[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-14] Vertical habitat shifts of juvenile Japanese jack mackerel (Trachurus japonicus) estimated by otolith microchemistry

*Megumi Enomoto¹, Shin-ichi Ito¹, Motomitsu Takahashi², Chiyuki Sassa², Tomihiko Higuchi¹, Kotaro Shirai¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Fisheries Resource Institute, Japan Fisheries Research and Education Agency)

Keywords:otolith, otolith oxygen stable isotope ratio, Japanese jack mackerel, vertical habitat layer shift, early life history, East China Sea

Japanese jack mackerel (Trachurus japonicus) are distributed in the semi-demersal layer, while larvae are distributed in the surface layer in the East China Sea. Smooth vertical habitat layer shift (HLS) is important for their survival. However, details (start timing and duration) of HLS have been unclarified. To elucidate HLS, otolith oxygen stable isotope ratio (δ¹⁸O_oto), of which the value increases at lower temperature and higher salinity, was analyzed in high resolution by 5 days segment. From the history of δ¹⁸O_oto, HLS timing was estimated by two types of indices; HLS moment and HLS duration, which were identified by the largest δ¹⁸O_oto gap signal and gradual δ¹⁸O_oto change, respectively. In addition, daily ages and water temperature variances fish experienced were compared among three life history traits; the metamorphosis, the behavioral change from planktonic to self-swimming and the swimming ability increase. As a result, average HLS moment was estimated to be 42.2±18.2 daily age. HLS started at 28.0±11.0 daily age and average HLS duration was 27.9±16.2 days on average. The start of HLS duration corresponded to the start of metamorphosis, and development of swimming ability was happened simultaneously with HLS moment. From the δ¹⁸O_oto value of edge, observed temperature and salinity in the layer fish caught and the relationship between salinity and δ¹⁸O_sw (δ¹⁸O of sea water) from the previous study, an equation between δ¹⁸O (δ¹⁸O_oto − δ¹⁸O_sw) and temperature was estimated; Temperature(°C) = −3.51 (±0.49) × δ¹⁸O (‰) + 15.86 (±0.29). Temperature variance converted from δ¹⁸O_oto suggested that juveniles gradually shift habitat layer as they experience early life history traits, not suddenly after specific trait.