Spatiotemporal structure of narrow-barred Spanish mackerel (Scomberomorus commerson) from the Red Sea and western Indian Ocean based on otolith micro-chemistry

Though the narrow-barred Spanish mackerel (Scomberomorus commerson) is considered to be migratory, the species is nevertheless thought to be locally overexploited in the northwest Indian Ocean. At the regional level, this local depletion is a major concern for food security. As the population structure and connectivity between sub-populations are poorly understood for this species, we examined the spatio-temporal dynamics of narrow-barred Spanish mackerel via elemental concentrations (P, Mg, Sr and Ba) along otolith transects using LA-ICPMS for samples from 6 sites: Egypt, Djibouti North and South, Somalia, Mozambique and South Africa. For homogeneous size class samples (70-90 cm), otolith chemical signatures immediately preceding capture were used to accurately group individuals sharing a spatial proximity and/or season of capture. Notable differences in otolith edge signatures were found among individuals from north and south of the equator and contrasting cluster compositions from nearby sites in the Gulf of Aden of individuals captured in summer versus winter. Otolith core chemistry identified two spawning chemical compositions. The first common composition was characterized by relatively high concentrations of Sr and lower concentrations of P, Ba and Mg. The second less common spawning chemical composition was particularly rich in P, Ba and Mg and corresponded primarily to individuals caught off Mozambique, Somalia and Djibouti. These results are broadly consistent on one hand with patterns of water mass circulation in the Red Sea and western Indian Ocean and on the other hand with the observed spawning seasons. Though further research using, for example, archival tagging is needed to clarify the mechanisms behind these patterns, these results reveal the potential of otolith chemistry to provide insights into the spatio-temporal dynamics of narrow-barred Spanish mackerel.