How to Tell If Your Frozen Fish Has Gone Bad

At 0 degrees Fahrenheit or below, frozen fish stays safe indefinitely. However, after a period of time, the quality of the fish decreases. Look for telltale signs on the surface of the fish to determine if you can use it. Depending on your recipe, the fish may work fine with a few modifications, or it may need to be tossed in the trash.

Different varieties of fish remain good in the freezer for different lengths of time. As a rule, lean fish retain their quality better than fatty fish: For example, salmon retains its quality for ‌two to three months‌, while cod stays good for ‌six to eight months‌. Most freezer packaging doesn't entirely exclude air from the food it protects, and the high levels of fat in salmon, herring, mackerel and similar fish will oxidize and develop "off" flavors after a few months.

The issue with frozen fish isn't one of food safety but of declining quality, since spoilage can't progress in the freezer. You should ideally date the packaging when you freeze fish and plan to use it within a few months. Alternatively, use the fish in a way that masks any deficiencies in its flavor and texture, such as a casserole, fish pate or seafood dip to serve with crackers or vegetables.

The signs of bad frozen fish depends on whether it's frozen or thawed. In frozen fish, look for:

• ‌Whitish or grayish-brown dry, flakes or patches‌, called freezer burn, at the edges of the fish or over the surface, indications that they fish has dried out. If the portions are thick enough, you can simply cut away the affected area and use the remainder.
  
• ‌Lighter weight‌ than the fish had when you put it into the freezer, a sign that moisture in the fish has evaporated. You can still cook the fish, but it might be more tough than tender.

Bacteria can't grow in the freezer, but if your fish was not frozen promptly (or if you don't follow good practices while you're thawing it) it's possible for spoilage to reveal itself once you're ready to cook. Look for these signs, and ‌throw the fish away‌ if you see them:

• A ‌slimy film‌ over the surface of the fish, indicating the presence of bacteria.
  
• An ‌off-odor‌ of ammonia or fishiness.

If your fish filet is thin enough, you can cook it directly from the freezer. A good rule is to increase the usual cooking time by roughly 50 percent, to compensate. Thawing is one point in the process where you can create opportunity for bacteria to grow, so cooking from frozen is innately safer.

If you opt to thaw your fish before cooking, there are three recognized options for thawing it safely. Whichever you choose, cook or refrigerate the fish immediately afterwards.

• Defrost the fish overnight in the refrigerator, so it remains at a food safe temperature from start to finish. Place it in a bowl or on a plate to catch drips, and then place it on the lowest shelf of your fridge, to minimize the risk of cross-contamination.
  
• Place the fish in an airtight bag and immerse it in cold water. If the fish takes more than 30 minutes to thaw, change the water to help ensure that it stays cold.
• Microwave the fish at intervals of one minute or less until it's just pliable; a few lingering ice crystals are fine.

If you use the microwave or water-thaw methods, the fish should be cooked immediately afterward to reduce the risk of foodborne illness.

Frozen fish can go bad if your freezer stops working during a power outage. If you suspect that the fish has thawed and refrozen, throw it away to be on the safe side -- bacteria grow rapidly within two hours at temperatures even a little above 40 degrees F, according to the U.S. Food Safety and Inspection Service.

When thawing frozen fish, never thaw it at room temperature on the kitchen counter. At room temperature any potentially harmful bacteria that might be present in the fish can grow rapidly, and can make you ill without ever showing any signs of spoilage.