[H-TT31] 環境トレーサビリティー手法の新展開

[HTT31-P11] 大槌域における河川水と淡水型イトヨのストロンチウム同位体比：現在の生息域と過去の移動の推定法として

*札本 果¹、申 基澈²、中野 孝教²、森 誠一³、陀安 一郎² (1.京都大学大学院理学研究科、2.総合地球環境学研究所、3.岐阜経済大学)

All living organisms have various interactions with surrounding organisms and their physical environment. For conservation and management purposes, it is important to understand the environmental condition not only in the present but also in the past. Strontium isotope ratio (⁸⁷Sr/⁸⁶Sr), often measured in otoliths of a fish, has been used as an indicator to estimate current habitat range and past movement of the fish. The method is based on two assumptions. 1, ⁸⁷Sr/⁸⁶Sr of a fish is almost the same as that of the ambient water. 2, ⁸⁷Sr/⁸⁶Sr of river water is characterized by the underlying bedrock geology. If there are significant differences among ⁸⁷Sr/⁸⁶Sr of the habitats in the study area, it is possible to estimate the past movement of a fish. However, the method cannot be used when there are no significant differences within the area. Wide study area potentially provides high probability that there are significant differences of ⁸⁷Sr/⁸⁶Sr among the habitats, so that most studies using this method subjected fishes that move widely. In this study, we propose to develop the ⁸⁷Sr/⁸⁶Sr method to reconstruct the movements of a small fish, threespine stickleback (Gasterosteus aculeatus). Japanese marine threespine sticklebacks can be classified into two genetically divergent groups, the Pacific Ocean anadromous form and the Japan Sea anadromous form. In Otsuchi area, Iwate Prefecture, both the Pacific resident freshwater form and the Japan Sea anadromous form of threespine sticklebacks occur, and there are many freshwater habitats including new coastal habitats that were made by a natural disaster of tsunami in 2011. For conservation and ecological understanding of threespine sticklebacks, it is important to know the past movement of the individual fish.
In this study, we analyzed ⁸⁷Sr/⁸⁶Sr of back bones of the freshwater form threespine sticklebacks and compared with that of ambient water in order to estimate the current habitat range and the past movement. Results and discussions are shown as follows.

1. ⁸⁷Sr/⁸⁶Sr of ambient water can be grouped into three regions (more than 0.709156, 0.708165~0.709156, and less than 0.708165). The result suggests that there are significant differences among the habitat of threespine sticklebacks in Otsuchi area.
2. There was a significant correlation between 87Sr/86Sr of threespine sticklebacks and that of environmental water in each sampling point. The result indicates that Sr of threespine sticklebacks mainly derived from the environmental water.
3. ⁸⁷Sr/⁸⁶Sr of threespine sticklebacks can be grouped into three regions as well as the ambient water. The result suggests that threespine sticklebacks moved only within each region.
4. There were significant differences between ⁸⁷Sr/⁸⁶Sr of threespine sticklebacks and that of the ambient water in each sampling point. The result implies that threespine sticklebacks moved among sampling points within each region. However, the detected differences were small, and the temporal changes of water ⁸⁷Sr/⁸⁶Sr need to be studied.
5. In the coastal area, ⁸⁷Sr/⁸⁶Sr of threespine sticklebacks was higher than that of the ambient water, probably because the water was affected by the sea water (⁸⁷Sr/⁸⁶Sr = 0.70918) at high tide. However, there was a significant positive correlation between them. The result suggests that although these sampling points were expected to have connected with each other at the flood, threespine sticklebacks did not move among the points frequently.