Abstract
Bluegill sunfish Lepomis macrochirus is an alien species introduced from North America in 1960, and currently widespread over Japan. It is designated as an invasive fish by which many domestic aquatic animals are threatened. Every effort to exterminate the populations of L. macrochirus has been continued, since 1998, in Mizorogaike Pond at northern Kyoto City, where the whole biological community has been protected as a natural monument of Japan. Capturing individuals (since 1998) and destroying spawning redds (since 2002) greatly decreased the population size in the pond: the estimated population size in 2005 was about one tenth of 1998. In that regard, we evaluate the changes in genetic structure caused by this rapid reduction of population size by using microsatellite polymorphisms. A microsatellite BG6X locus has shown that the genetic structure of the Mizorogaike population of 2006 was in Hardy-Weinberg equilibrium, but that of 2007 deviated from the equilibrium. Also, allelic richness decreased in 2007. According to the size frequency distribution, the specimens from Mizorogaike in 2006 were expected to have been born in the years of 2003 or earlier, while those of Mizorogaike in 2007 were expected to have been born in 2003-2005 when the destruction of spawning redds had been successfully performed. The results suggest that the majority of the Mizorogaike population in 2007 were originated from the small number of spawning redds that were escaped from the population control, and it caused the deviation from Hardy-Weinberg equilibrium.
