Recent Distributional Expansion of Florida Largemouth Bass in Japan

Largemouth bass (Micropterus salmoides), one of the most popular sport fish species in the United States, has been introduced into freshwater habitats around the world. In Japan, the northern subspecies of this species was introduced in a lake in 1925. This subspecies rapidly expanded its distribution area in the whole of Japan by illegal transplantation, and thereby led to serious damages to native aquatic organisms. In 1988, another subspecies, Florida subspecies, was also introduced into a reservoir lake (the Ikehara Reservoir) in central Japan. Establishing Florida subspecies in this lake was considered to be impossible because of its low water temperature in winter and the established population of the northern subspecies therein. However, establishment and interbreeding of the Florida subspecies with the northern subspecies were confirmed by the genetic research in 1996. Until 1990s, distribution of Florida subspecies in Japan had been limited to the Ikehara Reservoir and another reservoir connected with it. However, post 2000, Florida subspecies and their progeny have been also detected in other lakes and ponds in central Japan, including the Japan’s largest lake, Lake Biwa. Introductions of the Florida subspecies to the northern subspecies may lead to progeny with larger size and different ecological traits, which may thereby lead to greater damages to the aquatic organisms. Therefore, the distribution and ecology of Florida largemouth bass in Japan need to be investigated.

     In this study, in order to reveal the distribution of Florida subspecies and the genetic structure of the population wherein both the subspecies co-exist, largemouth bass from 28 lake or pond populations in the central Japan were collected by angling and analyzed by using mtDNA and two new nuclear DNA markers. The mtDNA haplotypes or nuclear alleles from the Florida subspecies were detected in 14 populations throughout the study area. Population structure analyses on three major lake populations where both the subspecies coexisted (the Ikehara Reservoir, the Nanairo Reservoir, and Lake Biwa) revealed that both the subspecies randomly mated, and frequency of the genes from the Florida subspecies had been constantly increasing there.  These results reveal that distributional areas of the Florida subspecies have been rapidly expanding in Japan, and that the northern subspecies population is being invaded by the Florida subspecies and is subsequently being replaced by the latter as a result of interbreeding. Hence, some positive selective advantages for ecological traits of the Florida subspecies probably exist in Japan’s freshwater habitats.