pg_dump warnings

Context:Postgresql 7.4.6 under linux gentoo

To move a postgres DB from a FreeBSD box to al linux box I pg_dumped the
archive from the FreeBSD box and pg_restored into the linux box: Now  when
I pg_dump the archive I get the following warnings for every table in the
archive:
.........................
pg_dump: WARNING: owner of table "atecu91" appears to be invalid
pg_dump: WARNING: owner of table "letture24btdom" appears to be invalid
............................

What should I do to eliminate these warnings?
Being a a posgresql newbye a step by step set of instructions will be highly
appreciated.

Ciao
Vittorio