Fresh fish only lasts one to two days in the refrigerator, according to the U.S. Department of Agriculture Food Safety and Inspection Service. Frozen fish, on the other hand, can keep for two to three months (for fatty fish) or up to six months (for lean fish). Defrost your frozen fish by leaving it in the refrigerator overnight or by leaving it under cold running water for up to an hour. Frozen fish can be very hard and dense, so cutting it can be difficult.
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Remove the fish from the freezer and any packaging. Lay it on a cutting board.
Place the blade of a sharp knife on the fish where you wish to cut it. Put your hand on the back of the knife and press down, leaning on the knife. If the fish is thin, this may be enough to cut through it.
Lay the blade of your serrated knife into the marks made by the sharp knife if you were unable to cut through the fish with the sharp knife alone. Saw back and forth with the serrated knife to cut through the fish. This may take some patience.
Use a cleaver or butcher's knife if all else fails. Lift the cleaver and bring it down quickly onto the spot you wish to cut. Keep control of the cleaver as you do this; do not wildly swing it from above your head. It may take a few blows before you make it through the fish.