Chemical marking of juvenile snapper, Pagrus auratus (Sparidae), by incorporation of strontium into dorsal spines

The aim of this project was to investigate the use of strontium as a chemic al tag in the dorsal spines of the marine teleost Pagrus auratus that would allow the mass tagging of juvenile fish. Previous studies in which the inc orporation of strontium has been experimentally manipulated for the purpose s of marking have generally concentrated on freshwater and anadromous speci es. This is the first study to investigate the tagging of spines with stron tium, the removal of which is nondestructive. Inductively coupled plasma-ma ss spectrometry (ICP-MS) was used to measure isotopic concentrations. The d orsal spines of juvenile P. auratus that had been immersed in salt water co ntaining 0.125 g/L SrCl2. 6H(2)O (5 x ambient strontium) and 0.250 g/L (10 x ambient) for five days incorporated Sr-86 at levels greater than those in control fish. The strontium signal was persistent in spines for at least 3 6 days and showed no sign of decay during the experiment. No effects of the treatments on fish health or growth were detected. Short-term immersion ex periments (6 hours to 5 days) indicated that treatments of 10 x ambient or greater for 4-5 days were required to tag fish reliably with strontium. Nat ural levels of strontium in the spines of juveniles varied among locations separated by tens of kilometres along the coast of New South Wales. Natural variations in strontium concentrations were not great enough, however, to obscure the differences between tagged and wild fish. It was concluded that strontium immersion is a useful and relatively environmentally safe method of tagging large numbers of small fish.