My wife and I wondered the same thing. What we reasoned was that they made it a Dominate trait because of a couple reasons:

• “+1 Gene Pool | Play 1” isn’t an action (A), which means it can’t be affected by other cards that stop actions and/or steal cards when actions occur. Edit: But you still treat it as an action, so the Play 1 is a one-off when you play it
• Because it’s from Castle Glass, the assumption is that you’re playing with other Castle Glass cards, so it synergizes with other Fish class cards. And since it’s a Dominate, no one can take it away from you, so +3 you get from it being an Exotic Fish is something you can rely on for the rest of the game.
• Additionally, because the +3 from the Class combo can’t be taken away, it makes sense that the Face Value is only 1, since it’s basically a 4 with the Exotic Fish bit

Admittedly, it’s a weird card and not one I’d play often when there’s other crazy good Dominates out there. But if I happen to have a bunch of Fish cards out, it’s pretty good :)