referential integrity and lookup

Hi all,

I want to enforce referential integrity between two fields in two separate
tables:

tbl FishTaxonomy (includes the full name, common name, and abbreviated
scientific name)
tb Fish (includes the abbreviated scientific name, length of each fish, sex,
etc.)

The name used in tbl Fish is from a lookup of tbl FishTaxonomy. However,
when I look at my relationships, there is no link between these two tables. I
try to join these fields and enforce referential integrity (I only want to
have one taxonomic description for each fish species, but lots of individuals
of each species). I get this error message:

No unique index found for the referenced field of the primary table.

I'm assuming that tbl FishTaxonomy is the primary table.

Also, the Edit Relationships box lists the relationship as 'indeterminate'.

Eek. I think there is something wrong with my design!

Help?

Thanks,

buggirl