The Department of Fisheries has warned people they face fines worth thousands of dollars for catching marron outside the season.
It comes after fisheries officers swooped on Wellington Dam last week following public reports a social group planned to catch marron as a part of an "annual get-together".
Officers set up a roadside checkpoint, inspected four vehicles, one of which allegedly contained 22 traps that had been used to take marron in what fisheries called a "significant marron poaching operation", and apprehended a man.
Earlier, officers seized 139 marron from two fishers on a night patrol on October 21, with the pair to front Collie Magistrates Court at a later date. The 139 seized marron were safely released back into the wild.
As recently as Monday night, fisheries officers caught more would-be marron thieves at Harvey Dam.
A pre-season initiative known as Operation Kerinci, in conjunction with WA Police and the Water Corporation, was focused on reducing the number of illegally-caught marron and educating the community about the importance of doing the right thing, according to supervising Fisheries and Marine officer Graeme Hall.
He said the marron season, which would run from midday January 8 to midday February 5, was short in order to keep fish stocks sustainable, with factors such as the number of licences and bag limits taken into account.
Thieves often went to great lengths to hide themselves and their catches, he said.
"They go to places they think we won't be, at obscure hours, and use illegal traps which is very damaging to the environment and fish stocks," he said.
Mr Hall said despite warnings and education campaigns about the importance of sustainable fishing, each year some people still did the wrong thing.
"Essentially it's greed, they're getting in there while the season's closed and taking stock that's for all recreational fishers, which totally goes against the idea of fishing for the future," he said.
Mr Hall said community members were eyes on the ground and an invaluable resource to help tackle those who fish unlawfully, and he urged them to contact Fisheries with information about illegal activity.
Meanwhile Mr Hall had a message for those who might try their luck at outsmarting fisheries officers.
"The second time we meet up with them is usually in a court of law," he said.