Periodicity of strontium: Calcium across annuli further validates otolith-ageing for Atlantic bluefin tuna (Thunnus thynnus)

Fisheries assessments depend on accurate age assignments, particularly those for late maturing and moderately long-lived species. For Atlantic bluefin tuna (Thunnus thynnus), otolith-based ageing techniques have undergone tests for accuracy and precision to validate longevity and reproducibility of ageing metrics, but the periodicity of annular growth band formation remains untested. In this study, wavelength-dispersive X-ray spectrometry was used to measure strontium:calcium ratios across alternating translucent and opaque zones of the otolith. Time series analysis was subsequently employed to compare periodicity of otolith annulus formation to oscillations in measured otolith strontium:calcium (Sr:Ca). Under the assumption that Sr:Ca oscillations represented seasonal temperature changes, the hypothesized periodicity of 1 cycle per annulus was confirmed as the highest intensity periodicity, corroborating visual assignment of annuli as a proxy for annual age. Minor but significant sub-annular cycles (3 and 2 cycles per annulus) were also detected.