The study on the conservation and management of ayu, Plecoglossus altivelis, populations in the Kasumigaura Lake system, Ibaraki Prefecture, Japan.

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Ayu, Plecoglossus altivelis recently proliferated in the Kasumigaura Lake was studied. From the allozyme analysis the population was identified as an amphidromous type with the neighboring rivers populations but making slightly different sabcluster. By the electron microplobe analysis(EPMA), Sr/Ca ratio in otolith was measured, and was found that the population of the lake system did not have the stage in the coastal area. In the juvenile stage, nauplius of copepods with other zooplankton were found to be the main food item, but in the later stage in the lake, cyclopoids and then mysids dominated as the main food items. After entering flowing Koise River, Ayu shifted food item to benthic algae with some addition of aquatic insects. From the analysis of daily growth ring on the otolith, the modes of growth of individuals from both the lake system and the neighboring rivers were compared. Because of extreme lowering of the water temperature of the lake system in winter, the growth of Ayu was scanted and the stunted growth resulted making a transparent zone without daily rings on the otolith. But the stunted growth was compensated by the faster growth by warmer water temperature in late spring to early summer and the body size in summer was not less than that of the amphidromous neighboring river groups. Estimation of the spawning date from the daily rugs on the otolith was made using individuals collected by January, thus lacking transparent zone on the otolith, the spawning season was determined as October to November. The growth rates an tie were compared and was found that those after the wintering was higher than those before the wiring. Analysis of mitochondria and genomic DNA suggested the genetic diversity of the Ayu population of the lake system was similar with that of the neighboring river population and there was no trace of the bottle neck effect on the population.