The restaurant admits it doesn't get in fresh fish every day
"A good, busy restaurant is going to have fish come in every day, and it's not going to be frozen. There's no need for it, and most restaurants that specialize in seafood should be flexible enough to serve alternatives if a certain fish becomes unavailable. You're going to want to eat fish fresh, not frozen. If it's frozen, it's either out of season, shipped in from far away, or it's a processing-plant product."
The menu lists the cost of the fish as "Market Price"
"You'll usually see 'Market Price' on menus in chain restaurants, or places that do a lot of touristy business. The seafood market doesn't go up and down in wild swings. They should know what the price is, and they should be able to put it on the menu. Usually restaurants put 'Market Price' on the menu because they want to give the illusion the fish is fresh. As if they're picking it up at the market every day, which is probably not the case. They're not going up to a boat in the docks, handing over cash, and asking the fisherman, 'What's it cost today?'