耳石Sr:Ca 比分析による栃木県那珂川に生息するサクラマスの回遊履歴の推定

抄録

The migratory histories of individual masu salmon Oncorhynchus masou collected from the Naka River, Tochigi, Japan were clarified from otolith strontium (Sr) and calcium (Ca) concentrations using X‒ray electron microprobe analysis. Mapping and line analysis indicated three migratory types of the species. In addition to the conventional types, such as freshwater resident and typical anadromous (spending one year in marine waters), a new migratory pattern of short marine term anadromous (remaining in marine waters for several months) was apparent. Detailed life histories of short marine term anadromous individuals were estimated from otolith Sr fluctuations and ageing, from both scale and otolith daily increments. Consequently, more than half of the Naka River population was estimated to migrate to a marine environment upon reaching a total length of 178‒252 mm (one year old) in December‒January, thereafter staying in marine waters for approximately five months, before returning to the river in May‒July at 345‒463 mm total length. These findings may help establish an enhancement method for this species as a recreational fishery stock. Future investigations, including tagging, verification of age evaluation methods, and gene analysis are necessary to confirm the existence of the short marine term anadromous form.