Ontogenetic vertical migration of grenadiers revealed by otolith microstructures and stable isotopic composition

Otolith δ18O and δ13C of six species grenadiers were analyzed to reconstruct the historical residing depths and metabolic activity. During the larval to juvenile stage, Spicomacrurus kuronumai, Hymenocephalus lethonemus, and Bathygadus nipponicus gradually migrated from the mixed layer to the thermocline downward over a vertical distance >140 m after which they moved within a narrow vertical range for the remaining life. The downward migration distance was less than 65 m during the larval stage of Hymenocephalus sp. and Coryphaenoides acrolepis, which showed a second descent period from the thermocline to deeper water as juveniles. Coryphaenoides marginatus stayed at the lower thermocline during the larval stage and the juveniles migrated downward in a relative short distance around 100 m and settled in deeper water (>600 m), followed by irregular movements over a vertical range of about 200 m during juvenile and adult stages. The otolith δ13C profile suggested that fishes (S. kuronumai, H. lethonemus, and B. nipponicus) with a longer migration distance had a higher metabolic rate in their early life-history stages than in the later stages. However, the metabolic rate did not vary for those fishes (H. sp., C. acrolepis and C. marginatus) living within a narrow vertical range during their larval to adult stages. The otolith microchemistry suggested that ontogenetic downward migration was an important strategy for grenadiers linking the life stages between pelagic larvae and benthic settlement. Furthermore, the migration timing and distance for the pelagic larvae varied between species and habitats. S. kuronumai, H. lethonemus, H. sp. and C. acrolepis might metamorphose and settle at the same time, B. nipponicus metamorphosed during migration and C. marginatus migrated as juveniles.