The distributional ecology of the ayu [Plecoglossus altivelis] in the marine life period
2005
Azuma, K.(Nishinihon Inst. of Technology, Nakamura, Kochi (Japan). Nakamura Office)
The ayu, Plecoglossus altivelis altivelis Temminck et Schlegel, an amphidromous osmerid fish with an annual life cycle, is distributed throughout the Japanese Archipelago. This fish is one of the most important species for fresh water fisheries and recreational fishing It spawns in the lower reaches of rivers in late autumn, Newly hatched larvae drift downstream to the sea, where they spend the winter months until ascending rivers as juveniles in spring. In the 1980's, Senta and Kinoshita (1985) found that ayu larvae and juveniles occurred abundantly in the surf zones of sandy beaches in western Japan. This discovery spurred the research on surf zones of sandy beaches as nurseries of ayu in various parts of Japan. In these studies, the nursery ground of ayu was confirmed to be shallow waters such as surf zones of sandy beaches, but the migration pattern of ayu larvae and juveniles between surf zones and offshore waters has not been revealed. Moreover little information has been gathered on the annual fluctuations of larval distribution in the sea. Such information is essential to encourage the propagation of ayu resources in the river. The present thesis deals with, 1) seasonal and annual changes of occurrence and developmental processes of ayu collected in surf zones of sandy beaches, 2) the larval migration of ayu in the sea (inshore migration to surf zone, diel movement in the surf zone, migration from surf zone to shallow waters, 3) annual changes of larval occurrence and stock fluctuation, and 4) the survival of released landlocked ayu in the sea using a biochemical genetic marker and the character of otolith daily ring.
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