a display having a screen providing historical water depth information and a screen providing fish school detection information is presented . the water depth information and the fish school detection information may be derived from an echo signal , and can be displayed simultaneously or alternated separately on the display by switching . when the water depth reaches or is less than a predetermined value , an alarm may be provided on the display and / or an alarm sound may be generated . as the alarm is displayed the brilliance of the display may be raised to a maximum brilliance , making the display brighter so the contents are easier to see .