As David mentioned on Thursday, discards are complex.
Not catching the small / unwanted dish in the first place is the obvious choice, be it net size or better specific targetting.
In waned fish will always be an issue, but reducing its key.
I have a real issue with boats tha are allowed to fill up their boxes with say, small, haddock. Then ditch it all if they bring up a net of either bigger haddock or say bass that has greater value per pound.