We tracked ten threeline grunts （Parapristipoma trilineatum）, using biotelemetry for 179 days around an artificial mound reef off the coast of Akune, Kagoshima Prefecture, Japan. Three resided around the artificial mound reef for about six months. Their swimming depth changed diurnally, being shallower during the day time and deeper the night time. However, during spawning periods, the cycle reversed: they swam deeper during the day time and shallower （30 - 35 meters deep） the night time. This appears to be a spawning behavior characteristic of threeline grunts.