DESCRIZIONE
An underwater detection device (1) is provided, which includes a frequency distribution generating module (14) configured to generate a frequency distribution of single fishes by categorizing the single fishes by one of body length and weight, the single fishes being detected based on echo signals corresponding to reflection waves that are caused by transmission waves transmitted underwater, a frequency distribution characteristic value calculating module (15) configured to calculate a frequency distribution characteristic value indicating a characteristic of the frequency distribution; and an alarm controlling module (20) configured to issue an alarm based on the frequency distribution characteristic value. [Image Omitted]