About the influence of the presence of gravel mixed in the digestive tract of Nagara River Ayu on the quality as a foodstuff

Abstract

　We investigated the sand and mud in the digestive tracy of natural Ayu (Plecoglossus altivelis) collected at six points of Nagara River and examined the degree of discomfort of sand by masticatory test. The sand and mud cause foreign substances in the oral cavity and bad taste. Gravel was recognized with a major axis of 0.28mm-0.48mm and was found of be difficult to chew with a major axis of 0.45mm-1.38mm. It is suggested that Ayu internal organs containing gravel larger than 0.5mm are not edible. More sand and mud were detected in the digestive tract of Ayu caught in the lower reaches (Sunomata area,Kagashima area,the vicinity of Aikawa Bridge and the vicinity of Godo Bridge) than in other areas. Especially near Aikawa Bridge and Godo Bridge, more than 50% of Ayu had gravel with a major axis of 0.5mm or more. Ayu in the lower reaches need to be cooked with the internal organs removed. The estuary and upstream Ayu,which are 85% free of gravel,may be cooked with their internal organs.