Sashimi Fish
A Sashimi Fish is a finned fish stripped to the bone. In an ordinary finned fish the underlying fish is complete and the fin is an extra. In a sashimi fish the fish is degenerate: remove the fin and one of the base units has no candidate left, so the pattern only exists because of the fin. Remarkably, the eliminations work exactly the same way.
The idea
Picture a would-be X-Wing on a digit where one of the two base rows has just a single candidate, and that candidate is the fin — sitting outside the two cover columns. Without it, that row is empty of the digit and there is no fish at all. The case split still holds, though. If the fin is true, cells seeing it lose the digit directly. If the fin is false, the remaining candidates form a genuine (if bare) fish that makes the same eliminations. So any cover cell that also sees the fin is safe to clear, just as in a normal finned fish.
A worked example
For the digit 6, picture a would-be X-Wing with base rows 2 and 8 aimed at cover columns 4 and 7. Row 2 supplies both 6s, at r2c4 and r2c7. Row 8, though, has no 6 in either cover column — its whole contribution is a single stray 6 at r8c5, the fin. Drop that fin and row 8 is empty of the pattern, so the fish collapses: this is a sashimi. Yet the case split still bites. The cover cell r7c4 shares box 8 with the fin, so either way it cannot be 6 — if the fin is the 6, its box-mate r7c4 is not; and if it is not, the bare fish clears r7c4 anyway. The sashimi shape delivers an elimination a clean fish could not.
A sashimi X-Wing on 6: base row 2 holds both cover 6s (blue), but row 8's only 6 is the orange fin r8c5. r7c4 shares box 8 with the fin, so either way it loses its 6.
Step by step
- Look for a bare near-fish where one base unit holds only the fin.
- Confirm it is sashimi — removing the fin would leave that unit empty.
- Restrict eliminations to cover cells that also see the fin.
- Remove the digit from those cells.
Common mistakes
- Treating it as a full fish. Only cells seeing the fin are safe; the degenerate base cannot promise more.
- Missing the sashimi case. Solvers who insist on a complete fish overlook these bare shapes, which are common on hard grids.
- Multiple stray candidates. Keep the fin small and ideally boxed; scattered extras usually mean no fish exists.
Which Sudoku types is this best for?
The same logic applies across variants, but it pays off more in some than others.