Abstract:
The chemical composition of otoliths (earbones) can provide valuable information about
stock structure and connectivity patterns among marine fish. For that, chemical signatures
must be sufficiently distinct to allow accurate classification of an unknown fish to their area
of origin. Here we have examined the suitability of otolith microchemistry as a tool to better
understand the spatial dynamics of skipjack tuna (Katsuwonus pelamis), a highly valuable
commercial species for which uncertainties remain regarding its stock structure in the Indian
Ocean. For this aim, we have compared the early life otolith chemical composition of young-
of-the-year (<6 months) skipjack tuna captured from the three main nursery areas of the
equatorial Indian Ocean (West, Central and East). Elemental (Li:Ca, Sr:Ca, Ba:Ca, Mg:Ca
and Mn:Ca) and stable isotopic (δ13C, δ18O) signatures were used, from individuals cap-
tured in 2018 and 2019. Otolith Sr:Ca, Ba:Ca, Mg:Ca and δ18O significantly differed among
fish from different nurseries, but, in general, the chemical signatures of the three nursery
areas largely overlapped. Multivariate analyses of otolith chemical signatures revealed low
geographic separation among Central and Eastern nurseries, achieving a maximum overall
random forest cross validated classification success of 51%. Cohort effect on otolith trace
element signatures was also detected, indicating that variations in chemical signatures
associated with seasonal changes in oceanographic conditions must be well understood,
particularly for species with several reproductive peaks throughout the year. Otolith micro-
chemistry in conjunction with other techniques (e.g., genetics, particle tracking) should be
further investigated to resolve skipjack stock structure, which will ultimately contribute to the
sustainable management of this stock in the Indian Ocean.
