Estimating the identity of the whitebait collected by Ajiro-weir installed in the Uji River in the Heian period

Abstract

 During the Heian period to the Kamakura period, numerous tankas, or Japanese poems of thirty-one syllables, were composed regarding wicker works known as Ajiro-weir installed along the Uji River. Historical records suggest that during this period, Ajiro-weir was actively used for whitebait fishing, primarily conducted from late September to early February of the following year, serving as a supplier to the aristocracy. In present-day Omi region of Shiga Prefecture, whitebait refers to young ayu (Plecoglossus altivelis) caught in Lake Biwa before developing pigmentation on their body surface. Ayu, which are vulnerable in their larval stage, inhabit still waters such as lakes and are not typically found in the flowing waters of the Uji River. To estimate the identity of whitebait caught in Ajiro-weir, Sr/Ca analysis and daily growth analysis using otolith were conducted on upstream migrating ayu passing through the estuary barrage of the Yodo River. The findings indicated that some of these migratory fish hatched in rivers flowing into Lake Biwa and spent several dozen days in freshwater. Our result estimates that in the past, Ajiro fisheries likely targeted larvae of land-locked ayu that accidentally flowed out of Lake Biwa.