Characteristics of the feeding behavior and relationship between the standard length (SL) and the size of scrape marks of ayu Plecoglossus altivelis altivelis were studied using 123 cultured specimens in aquaria. Feeding behavior was observed individually, 116 specimens (94%) have mouth laterality for feeding. Also, feeding behavior were found two types. The first was eating the algae directly from the shelter in 91 specimens (78%). The second left the shelter and ate the algae after changing direction back to the shelter (22%). One side of the lower jaw when feeding on the acrylic plate side was turned downward and the other side of the jaws was closing. On the scrape marks, 11 to 14 and 11 to 13 level lines were visible caused by the upper and lower jaws and the algae was not scraped between the upper and lower parts. The comb–like teeth groups are arranged as 13 to 14 rows and 12 to 14 rows on the upper and lower jaws, respectively. Therefore, it is suggested that they use almost all of the comb–like teeth groups except the back teeth row on the upper and lower jaws for feeding. The lengths of upper and lower jaws were found to have power regression with the SL, respectively. Also, the relationship between the SL and full width of the scrape marks (FWSM) was： FWSM = 0.3939SL^0.6767 (R^2= 0.8392, 64-212 mm SL).

Recently overbrowsing by fish in cultivation of the wakame Undaria pinnatifida has become problematic and the black scraper Thamnaconus modestus is suspected of the feeding damage for the cultured U. pinnatifida as well as a few herbivorous fishes and the Japanese black seabream Acanthopagrus schlegelii, but aspects on feeding behavior and bite scars of the species are extremely limited. We examined characteristics of bite scars left on thalli of the cultured U. pinnatifida by feedng of T. modestus in a tank experiment. The species showed active feeding behavor of the cultured U. pinnatifida and conspicuously consumed juveniles rather than adults. Bite marks left on blades were categorized into three shapes of arc, ellipse and serration and particularly the ellipse-shaped bite mark is characteristic of T. modestus, unknouwn for other overbrowsing fishes. Additionally cut surfaces of those bite marks were edged with irregular gentle concaves. The shape of bite marks on the thalli was thought to be useful for identifying the fish species that caused the feeding damage of the cultured U. pinnatifida.