Mitochondrial DNA Analysis Reveals Urgent Conservation Needs for the Southernmost Population of Ayu (plecoglossus Altivelis)

We hypothesized that populations of ayu (Plecoglossus altivelis, an ecologically and commercially important fish) living in marginal habitats are under less favorable conditions, and are, thus, more vulnerable to extinction. To test this hypothesis, we analyzed the mitochondrial DNA control region of ayu collected from different localities, including central and marginal populations. The results showed that the ayu populations in southern marginal habitats (Tien Yen River in Vietnam; Yakugachi and Kawauchi Rivers in Amami-Oshima Island) displayed lower genetic diversity (haplotype diversity, h = 0.67; 0.54 and 0.80; nucleotide diversity, π = 0.006, 0.002, and 0.009, respectively) compared to the central populations. The genetic differences were highly significant between the southern marginal and the central populations (Fixation index (FST) values ranged from 0.412 to 0.543). However, ayu populations at the center of their distribution range (Shinano River in Honshu and Hamoji River in Sado Island) and the northern marginal habitats (Yoichi River in Hokkaido and Aonae River in Okushiri Island) exhibited high genetic diversity (h = 1 for all rivers; π = 0.024–0.029). The genetic differences between the populations in the central and northern marginal habitats were not significant (FST values ranged from 0.023 to 0.076). This study suggests that, in ayu, the central-marginal hypothesis is valid in the southern direction but invalid in the northern direction. Urgent conservation is needed for the genetically distinct populations in the degraded southern habitats with severely low genetic diversity.