Three boats were caught taking blue cod on-board inside the Sounds at the top of the South Island over Labour Weekend.
It is currently closed season for catching blue cod in the area, which runs from September 1 to December 19.
Fishery officers said one of the boats had whole blue cod on board, while another two had fillets. It is an offence to fillet the fish at sea, as they cannot be accurately measured.
Ministry for Primary Industries spokesman Ian Bright said it was disappointing to find people ignoring the closed season.
"Everyone wants to re-build blue cod numbers in the Sounds. The closed season is the breeding season. Taking blue cod during this time doesn't help the re-build," he said.
Mr Bright said one commercial fishing boat was found to be in breach of regulations - for not reporting its catch correctly - and officers will be further investigating the incident.
Officers also found people with undersize scallops, undersize paua and another with shucked paua, he said.
"It's a timely reminder for fishers that scallops, paua and oysters must be in a measurable (in the shell) state at sea. It is an offence to possess these shellfish seaward of the high water mark in an immeasurable state.
"And a big thumbs up to the majority of fishers we inspected who were following the rules and looking after their fishery," Mr Bright said.
The Government kicked off the periodic blue cod fishing ban in the Marlborough Sounds in 2012, stating the region's stock was "in serious trouble".
The ban covers all enclosed waters in the Sounds including Pelorus, Kenepuru, Queen Charlotte Sound and Tory Channel.
Fishery officers are able to issue infringement notices, including instant fines of up to $500.
MPI encourages people to report any suspicious fishing activity confidentially on 0800 4 POACHER (0800 476 224).
Labour Weekend patrolling figures
• 101 recreational vessels inspected
• 356 people spoken to over two days
• 5 blue cod offences
• 2 scallop offences - undersize
• 2 paua offences - undersize, and having shucked paua on a vessel
• 1 crayfish offence - taking crayfish in the softshell state.
• 2 commercial inspections and one commercial offence - for not reporting catch correctly
