Catching salmon parr with electricity is a method to estimate n.of fish per areal and collecting samples for analyses of growth, stomac content etc.
This video is from a small Icelandic salmon river Leirvogsá, loaded with salmon parr of natural origin. Growth is slow because density is to high, to much spawning, 15 females/ kilometer. This results in high mortality and low production. Remedy: Decrease the spawning stock, give up catch and release and increase fishing pressure.