Franken Fish in Sudoku: An Advanced Fish Guide

Review the base and cover idea behind every fish

Fish techniques follow one candidate at a time. In a basic Swordfish, for example, the candidate is restricted within three base rows to three cover columns. Each base row must eventually contain the candidate once, so the three required placements must occupy the three cover columns. Other candidates in those cover columns, outside the fish, can then be removed. The number of base and cover houses gives the fish its size: two produces an X-Wing, three a Swordfish, and four a Jellyfish. A house is any row, column, or 3 by 3 box. Basic fish use rows as bases and columns as covers, or the reverse. Franken Fish extend the idea by allowing at least one box to replace a row or column among the base or cover houses. The creature has a dramatic name because Sudoku apparently needed marine biology with laboratory equipment, but the underlying counting rule remains unchanged.

Recognize a Franken Swordfish structure

Choose one shape and look for three base houses, perhaps two rows and one box. Collect every candidate for that shape inside those bases. If all of them lie within exactly three cover houses, such as three columns, the structure may be a Franken Swordfish. Each base house must contain the shape once in the final solution. Because the three placements are confined to the three cover houses, those covers cannot contain another copy of the shape outside the base structure. Those outside candidates are the targets. The box base acts as a grouped unit: several candidate cells inside it may collectively represent one required placement. You do not need to know which cell in the box is correct. You only need the box to contribute one placement somewhere within the chosen covers. The same logic works with columns and boxes swapped, and larger Franken Jellyfish use four bases and four covers.

Check house counts before making eliminations

Complex fish are easy to overdraw. First confirm that the number of base houses matches the number of cover houses. Next, verify that every candidate in each base lies inside the selected covers. A forgotten candidate outside the cover set breaks the fish. Then identify targets that belong to a cover house but not to any base house. Only those candidates may be removed in a clean, unfinned Franken Fish. Do not include extra boxes simply because they make the picture look symmetrical. On an Extreme Shapedoku board, scan one shape at a time and use complete Notes, since a missing mark can manufacture a false pattern. Franken Fish are rare and often replaceable with grouped chains, so there is no need to force one into every stalled puzzle. When a valid structure does appear, make the smallest justified elimination and return to ordinary techniques. The removal may create a hidden single or a simpler fish nearby.

1. Choose one shape and map all of its remaining candidates.
2. Select N base houses, including at least one 3 by 3 box.
3. Find N cover houses containing every candidate from those bases.
4. Confirm that each base must contribute one placement.
5. Remove the shape from cover-house cells that lie outside all base houses.
6. Recheck the pattern for fins or forgotten candidates before changing the board.