Fish habitat study using radiocarbon of otoliths with stepwise dissolution technique

Full text: Fish habitats have been studies using instrument loggers tagged directly. Yet, their life-long migration histories are often difficult to reconstruct due to various obstacles including size of instruments and low recovery rates after catch and release. Chemical traces including radiocarbon, however, likely can overcome these problems to fill the gap of our scientific knowledge (eg., Larsen et al., 2017). Spatial variation of radiocarbon in North Pacific ocean is particularly large and hence we selected the area to demonstrate for proof of concept. Radiocarbon in the Pacific deep water depleted its values significantly since the water aged after isolation from the surface, thus depth information of water can be deduced. Surface variation of radiocarbon concentration can be found where different water masses meet. Off Japan is a such location where radiocarbon depleted Oyashio cold current (Δ¹⁴C= -50‰) and radiocarbon less depleted Kuroshio warm current (Δ¹⁴C= 50 to 70‰) is converged (Larsen et.al., 2017). Since otoliths of fish are recorded mainly by DIC in ambient seawater (eg., Kubota et al., 2015; Hane et al., 2020), analyzing the radiocarbon concentration for the calcium carbonate contained in the otoliths provides information of seawaters where the fish migrates. The otoliths of the western Pacific roughy (Hoplostethus japonicus) caught in Suruga Bay (Shizuoka prefecture) were treated by a stepwise dissolution method using phosphoric acid (Barr et al., 1993; Yokoyama et al., 2000), and carbon dioxide was extracted to make graphite. Each graphite samples converted from CO₂ extracted in step from the otoliths and analyzed using a single-stage accelerator mass spectrometer at the Atmosphere and Ocean Research Institute of the University of Tokyo (Yokoyama et al., 2019). The results show that radiocarbon concentrations in the otoliths are gradually decreased, indicating that the depth of habitat of the analyzed individuals gradually shifted from deep to shallow water.