Everyone loves fresh fish. Yet what most people don’t know is that a fish’s “freshness” often has little to do with when it is caught or the fact that it isn't frozen.

Here’s why: The shelf life and quality of fish is a function of several factors, but most importantly handling and temperature. Fresh halibut, for example, has a shelf life of more than 18 days chilled on ice. But if improperly handled and stored, its shelf life shortens significantly.

FRESH

Here are the four main factors that determine seafood quality and freshness:

• Handling: If fish aren’t properly bled, cleaned and handled, their shelf life declines appreciably. Rough handling, for example, damages the flesh, causing enzyme breakdown and bacterial spoilage.
• Temperature: Keeping fish at a cool, consistent temperature ensures that it will maintain its quality for its maximum shelf life. For every hour that a salmon is left at 10 degrees Celsius (50 F), its shelf life declines by four hours.
• Oil Content: Fish with high oil content are prized because of the health benefits of Omega-3 fatty acids. However, the high oil content shortens the shelf life.
• Species Characteristics. Other factors also affect a species’ shelf life. For example, big fish stay fresh longer than small fish, flat fish keep better than round fish, fish caught in warm, tropical seas spoil faster than those caught in cold waters, and bony fish are edible longer than cartilaginous fish.

FROZEN

Just because a fish is frozen doesn’t mean it isn’t fresh. Indeed, freezing a fish shortly after it is caught locks in its freshness. Sushi-grade tuna, for example, is typically frozen at sea and then thawed—a fact not widely known by seafood lovers. Sushi chefs prize fish frozen at sea because it meets strict health rules on eating raw fish and because it is often the top quality on the market. A fish frozen a few hours after being caught doesn't have time to begin to spoil.

Still, some frozen fish has a bad reputation for good reason. Old fish can be frozen to extend its shelf life, which means it may spoil shortly after you thaw it. If improperly stored, frozen fish can also become freezer burned.

When you trace your seafood, ThisFish tells you its condition (live, fresh or frozen) and the date when it is landed. This can be valuable information, but nothing beats a good pair of eyes and a keen nose in determining the quality and freshness of your seafood.

Sources:

• U.N. Food and Agriculture Organization

• John P. Doyle. Seafood Shelf Life as a Function of Temperature. Alaskan Sea Grant: No. 30, 1989.