2023 年 70 巻 1 号 p. 1-20
Gnathopogon caerulescens is an endangered species endemic to Lake Biwa that is suffering as a result of the deterioration of its spawning habitat (lakeside vegetation zone). To determine the current distribution of spawning sites along the coast of Lake Biwa, a field survey was conducted in 2020, mainly in the North Basin, following the previous published surveys conducted in 2019 in the South Basin. A total of 1,950 eggs (67 egg populations) were collected from the submerged roots of Salix trees at 180 points between May 6 and June 23. DNA analysis of 524 eggs, sampled approximately equally from all egg populations, identified 331 G. caerulescens eggs, which were found at 56 of the 67 points and distributed in 19 of the 28 areas investigated (the entire lakeshore was divided into 51 areas spanning 8 shore regions). Combined with the findings from the 2019 survey, the present analysis demonstrated that G. caerulescens eggs were distributed in 29 of the 41 surveyed areas and 7 of the 8 shore regions. A review of the published literature from five time points since 1940, however, revealed that the spawning sites were distributed across almost the entire lakeshore around 1940, but that the distribution area continued to decline considerably until the mid-1990s when spawning sites were found only in 17 of the 50 literature-verifiable areas and 5 of the 8 regions. These results verified that this species has experienced significant declines and recovery with respect to spawning site distribution over the last 80 years. The timing and degree of the fluctuations, however, differed across the eight regions, likely reflecting the history of land modifications and the original spawning levels in each region. Artificial water level control since 1992, thought to have resulted in a catch decrease, was not considered responsible for the decline in the distribution of spawning sites, since the latter had already occurred prior to the water level control.