The Ministry for Primary Industries is widening a crackdown on the sale of crayfish on the black market with a plan to get recreational fishers in the upper South Island to tag their catch.
MPI said illegally caught crayfish was often sold more cheaply than that caught commercially, which was encouraging an underground market.
Compliance manager John Slaughter told Nine to Noon marking the crays by clipping part of their tail made it easier to trace illegal fishing.
"If we can discourage it - if we can make it harder to sell fish and take that temptation away to poach for the purpose of sale, then yes, we must be helping to enhance that fishery," Mr Slaughter said.
The tagging programme has been operating in Kaikoura, and there was a plan to extend it to the Cray Five area around the top of the South Island, as far as Farewell Spit, he said.
Mr Slaughter said the method relied on the honesty of fishers but it had proved effective in Kaikoura.
A tail clip denoted a cray had been recreationally caught and was not able to be sold. He said if a fish marked entered the commercial chain, it was proof that it had been taken by a recreational fisher.
MPI said it was aware of some "very organised individual and groups, and some small-scale operators" poaching, said Mr Slaughter.
The public is invited to give feedback on the proposal to extend the monitoring area. They are also asked to pass on information about any suspicious activity via the 0800 4 POACHER hotline, or by contacting their local MPI office.