"This is simply another vehicle that people use to illegally sell recreational catch," he said.
"Both buying and selling recreationally caught seafood, as well as swapping it, is against the law. These are offences against the Fisheries Act and can result in fines of up to $250,000."
Keys said it was crucial MPI cracks down on every case to ensure the sustainability of New Zealand's fisheries as the warmer weather sets in.
He said it was also crucial to ensure seafood met food safety standards.
Earlier this month Ten cases of paratyphoid were confirmed in Hawke's Bay, linked to contaminated mussels.
Keys was encouraging people to report illegal sales to MPI through email or the department's Facebook page.