As an ex exporter of shellfish and live lobster to the USA I discovered they have domestic standards and importer standards, some official, others defacto. I was blocked from shellfish from the Irish sea cos of english radioactivity although the local Maine figures were higher.

I was told "if you don't like the USDA decision, take it to court".
UK has much to learn about accessing the hyper protectionist USA.

The USA has very extreme barriers to free trade, between its states and between itself and others. It has USMCA with its needed partners north and south. Its GDP is less than 10% dependent on international trade.
It is a tough market to access

A small exporter to USA may be impeded, often illegally by state and/or federal organs.
The dispute can only be resolved in USA courts. Needless to say, an expensive option that may take years.
USA is a very one-directional market. It wants to export,only imports what needs.

when USA accepted Irish beef was not associated with British-BSE beef, there was hope our premium grass fed, hormone-free, antibiotic free beef would find a market. $10 million annually. That's it. USA market is a waste of time and energy.

I had to have a declaration of radioactive isotope load . I had to pay for those tests. The local , fishing in local waters had no legal requirement although the local waters are higher due to nuclear washouts that occur.