Sashimi Fish

Prerequisites: X-Wing · Finned X-Wing

Description

Sashimi Fish is an advanced “candidate elimination” technique. You can think of it as: a more degenerate version of a finned fish — the green fish body is not quite a complete standard fish, but because all fins (yellow) are confined to a single box, we can still derive a small, reliable elimination.

It usually has three key parts:

• Body (green): almost a normal fish, but with a missing corner / a missing cover line
• Fins (yellow): extra candidates of the same digit, and they must all be in one box
• Elimination (red): typically inside the fin box (more local than a standard fish)

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Walkthrough

In the diagram, the target digit is 3:

• Green body candidates: r3c3, r3c6, r7c6
• Yellow fins: r8c3, r9c3 (in the same box)
• The red target is candidate 3 in r7c1

Why can we eliminate candidate 3 in r7c1? A short contradiction is enough:

1. Assume r7c1 = 3 (treat the red candidate as true).
   Since r7c1 shares a box with the fins r8c3 and r9c3, both fin candidates must be false.

2. Now look at column 3: in this position, digit 3 only appears as candidates in r3c3, r8c3, r9c3.
   With both fins ruled out, r3c3 must be 3.

3. If r3c3 = 3, then r3c6 cannot be 3 (same row).
   And in column 6, digit 3 only appears in r3c6 and r7c6, so r3c6 ≠ 3 ⇒ r7c6 = 3.

4. That’s a contradiction: we assumed r7c1 = 3, but we’re also forced into r7c6 = 3 —
   two 3s in the same row is impossible.

So the assumption fails, and candidate 3 in r7c1 can be eliminated.

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Example

Here is another Sashimi Fish example. Use it to practice spotting “body (green) / fins (yellow) / elimination (red)”:

Sashimi Swordfish and Sashimi Jellyfish follow the same idea: the net is larger, but the core remains “incomplete fish body + fins in one box ⇒ local eliminations”.

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How to Spot a Sashimi Fish?

One sentence: look for an “almost-fish” with a missing corner, confirm the fins are in one box, then focus eliminations inside that fin box.

A practical checklist:

1. Pick a digit d
2. Look for a fish-like alignment (green), but with an obvious missing corner / missing cover line
3. Find extra candidates of d (yellow) and confirm all fins are in one box
4. Check that fin box for eliminations of digit d (often exactly the red-marked candidates)