Watch electrofishing method used to survey fish populations

Electrofishing is exactly what the name alludes to, but it's more nuanced than just using electricity to stun fish. Sciencedirect.com has an excellent breakdown on how the process works, in their synopsis of William L. Thompson, Gary C. White and Charles Gowan's book "Monitoring Vertebrate Populations":

Electrofishing gear consists of three major components: a power source (a generator, usually producing alternating current, or a battery), a transformer to convert current from the power source to different voltages or to direct current, and electrodes placed in the water to create an electrical field.