Fly fishing and protected species
While some fish in UK waters are subject to a legal minimum size limit for fly fishing, because they are commercially valuable species, others are not. For example, pouting, rockling and some others aren’t legally protected because they have no commercial value, though they have value to anglers for sport.
There is a list of minimum species size in sea angling, however. We urge you as anglers to use the list so as to avoid the risk of making an illegal catch. You could, for example, face a large find for retaining an undersized fish. In terms of measurement, this is determined by reading from the tip of the nose to the tip of the tail. You can get custom-made measures with a stop board at one end. These are principally used by competition anglers, however.
And always observe the upper limit for the larger species especially bass (6lb). If you kill a bass you will have a greater negative impact on fish stocks than killing an immature fish. You can find specific, detailed guidelines on the internet, or ask a fly fishing expert in your chosen area for advice before you start fishing.