Spatial segregation, dispersion and migration in early stages of polar cod Boreogadus saida revealed by otolith chemistry

Arctic marine food webs are centered on polar cod (Boreogadus saida), a small, largely pelagic gadid, which movement and migration remain unclear, especially for the early life stages. The present study examined the otolith chemistry of juvenile polar cod from six oceanographic regions of the Arctic Ocean in order to document patterns of spatial segregation, dispersion and migration during the species early life. The freshwater winter refuge hypothesis, suggesting that polar cod larvae start to hatch in winter in freshwater-influenced regions but only later in the season in purely marine regions, was also tested. Five elemental ratios (Li/Ca, Mg/Ca, Mn/Ca, Sr/Ca and Ba/Ca) were analyzed by laser ablation inductively coupled plasma mass spectrometry in three otolith zones representing the egg, larval and juvenile stages. The concentration of each of the five elements at the edge of the otoliths, corresponding to incorporation shortly before capture, was significantly correlated with surface salinity and temperature at capture site and date. Otolith chemistry differed between juveniles from freshwater-influenced regions (Laptev Sea, Hudson Bay Amundsen Gulf) and those from purely marine regions (Lancaster Sound, Baffin Bay, Frobisher Bay), in agreement with dissolved concentrations of at least some of the target elements in the Arctic Ocean. Discriminant function analyses including all five elements provided valuable information on the species population structure and dispersion of early stages. The correspondence between otolith Mn/Ca, Ba/Ca and vertical profiles of dissolved Mn and Ba in the water column may reflect the ontogenetic vertical migration of juvenile polar cod in late-summer and fall.