this ... have

Daab

Senior Member
Korean
This, in addition to other methods that decrease the overall amount of uneaten food, have helped aquaculture to clean up its act.

I got the above sentence from a book "Do Fish Feel Pain?"
Is 'have' there OK? I think it should be "has" because the subject is 'this.'
Is 'have' OK because the subject is considered 'this and other methods?'
 
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