Using Mg as a Proxy for Crystal Structure and Sr as an Indicator of Marine Growth in Vaterite and Aragonite Otoliths of Aquaculture Rainbow Trout

Otoliths of rainbow trout Oncorhynchus mykiss reared in marine cages were analyzed by laser ablation-inductively coupled plasma mass spectrometry to determine their Sr and Mg concentrations and by micro-Raman spectrometry to assess mineralogy. Analyses of transects from the core of the otolith to the growing edge (incorporating otolith material produced during freshwater hatchery and marine cage growth) revealed that these transects were composed of aragonite, vaterite, or both. The concentration of Sr varied significantly between mineral polymorphs and environments (hatchery or marine). In vaterite transects. Sr concentrations increased an average of 130 mu g/g between freshwater hatchery growth and marine growth. This increase represents a near doubling Of the Sr concentration in vaterite and made Sr a Suitable indicator of a change in the environment as reflected in vaterite transects. In aragonite transects. Sr concentrations increased an average of 1,330 mu g/g between hatchery and marine growth, which simplified the identification of the timing of marine transfer. The concentrations of Mg also varied significantly between mineral polymorphs, the difference in the average concentration between aragonite and vaterite being 1, 179 mu g/g. This allowed for the use of Mg as a proxy for mineralogy. While there was an increase in Mg concentration between hatchery and marine growth in some vaterite otoliths, the change was small compared with the change as a result Of mineralogy. The average aragonite-vaterite partition coefficients were 5.238 and 8.096 for Sr and 0.057 and 0.128 for Mg for hatchery and marine growth, respectively.