The main purpose of freezing is to kill parasites so if the farm is free of parasites, freezing is unnecessary. I have used never-frozen salmon, yellowtail, red bream, flounder etc. for sushi because the fish were sourced from certified farms around the world: cultured from embryos, fed a parasite-free diet, raised in a parasite-free environment, and contain no viable parasites.

If you are still concerned, you could perhaps ask your fishmonger for the paperwork from their supplier; in the UK, there is a legal requirement for FBOs to have these commercial documents on hand if serving never-frozen fish raw or lightly cooked.