Otolith chemistry reveals fine population structure and close affinity to the Pacific and Atlantic oceanic spawning grounds in the migratory southern blue whiting (Micromesistius australis australis)

Trace elemental signatures of otolith nuclei and edges were analysed by LA-ICP-MS in 514 specimens of the highly migratory oceanic fish southern blue whiting, Micromesistius australis australis, that were caught in their two distant spawning grounds located in the Southwest Atlantic (Patagonian Shelf around the Falkland Islands) and Southeast Pacific (shelf and slope of the southern Chile). The elemental fingerprints in the otolith edges, accumulated during the adult life of M. a. australis were spawning-site specific. Using just four elements (Li, Mg, Sr and Ba), the correct allocation of fish to their sampling areas situated in either Chilean or Falkland spawning grounds was about 90%, meaning the adult fish have a close affinity to their respective spawning grounds. Despite this, chemical concentrations in their otolith cores revealed two clusters of signatures, with the majority of Chilean fish (more than 80%) grouped into Cluster 1, which is characterized by lower Sr concentrations, and about 80% of Patagonian fish grouped into Cluster 2, which is characterized by higher Sr concentration. This most likely indicates that the Chilean spawning population consists of about four fifths of the native fish plus about one fifth of migrants from the Atlantic, and vice versa. Possible ways of the population interchange and migrations of Chilean and Patagonian fish were discussed contributing to identification of their stock status.