Kraken Fish

In a finned fish either the plain fish exists, or one of the fins is true. A Kraken fish uses chains to deal with the fins. Type 1: a candidate of the digit that the plain fish would eliminate, but which does not see the fins. If a chain from every fin leads to that candidate being false, it can be removed in every case.

Type 2: for a cover unit, find chains from all the base candidates in it (and from every fin) that lead to one and the same candidate being false. In an ordinary fish exactly one of those base candidates is true, so that candidate is false whichever it is. Type 2 may eliminate any digit, not only the fish digit. The search is slow, and Kraken fish are among the hardest techniques HoDoKu looks for.

Notation

Kraken Fish Type 1: 9 can be removed from r1c5. chain: 9 in r9c6 → 9 in r9c9 → 9 in r1c9 → 9 in r1c5.