A regular fish is a set of n lines with candidates of a single value on n crossing line positions. In the solution, each true candidate will occupy one of the n crossing positions. Outside candidates of that value on the crossing lines must be false and are removed.
A finned fish is a crippled fish, whose lock on crossing line positions is broken by the presence of one or more extra candidates on its n lines. These extra candidates are the fins. However, an outside candidate on a crossing line that sees all of the fins is removed, because if it were true, it would remove all the fins and be removed by the repaired fish. Finned fish add to the possible fish removals.
In this 17 clue puzzle suggested by reader Gordon Fick, the line marked grid looks impenetrable. However, the low corner slink marks suggest we do some fishing on the 1-panels and 2-panels.
These blank line tallied panels reveal a finned fish on rows 3,4 and 9, and a complementary one on columns 3, 6 and 7. “f’ marks the victim and “v” the victim.
After the removal, another finned fish remains on rows 2, 3 and 5, with a fin on r2c3. This time, none of the 3 potential victims (on the + columns, and not on the – lines), see the fin.
But on the 2-panel, there’s another finned fish, and the removal starts the collapse
Multiple fins can work as well. This finned fish was superseded on KrazyDad SuperTough v.5, b.5, #5 by a 6-node remote pair.
The two fins spoil the X-wing in c34.
Two outside candidates on crossing line c6 are in the same box with the fins. If either one is true, the fins are false and the X-wing is valid.
1r3c9 is on a crossing line r3,but that line is not limited to c34 because a fin may be true.
Kraken Fish
When the potential victims are not in the fin box, they may be removed by “seeing” the fin by an AIC forcing chain, an effective weak link. We call this type of finned fish a kraken fish.
Before the remote pair was found KDST 555 also featured this simple kraken 6-wing.
Potential fish victim 6r3c3 sees the fin spoiling the r45 6-wing. So, if its true, the repaired 6-wing removes it. 6r6c3 escapes the fish. it cannot see the fin. If true, it would confirm it.
This finned jellyfish from World’s Hardest Sudoku, Very Hard 65 is a kraken analysis challenge. The fin is in the West box with victim 9r5c9, but but there are many potential kraken victims as well. Are there any?
To check your kraken analysis, see the review post of 6/5/18.
Sashimi Finned Fish
There is a type of finned fish that violates normal rules and gets away with it. When scanning lines for an n line fish, you fail to come up with the nth line matching only the crossing positions. But there is a line matching some positions. Consider the remaining X candidates on that line to be fins, and treat it as a finned fish. If you have a victim seeing these fins, it’s a sashimi fish. The “seeing” can be by fin box or by an inference chain.
From Weekly Extreme 430, posted 4/18/15, here is a blue finned swordfish on columns 1, 3 and 9, with a single fin.
In the red Sashimi 7-wing column 7 lacks an upper right right corner candidate, but the 7r13c7 fins are seen by a potential 7-wing victim. If 7r2c9 were true, 7r6c7 and 7r2c2 would be true and 7r2c9, false.
On the X-panel, X-wing sashimi with a single fin are often taken to be X-chain ANL called skyscrapers.
Here are two skyscrapers from KrazyDad SuperTough v.5, b.6, #5. Each of the three removals can be attributed to a Sashimi X-wing. The Sashimi on the previous diagram is also a 7-chain ANL, with one terminal being a 7 group.
Most Sashimi are modified X-wings.
As an example of a higher order Sashimi, here is the blank line tallied X-panel of a Sashimi swordfish from Andrew Stuart’s The Logic of Sudoku. Since two lines contain two candidates, there’s an ANL version.
Systematic Sudoku 