Fish butchery knife for removing fish heads, splitting fish through the spine, filleting from the skeleton, and breaking through cartilage at bone joints. The thick spine (5–8mm at the heel) provides the mass and rigidity to push through bone without deflection. The A blade ground to an edge on only one side, with a flat or hollow back; it cuts straighter and finer but takes more skill to sharpen than a double bevel. geometry keeps the fillet side of the fish flat against the blade as it pushes along the skeleton, maximizing yield.
Primary tasks: breaking down whole fish, including removing heads through the spinal column, separating fillets from the rib cage, and portioning large fish. The heavy spine is what sets the deba apart: it is a fish-breaking knife, not just a fish-slicing knife.
Size matched to fish: 150mm for mackerel, 210mm for sea bass, and 270–300mm for large whole fish.
Ideal steel: toughness to handle lateral force through fish spines, so Shirogami #1 or #2 is the classic choice. The deba is traditionally made in Aogami or Shirogami carbon steel at 62–64+ HRC. That extreme hardness works here because the thick geometry, rather than the edge alone, handles the impact loads.
Variants: ko-deba (120–150mm, small fish), the mioroshi deba (longer and lighter, a hybrid with the sujihiki), and the oroshi deba (very large and heavy, for tuna-scale fish).