Using otolith chemistry to reflect population structure of the Subantarctic myctophid Electrona carlsbergi in the Antarctic Circumpolar Current and Antarctic Slope Current off the South Shetland Islands

The large-scale circulation in the Southern Ocean features eastward flow in jets along fronts in the Antarctic Circumpolar Current (ACC), and westward flow in the Antarctic Slope Current (ASC) and the associated Antarctic Slope Front (ASF). These oceanographic features structure populations of Antarctic krill (Euphausia superba) and dominant mesopelagic myctophid Electrona antarctica. Subantarctic myctophids spawn in temperate regions but widely distribute in the Southern Ocean, implying connectivity influenced by oceanographic processes. Yet little is known about their population structure. We examined otolith chemistry of Electrona carlsbergi, a subantarctic species closely related to E. antarctica, collected as krill fishery bycatch off the South Shetland Islands (SSI), where the ACC adjoins the continental slope and the ASC flows along the shelf break. Otolith edge chemical fingerprints differentiated fish with lower MgCa^-1 and higher BaCa^-1 in the ASC. Otolith nucleus chemistry reflecting early life and first migration differed between the ACC and ASC in MgCa^-1, BaCa^-1 and LiCa^-1. Discrepancies in length, age and chemistry indicated a spatial discontinuity associated with the ASF, rather than random mixing. Strong chemical differentiation between the ACC and ASC suggests separate migration pathways involving large-scale circulation transport. We propose that spatiotemporal structuring over the life history could explain heterogeneity observed in the nucleus chemistry. This study highlights the potential of otolith elements as natural chemical tags for elucidating fish life history and population structure, and offers insights into how fish may respond to environmental change and evolve over geological time through chemical records preserved in fossil otoliths. The study further highlights the importance of local hydrography in modulating population structure of marine species in the Antarctic Peninsula, and even different regions of the whole Southern Ocean. We therefore recommend that the role of local hydrography in modulating distribution of marine species cannot be neglectable in exploring the stock hypothesis.