Comparisons of strontium isotope ratios in the water and otoliths suggest narrow habitat ranges of freshwater three-spined sticklebacks in Otsuchi, Japan

Abstract Stable isotope analysis is useful in elucidating the flow of elements among organisms and environments. Understanding the movement of organisms is important for the conservation of ecosystems, and radiogenic strontium stable isotope ratios (⁸⁷Sr/⁸⁶Sr) are used as a tool to elucidate the relationship between fish and their habitat water and to investigate the movements and habitat ranges of fish. In this study, we compared ⁸⁷Sr/⁸⁶Sr ratios of whole otoliths of sticklebacks with those of ambient water to examine the habitat range of freshwater-resident ecotype of three-spined sticklebacks in the estuary of Otsuchi town, Iwate Prefecture, Japan. Using cluster analysis, we classified the sites in the Otsuchi water areas into four freshwater strontium isotope groups (SIGs) with non-overlapping ⁸⁷Sr/⁸⁶Sr ratios except for the tidal sites, which were identified based on ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations. Another cluster consisting only of the tidal sites and seawater was formed, distinct from that of freshwater. When ⁸⁷Sr/⁸⁶Sr ratio of the incoming freshwater was close to that of the tidal cluster, the influence of the tides on otolith classification was small. We calculated the range of ⁸⁷Sr/⁸⁶Sr ratios for each SIG from water ⁸⁷Sr/⁸⁶Sr ratios and compared otolith ⁸⁷Sr/⁸⁶Sr ratios with those ranges. All 17 sticklebacks in one tributary were classified into the same SIG as that of the water at the sampling site, suggesting a narrow habitat range, whereas six out of 24 or one out of 38 sticklebacks in two other habitats were classified into different SIGs from those of the water at the sampling sites, suggesting movement from other habitat groups. The remaining habitat, which was a tributary include the tidal zone, had a point of change in classification results between the upper (all 40 sticklebacks were classified into the same group as that of the water of sampling sties) and lower reaches (sticklebacks were classified into multiple SIGs), suggesting the presence of a movement barrier. Our study showed that dividing study area into SIGs based on water ⁸⁷Sr/⁸⁶Sr ratios and comparing ⁸⁷Sr/⁸⁶Sr ratios of otoliths with those of SIGs were useful to elucidate the fish habitat use.