A superbly healthful food, fresh fish provides both high-quality protein and heart-healthy omega-3 fatty acids. Unfortunately it's also a spectacularly perishable food, and just a day or two in your refrigerator -- or an unplanned stop on your way home from the store -- can spoil your meal. You can often detect bad fish easily by smell or touch, but that's not always the case.
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Look, Smell, Touch
You can judge the freshness of whole fish though visual and olfactory cues. As they age their eyes become flat and dull, the skin fades, scales come loose and the vividly red gills fade to purple-brown. No fish, whole or cut up, should ever smell "fishy." Fresh fish has a clean, briny smell, like seawater, and any hint of whiffiness is a sign of spoilage. When the fish is distinctly past its prime it will also feel sticky to the touch and may be covered with a slimy film. That's a sure sign of bacterial activity in cut-up fillet portions, and the fish should be discarded. Some whole fish have a naturally occurring protective coat of mucus, so it's a less certain guide with those.
Not Always Easy
In some instances, a piece of fish may look and smell perfectly fresh but still pose a threat to your health. Tuna, mackerel and some other forms of fresh fish host a bacterium that causes scombroid poisoning, an unpleasant but non-dangerous illness that can be treated with antihistamines. Your fish might also carry more conventional forms of food-borne illness, such as salmonella. You won't detect these common pathogens by eye or nose, so prevention is your best defense. Keep the fish at refrigerator temperatures at all times, and use it within a day or two to minimize the potential for bacterial growth.