Otolith shape variation reveals novel stock structure of range-shifting Yellowtail Kingfish (Seriola lalandi lalandi) in Aotearoa New Zealand

Climate-driven range expansion is reshaping species distributions worldwide, altering population connectivity and complicating stock assessment in many marine taxa. In Aotearoa New Zealand, Yellowtail Kingfish (Seriola lalandi lalandi) have extended southward from their historically northern distribution, prompting renewed questions about stock boundaries and connectivity across the range. Here, we present the first comprehensive evaluation of sagittal otolith shape variation in this species and assess whether otolith morphology can serve as an indicator of ongoing range-expansion dynamics. We analysed otoliths from 112 individuals collected across five regions using wavelet-based contour descriptors and traditional shape indices to test for spatial structure and signatures of expansion. Wavelet-based contour descriptors differed significantly between Auckland West— the traditional home range of Yellowtail Kingfish— and all other sampled regions, which represent the species’ ongoing range expansion. Classification analyses further achieved the highest assignment success for Auckland West fish, underscoring the utility of otolith shape for identifying individuals from this historic northern range. Together, these findings demonstrate the value of otolith morphology as a tool for delineating stock structure and supporting adaptive fisheries management under accelerating climate-driven distributional shifts.