  Quantum shell effects stabilising fission fragments with various shapes have
been invoked as a factor determining the distribution of nucleons between the
fragments at scission. Shell effects also induce asymmetric shapes in the
nucleus on its way to fission well before the final fragments are (pre)formed.
These shell effects are studied in fission of $^{180}$Hg, $^{236}$U and
$^{256}$Fm with constrained Hartree-Fock-Bogoliubov calculations using the D1S
parametrisation of the Gogny interaction. Strutinsky shell energy correction
and single-particle energy level density near the Fermi surface are computed.
Several neutron and proton shell effects are identified as drivers towards
asymmetric fission. Shell effects are also used to identify the preformation of
the fragments in the later stage of fission.
