I often fish with a colleague who has longer experience in fly-fishing. He’s an expert in barbels and greylings, The first ones are beyond my reach so far as I hardly use nymphs, but I do always try to catch some graelings (they are unexpectedly numerous in the rivers here). So far, however, my results are mediocre at best, I only had 3-4 this year. Probably the size of flies has a lot to do with it, when I fish in the sun I must use bigger flies to see them. My colleague catches them much more often and his last fish is great also literally, almost 50 cm! Of course it was released in good shape.
I support a size limit for taking fish home say maximum 40 cm for the trout and greylings. Bigger fish should be released, as well as those not having the minimum size, naturally.