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.