So far these are wild fish so i don't think it is dye, it is the same natural colour as the rest of the blue sheen on the gill plates, just a concentrated area.
Fish were caught from the Annan, the Don and the Deveron, Ricardo had a photo of one as well, the same, off the eden i think, on FB
You can see it quite clearly in real life, the photo's actually hide it a bit, the underwater picture Mike has of the first fish shows the spot really distinctly, much more like real life was.
Some are more intense than others and some fish don't have it all, but it does seem to me to be something i'm noticing more and more, perhaps it is something to do with the season progressing?
Sandy