It wouldn't seem to be particularly difficult to drop a few million or even billion little codlets back into the North Sea. Together of course with the smaller ones which are crushed in the net and so die anyway. Previous answers don't seem to hit the mark to me. This seems like a good idea to me too.
Problem is that fishing grounds tend to be fished by different fleets of many different nations.
Why would you bother to produce and release eggs when somebody else could come along and harvest your crop of adult fish.