Different growth patterns reveal the potential origins of two Pacific saury (Cololabis saira) groups in the northwest Pacific Ocean

Abstract

Identification of stock composition is essential for fish stock assessment and management. Previous studies have suggested that a mixed stock composition of Pacific saury Cololabis saira exists in the northwest Pacific Ocean, but the origins of the stock’s components are poorly understood. We analyze the otolith microstructure of Pacific saury collected in high-seas fishing grounds from June–November, 2016–2019, and estimate hatch date and reconstruct otolith growth trajectory before hyaline zone formation. Results confirm that Pacific saury recruiting to the high-seas fishing ground are mainly from a winter-spawned cohort, secondarily from an autumn-spawned cohort, but also with a small proportion from a spring-spawned cohort. Ages of medium (knob length 240–280 mm) and large (knob length > 280 mm) individuals are mainly 8–12 and 18–21 months, respectively. Two growth patterns are reported for individuals with different otolith growth axes in their early life stages. Each year, unsupervised random forest clustering analysis of otolith daily increment widths reveals two Pacific saury groups. One group, with younger age and a later hatching period, shows a consistently higher growth rate than the other, suggesting that the two groups have different hatch dates and locations. These findings permit more accurate prediction of Pacific saury recruitment, and improve the understanding of short-term stock fluctuations to enable more sustainable management of fisheries.