Structure and composition of yellowtail kingfish (Seriola lalandi) otoliths: implications for using aquaculture-reared fish to reconstruct life histories.

Understanding fish life history is essential for effective management of fisheries, but continuous tracking over lifetime temporal scales can be difficult. Fish otoliths contain a natural biogeochemical record of ambient environmental conditions and habitat use over such scales. However, ecological interpretations of these elemental compositions can be influenced by the structural composition of calcium carbonate otoliths, which can vary between wild and aquaculture fish as well as across species. As such, we used in situ confocal micro-Raman spectroscopy and powder X-ray diffraction to assess and quantify, respectively, the calcium carbonate polymorphs of the otoliths of aquaculture and wild yellowtail kingfish (Seriola lalandi) to validate whether the otoliths from aquaculture fish can be used to interpret patterns seen in the otoliths of wild fish. We then analysed major and minor trace elements using wavelength dispersive X-ray spectroscopy to investigate the potential habitat use of wild S. lalandi. The elemental composition of S. lalandi otoliths was examined across three cross-sectioned growth zones: the first (i.e. oldest) annulus (growth ring), the outermost (i.e. most recent) annulus and mid-way between these. All otoliths were shown to be composed of aragonite, with the exception of small proportions (0.5 wt%) of vaterite in a single wild S. lalandi otolith. This finding indicates that otoliths of aquaculture S. lalandi, reared in known conditions, are appropriate to interpret stable isotope analyses in wild S. lalandi. Wild-caught S. lalandi otoliths had concentrations of barium and manganese below detectable limits (<0.2 and <0.01 wt%, respectively) inferring that S. lalandi were not in estuarine or brackish environments when the analysed annuli were formed, suggesting these environments are not used for spawning.