Fresh tuna has a slightly salty, distinctive flavour that lingers in your mouth as you bite into the cooked fillet. But bad tuna can create stomach problems and other troublesome symptoms because of potentially deadly bacteria, such as E. coli. Thankfully, it's simple to distinguish between rotten and fresh tuna fish. Follow a few simple steps to ensure you can enjoy tuna sandwiches, salads and other dishes without worry.
Smell the tuna to see if it gives off a foul odour. Tuna and other types of fish have a naturally fishy scent, but it shouldn't smell foul.
Touch the tuna to see if it has a firm texture. Tuna becomes squishy and less firm as it rots from the inside. Fresh fish has a moist, but non-slimy feel. Tuna gradually develops more slime on the exterior as it begins to go bad.
Rub the scales of the fish to see if they rub off easily. Rotting tuna begins to fall apart as decay sets in, but fresh tuna has tight, firm scales that don't rub off easily.
Look at the eyes of the fish. Fresh tuna displays clear, bright eyes without any cloudiness or dark spots.
Look for dried or brown blood coming from the fish. Blood begins to dry out as fish starts to decompose. Red, runny blood should come out of fresh tuna, unless it has been frozen beforehand.