This is a Bluefin Tuna I did a while back, lousy photograph but they are a gorgeous fish. Speaking of which I need to work on photographing watercolours, I think there is an issue with them and reflectivity. For example the cadmium yellow borders on the flank is not visible in reality, but is dramatic in the photograph.
Still they are a nice fish, and sad that there are not that many left. Also it is a good example that watercolour does not always have to be big broad wash landscapes. This image was based on measurements from a research paper, so it is pretty authentic. The most confusing thing is that we don't normally see them with the first dorsal erect.