noun
fishfinder; fish-finding sonar
A device that uses sonar to detect schools of fish underwater. Common in fishing and marine contexts.
漁師は魚群探知機を使って魚の群れを見つけた。
The fisherman used a fishfinder to locate a school of fish.
ソナー is a broader term for any sonar system, while 魚群探知機 specifically refers to equipment for detecting fish.
Compound of 魚群 (gyogun, 'school of fish') + 探知機 (tanchiki, 'detector').