Ive actually gone out with the IDNR before, while they conducted an electrofishing survey.
Electrofishing surveys are great for collecting numbers, but they tell you little about the quality of the fishery. As Blu stated, most surveys are brief. Usually only 2 hours or so. They have several electrode rods sticking in the water off the bow, and unless a fish crosses between two of the rods, the fish will not receive a full shock. You can see most of the fish feel the stun and take off before they even get close enough to be incopacitated. They pretty much just cruised the bank, while guys in the front of the boat netted as many fish that came up as possible. Larger fish are more resistant, and the shock does not go very deep in the water, so in water in excess of 5-6 ft, if laying on the bottom can usually get away.
The surveys I have observed have taken place late summer / early fall.
I wonder if they inluded findings from hoop nets in this statistic??
Hoop nets also collect large numbers of fish, and tend to get the larger fish as well. And you can check a lot of hoop nets in an hour!!