MARKING SALMON FRY WITH STRONTIUM CHLORIDE SOLUTIONS

Chum (Oncorhynchus keta) and sockeye salmon (O. nerka) fry immersed in strontium chloride solutions for 24 h had greater amounts of strontium in their otoliths, vertebrae, and opercula than controls. Exposure to a hyperosmotic bath prior to marking, and dimethyl sulfoxide and heat during the immersion period, slightly increased the uptake of strontium in some cases. Hemisections of otoliths taken from treated fish possessed a highly visible ring of deposited strontium when viewed with backscattered electron microscopy. The peak concentration of strontium in a ring was often two orders of magnitude higher than that found in otoliths taken from control fish or in preexisting growth areas of a treated otolith. Treated fish that had been reared for 21 months in freshwater were easily identified when their otoliths were examined with backscattered electron microscopy or Wavelenth Dispersive Spectrometry. When marked fish weighed less than 15 g their otoliths were also distinguished from control specimens by using bulk Inductively Coupled Plasma Mass Spectrometry. This study showed that 24-h immersion in strontium chloride solutions produced easily detected and long-lived marks in Pacific salmon fry.