The relationship between otolith and somatic size was examined for ayu Plecoglossus altivelis altivelis sampled in Lake Biwa. Combined allometric (otolith radius < 400 µm) and linear (otolith radius ≥ 400 µm) equations showed the closest fit relationship between otolith radius and standard length. Slower growing ayu tend to have larger otoliths than faster growing ayu at the same somatic size. This phenomenon has been called the growth effect and may be a cause of bias in the back-calculation of fish length from otoliths.