A popular seafood chain accused of misleading customers by swapping "fish-of-the-day" dory fillets for cheap Vietnamese catfish is linked to notorious outlaw motorcycle gang the Mongols.

The Port Melbourne-based club, which has been the subject of recent police investigations into drugs, firearms and arson offences, is considered one of the most powerful bikie gangs in the state. Its members are already linked to businesses in the construction and tattooing industries.

Hunky Dory's signature fish and chips.

Fairfax Media can reveal high-ranking Mongol member Sherif Derias has a significant financial interest in the lucrative Hunky Dory fish-and-chip chain, which operates seven outlets across Melbourne.

While company records show no direct connection between Mr Derias and the business, police, underworld and seafood industry sources say the former body builder is "heavily involved" in the chain, which was founded by long time friend Greg Robotis.