These days, in large non-coastal cities, I've heard that it is often other than you would expect. In cities where there are large international airports, fish is flown in daily, to the point where the fish served in nicer restaurants will not be much less fresh than on the coast. The Denver Post article linked below goes into how fish is flown to Denver within hours of being caught, every day.

https://www.denverpost.com/2009/01/26/how-does-fish-get-from...