Fish specialist Mark Bittman shares his insights on fresh as compared to frozen from his book Fish.
Recent technological advances have allowed modern fishing fleets to clean and flash-freeze fish - at -60 degrees F, or even colder - within minutes of its capture. When thawed, such fish is in frequently better shape than that which spent several days sitting in a hold before reaching the dock, at which point it was sold as "fresh." Much of the fish that you eat in restaurants including 98% of all shrimp sold in this country, has been treated this way.
If you buy fish straight off the boat or from a reliable market, you should continue to do so; generally speaking, such top quality fish is fresh and not frozen, but keep in mind that much frozen fish makes good eating. Advocates in the industry like to say that their procedure "captures a moment in quality." Indeed, well-frozen fish has good color, texture and flavor. When frozen quickly and thawed properly the liquid from the ice crystals remains in the flesh, and the fish is nearly as moist and flavorful as it was when it was first frozen.
Allow fish to thaw in the refrigerator for 24 hours or, if you're in a hurry, you can run the tightly wrapped fish under COLD water. Thawing too fast can change the cell structure allowing fluids - and flavors - to escape from the cells. Cook it as soon as possible to minimize this drip loss.
Some fish simply freeze better than others. Shrimp, squid, and other small-celled, flexible fish generally do well, and small cuts of fish, because they freeze more quickly, retain their quality better. Flounder takes to freezing better than cod, for example.
How do you know when a fish has been frozen well? Frozen fish should be somewhat shiny, not flat-looking, and it should have none of the white spots that indicate freezer burn. It should also be as hard as a rock and have no evidence of prior defrosting, look for even color and texture. Finally, frozen fish should be well-wrapped.