Riverine longitudinal distribution of native ayu Plecoglossus altivelis altivelis in the Syubuto River, where no migration barriers are installed, in Hokkaido.

Abstract

In order to clarify the riverine longitudinal distribution of native ayu, especially how the upper limit is determined, the density survey by underwater line transect was carried out at 12 fixed points for 9 years from 2013 to 2021 in the Syubuto River where are no structures which obstruct upstream migration of ayu and stocking is not performed. In 2014, ayu was collected from the downstream, midstream and upstream areas for otolith analysis. Otolith Sr/Ca ratio for ayu (n＝52) was used to estimate the period of starting upstream migration in the river and relation between the period and settled position of ayu was examined. The estimated population of ayu in 9 years fluctuated from 46,000 to 1,320,000. The upper limit of the distribution and upper limit at density of 0.3 /m² (density at which all individuals can feed sufficiently) varied between 21 and 37 km and 4 and 37 km from the river mouth, respectively. The distance from the river mouth to the upper limit of distribution and to the upper limit of the density of 0.3 /m² extended with the increase in the estimated population of ayu. Estimating from the length of otolith after starting upstream migration, most of ayu which had been started upstream migration at early period settled in the upstream, but some of them also settled downstream. On the other hand, there were a lot of individuals which had been started upstream migration at late period in the downstream, but the individual who went upstream was admitted, too. These facts suggest that ayu is not simply moving upstream resulted from extrusion by the individuals recruited later in the season. In addition, the upper limit of distribution in 2018 when the population was the lowest was 10-15 km downstream compared to other years. It is presumed that ayu during upstream migration minimized the cost of migration if satisfactory feeding conditions were obtained.