Study on the early life history of ayu [Plecoglossus altivelis] in the Shimanto estuary, Japan
2005
Takahashi, I.(Takahashi Research Office of Freshwater Biology, Yasu, Kochi (Japan))
The ayu Plecoglossus altivelis altivelis, is an amphidromous fish with a life span of only one year. This is one of the most important species for both commercial fisheries and sport fishing in Japan. Hatching occurs about two weeks after spawning in the lower reaches of rivers in autumn and yolk-sac larvae drift down into the coastal waters, where they remain throughout the larval and early juvenile stages. In spring, juveniles ascend the rivers, where they feed attached algae on rocks and grow over the summer. They mature the following autumn, and spawn in the lower reaches of rivers after a downstream migration by adult. Much works has been on the ecology of the ayu in a river, but little is done about the early life history of ayu before 1980s. Studies on the early life history of ayu have progressed since the 1980s when the main nursery ground of the ayu was found to be in the shallow waters such as the surf zone. In recent years, some studies on larval and juvenile ayu proved us the information on growth, migration and so on forth in the coastal waters, However, the habitat, development, mortality and so forth in the early stages are not yet fully understood. Since 1985, I have conducted the survey of larval and juvenile ayu in the Shimanto Estuary, and found that a number of the larvae and juveniles of ayu remain in the estuary, and achieve higher growth rates. This indicates the importance of the estuary as a nursery ground for ayu. In the present study, I try to detail the early life history of ayu, especially for migration, mortality and development, which are still unknown, in the Shimanto Estuary, and clear the significance of the estuary for ayu.
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