Vertical habitat shifts of juvenile jack mackerel estimated using otolith oxygen stable isotope

Abstract

Japanese jack mackerel (Trachurus japonicus) is distributed in the semi-demersal layer of the continental shelf of the East China Sea (ECS), whereas their larvae are distributed on the surface. Notably, a smooth vertical habitat layer shift (HLS) to a deeper layer during their juvenile stage is crucial for the survival of this species. However, the determination of the start and duration of the HLS and the factors controlling HLS are not yet optimized. To elucidate the timing of the HLS, we analyzed the otolith oxygen stable isotope ratio (δ18Ooto), which increases at lower temperatures and higher salinity (the conditions that the fish experience in the deeper layers of the ECS), at a high temporal resolution of minimum 5 days intervals. From the profile of the δ18Ooto in otoliths of juvenile T. japonicus in the demersal layer, the HLS timing was estimated using two different indices, namely, HLS moment and HLS duration, which represent the highest δ18Ooto gap signal and gradual δ18Ooto change, respectively. Consequently, the average HLS moment was estimated to be 35.1 ± 10.3 days post-hatching (dph). The HLS started at 23.1 ± 9.6 dph, and its duration lasted for 31.3 ± 17.2 days on average. Generalized additive model (GAM) analyses indicated that the standard length and growth conditions influenced the HLS timing, which suggested that energy demand might regulate the HLS timing. To determine the order of HLS and life-history events, the ages of T. japonicus were compared among HLS, metamorphosis and maximum growth rate. The results showed that the start of HLS duration, metamorphosis, the HLS moment, and maximum growth rate occurred in that sequence. The experienced change of δ18Ooto suggested that T. japonicus juveniles shift their habitat layer gradually, rather than rapidly at a specific life-history event.