It isn't a matter of whether they live or die, you will find that the legislation is "that it is illegal to LAND any fish that is below the MLS for that species", anglers must throw them back DEAD OR ALIVE or take a chance on being prosecuted for possessing undersized fish.
Likewise commercial fishermen have to return all undersized fish, plus, if they catch fish that dont have any quota left on them, they also have to be returned dead or alive, regardless of size, e.g. If a commercial boat caught a BFT of 1000kg AFTER the close of the season, it would have to be returned as a DISCARD even if it was dead! Its the EU's crazy way of dealing with problems

It would make sense to land fish that are caught, then deduct them from the next quota so that only the right amount are removed, but nothing is wasted, unfortunately that makes sense, so it wont be entertained!

blueskip