They swooped on the vehicle carrying the consignment at Oban Ferry Terminal after it arrived from Barra.
Shellfish farmed in unclassified areas are deemed to be legally unfit for human consumption and cannot enter the food chain.
Argyll and Bute Council's environmental health team found that the vehicle was carrying 815kg (128 stone) of illegally harvested razor fish, which came from unclassified waters around Barra.
They were granted an order for destruction of the consignment by a justice of the peace and it was taken to be destroyed by a local waste contractor.
Argyll and Bute Council spokesman David Kinniburgh said: ''The council is advising shellfish harvesters, processors and food businesses to ensure that any shellfish intended for human consumption should be harvested from approved waters and be accompanied by the appropriate documentation.
''There are significant risks attached to the consumption of unsafe shellfish, not only for the consumer but for the local and national shellfish industry.
''We will be working with our partners to target similar activity by irresponsible harvesters and would welcome any information which would assist our environmental health officers in their enforcement work.
''We aim to ensure that products entering the food chain are safe and that we protect responsible businesses.''
The council worked with partner agencies including the Food Standards Agency Scotland, Marine Scotland and Police Scotland on the operation on February 27.
They detained the consignment using powers under UK and European food safety legislation.
WWF Scotland director Lang Banks said: "This is a welcome intervention by the authorities. Illegal or unregulated fishing and harvesting of seafood can lead to major damage to the marine environment.
"In addition to the threat posed to the unwitting consumer, illegal harvesting is often carried out in ways that can put the health and safety of those involved at risk.
"To protect the marine environment and the livelihoods of those communities that depend upon healthy seas, it is important any illegal activities are stamped out."