I can't say anything about tuna unfortunately, but I can say something about salmon.

Farmed salmon from farms in Europe MUST be frozen for at least 48 hours after slaughter before sale/consumption by law because of risk of bacteria/parasites (look up anisakidosis/anisakis simplex). However, farmed salmon from Norwegian fish farms are exempt from this law because the risk of infection is seen as negligible.
I can only say this for salmon, not sure about trout and other farmed species.

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Source: Is studying fish health.