Fish, unlike most mammals absorb and retain any toxins that might be in the water they are caught in. I will not even think about eating fish caught in vietnam , thailand or other countries that have open sewers draining into the water, let alone toxins from industry.
What has that got to do with putting the onus on retailers, never mind expanding state power to bully them unnecessarily? There are other ways to achieve such goals. This is not one of them.