliths of the mackerel 

 Scomber japonicus colias decreased with age. Dilu- 

 tion from growth, dietary changes with age, and 

 otolith compositional changes were mentioned as 

 possible causes. Nevertheless, the duration of most 

 tagging experiments does not generally exceed a pe- 

 riod of several years, and growth or age-related prob- 

 lems are unlikely to be a major concern over such 

 time scales. The differences between the strontium- 

 tagged and wild fish are great enough for there to be 

 little confusion between the two groups if samples 

 are taken from the uppermost spine region and tag- 

 ging experiments are not designed to extend over the 

 very long term. 



Strontium in tagged and wild fish 



It was a major concern for our study that naturally 

 occurring variation in the levels of strontium in wild 

 fish might obscure the strontium signal in fish that 

 had been tagged by immersion in SrCl2. Adult snap- 

 per may undertake movements associated with feed- 

 ing or spawning (Crossland, 1976), extending the 

 range of water chemistries experienced by the fish. 

 Moreover, juveniles may remain resident in shallow 

 bays and estuaries that are subject to variable inputs 

 of fresh water (Kingsford and Suthers, 1994) and that 

 are likely to have differences in water chemistry. 



Despite the observed variation in natural levels of 

 strontium in wild snapper, the treatments with Sr 

 lOx for 5 days (Fig. 3) and Sr 40x for 24 to 48 hours 

 (Figs. 2 and 3) had unequivocally higher readings. 

 Other treatments also had higher readings. They 



were, however, not considered great enough for ef- 

 fective tagging owing to the increased variation in 

 spine strontium expected to result from growth fac- 

 tors and exposure to the variable marine environ- 

 ment. The minimum concentration recommended is 

 Sr^'^ lOx ambient, with an exposure period of at least 

 4-5 days at this concentration. Shorter exposure 

 periods will require concentrations greater than Sr 

 lOx, and snapper significantly larger than those used 

 in our study may also require higher treatment con- 

 centrations. Although the comparisons were made 

 with wild snapper captured within a period of six 

 months in the general region of Sydney across a range 

 of less than 40 km of coastline, it is unlikely that 

 fish tagged with strontium at this recommended level 

 would be confused with wild fish from any latitude 

 or time. 



It is not surprising to find differences in naturally 

 occurring strontium among fish from different estu- 

 aries. For example, Edmonds et al. (1989) investi- 

 gated eight elements, including strontium and found 

 elemental compositions to be specific to the geo- 

 graphical origin of nonmixing populations of snap- 

 per in Western Australia. Similarly, Port Jackson, 

 Botany Bay, and Port Hacking are distinct estuaries 

 which would also be expected to contain nonmixing 

 juvenile snapper populations. On the other hand, 

 Middle Harbour is within Port Jackson, and inter- 

 estingly no significant difference has been observed 

 between these two groups. 



A variety of factors influence natural variation in 

 the Sr content of otoliths. Salinity is perhaps the 

 strongest determinant, and analysis of strontium 

 levels has long been used to interpret the salinity 

 histories of anadromous or catadromous fish species 

 (Castonguay and Fitzgerald, 1982; Coutant and 

 Chen, 1993; Limburg, 1995; Secor et al, 1995; Pender 

 and Griffin, 1996). Decreasing water temperature 

 causes an increase in otolith Sr/Ca ratio, with the 

 greatest effect occurring at low temperatures. This 

 knowledge has been used in attempts to reconstruct 

 temperature histories of wild fish (Radtke and 

 Targett, 1984; Radtke et al., 1990; Townsend et al., 

 1989, 1992, 1995, Arai et al., 1996). It has been sug- 

 gested that low temperatures may impair physiologi- 

 cal mechanisms that exist in fish and inhibit the 

 uptake of strontium (Kalish 1989; Townsend et al., 

 1992). Other environmental and biological factors 

 that influence the natural markings of calcified struc- 

 tures include pH(Moreauetal., 1983;Wickins, 1984), 

 fish age and size (Papadopoulou et al., 1980; Gauldie 

 et al., 1995), developmental and reproductive events 

 (Francis, 1994; Kingsford and Atkinson, 1994), physi- 

 ology (Kalish, 1991), periods of stress (Townsend et 

 al., 1992), dietary differences (Edmonds et al., 1989; 



