foreign key problem

Hi,

I need a foreign key (or equivalent) where the referenced table cannot have a unique constraint.

For fisheries surveys we have a table of catch records. For a given event, several species are caught, and are weighed
byone or more methods. Thus a given event may have more than one record for the same spaecies, as parts of the catch
maybe weighed differently.  

When we sample a species for measuring, the event is recorded, but as the sample may include individuals from
throughoutthe catch, it does not necessarily relate to the catch-with-method table. 

It is possible to have catch/sub-catch tables where the catch table no method & stores the aggregated weight of each
sub-catch(with method), but I prefer to avoid this if possible. 

How can I impose a constraint on the table of length data, so that the event/species has teh equivalent of a
referentialon catch(event,species) when I cannot have a unique index on catch(event,species)? 


Thanks,