This year's disastrous sockeye salmon runs have prompted the closure of all commercial and recreational fisheries.
But enforcement officials say they've pulled up dozens of illegal nets and seized several vessels in the last week alone.
"We're facing a fair number of nets that are fishing at night,” said Herb Redekopp, chief of conservation and protection for the Department of Fisheries and Oceans.
"There's a real pent-up demand for these fish and the market price on the black market is very high."
Officials have stepped up enforcement on the river this week, including night patrols and aerial units.
"We're hitting the river hard,” Redekopp said.
“We've doubled our enforcement strength on the Fraser River from Surrey up to Hell's Gate and our primary focus is to move all the sockeye that are in the system right now up to the spawning grounds.”
The DFO says it's also looking into complaints of illegal fish sales.
Poachers can face a fine of up to $100,000 or up to two years in jail if found guilty.