Comment 2 for bug 80544

Some investigations:

fish doesn't install anything into *omf*:
daniel@bert:~$ grep omf /var/lib/dpkg/info/fish.list
daniel@bert:~$

The fish.prerm script merely runs:
        install-docs -r fish

from install-docs(8):
       -r docid | --remove docid
           Remove the documentation identified by the document identifier docid. Document identifiers are tags which are set in the control file, and usually correspond to the package name.

./debian/fish.doc-base contains:
Document: fish
Title: Debian fish Manual
Author: Axel Liljencrantz <email address hidden>
Abstract: This guide documents fish, a shell
 geared towards interactive use.
Section: Apps/Shells

Format: HTML
Index: /usr/share/doc/fish/index.html
Files: /usr/share/doc/fish/*.html

To be honest, I suspect a problem in either doc-base (install-docs) or fish.doc-base