Not long ago I watched a tv show from Spain that wanted to make some awareness about this topic. They bought some fish and prawns and more seafood that should contain "lots of microplastics".

They took all of it to a laboratory and the laboratory found nothing at all. The presenter told the audience that it was surprising for him, and that they thought that the results were a letdown for the purpose of the documentary but that it was "good news" that we can still enjoy food without microplastics...

I would say that the results will vary vastly depending on the place you obtain the samples, but anyway, we should change our relationship with plastic...