One of the key tools available for fish assessment is that of electrofishing.
Electrofishing is a reasonably simple concept to explain practically,
but theoretically it can be rather confusing. Simply, a field of
electricity is passed through the water that causes a muscle response
reaction from the fish forcing them towards the netsman. A full
explanation can be found later on this page. The main purposes for
electrofishing are stock assessment, sampling/health surveys, tagging,
catching spawners, anaesthetising or eliminating species http://www.fisheriesmanagement.co.uk/electrofishing.htm