I have been electrofishing in Iceland since 1975. I constructed an electrofisher in 1986 which has proved to be very effective. In Iceland, only DC fishers are used. They are driven by gasoline generator, placed on the river bank or carried on a back pack.
Here is a circuit diagram of the unit and some hints on its construction, presented as a poster at Sampling Sear conference in Budojevice 2010.
Here is the manual I wrote on the 300/600V DC unit back in 1986.
We made a video on electrofishing using a back pack unit, when sampling a small river near Reykjavik. The video is on youtube and I have got lot of comments on it, mostly asking if this kills the fish, which it does not.
Here is another video, showing sampling for salmon parr and catch of precautious smolts, small dwarfed salmon males which participate in the spawning later in the fall. They sucsessfully fertilize the big females.