I only think that fishes, being on top of the trophic chain, accumulate more heavy metals!

Thank to biomagification studies, we know that, since higher trophic levels consume more organic matter, they will accumulate more heavy metals (since they are difficult to dispose of).

shellfishes better bioacummulators of heavy metals because it is closed to the soil where heavy metals sink

Hello 

I think it is depended to you that which of mollusk or fishes you want to select  for your study. if you want  fishes for study , benthic fishes such as halibut family very good for your survey also filter feeder mollusk has the best in between mollusk. 

  Sincerely yours

Thans for ur suggestive answers..