My opinion is Murano on the fish. I love this style of fish and have seen several over the past 2-3 yrs for sale, but unfortunately have been unable to purchase due to lack of cash. So they go for a good price!
The style reminds me of some Cenedese and SVdA pieces I have seen (the back fins and the simplicity of the shape). Not saying yours could be one of them though as I have not seen anything similar to you piece. But it could be a direction to look in. I would also consider Murano due to the sommerso style.
Absolutely stunning piece. Am really jealous! Me, still on the lookout for a good SVdA fish I can snap up when I have the cash at the right moment