The common torpedo (torpedo torpedo) is not a commercial species. It lives in shallow coastal waters over soft substrates and preys by night on small bony fish and crustaceans. It's downfall in terms of it becoming part of bycatch is its method of attack and defence.
The torpedo can deliver hefty electric shocks, up to 200 volts according to fishbase.org and the wikipedia. The shocks diminish in intensity as the animal dies and cease as it does. It is not practical to attempt to extract the torpedo from the net while it is evidently alive, and even when it appears dead it can often spark up again. And so it can never be returned to the water alive, unless it drops from the net as it comes over the roller and can be teased out through a scupper with the toe of a welly, or grabbed by the tail and flung. It appears to be the torpedo's back that shocks or the distinctive blue spots themselves, I've never touched one in such a way as to be shocked intentionally. Please excuse this lack of scientific fibre, I'm no Stephen Maturin.
Though they are widely thought to be uneatable, fishermen have always cooked them and I've found they make fine food. The difficulty, if there is one, is in preparing them for the pan as you have to peel off the tough skin—a task best performed on the dock, or outside at least. Many fishy jobs can cause havoc in the kitchen.
Torpedo 'wings' with torpedo livers and sea snails.