MPI officers said Tuatua beds were far more plentiful and accessible on the beaches south of Dargaville than for many years. Small toheroa and mature tuatua look similar and often sit together in the same beds, a spokesman said.
MPI compliance staff are regularly meeting people on the beach who they have spoken to previously and "99 per cent of people have it right the second time".
Toheroa shells are more brittle and slightly rounder than tuatua. The usually smaller tuatua has a slightly glossy shell, compared with the dull shell of the toheroa.
Another simple test is to sit the shellfish on their base on the sand, with the sharp end standing up. A tuatua should stay standing, balanced on the flat base, while the toheroa, which has a lump on the base of the shell, should fall over.
Honorary fisheries officers spend a lot of time at Baylys Beach and on average there are about 10 incidents investigated a week.
"Evidence suggests there are some repeat offenders and people who are intentionally targeting the Toheroa. These people have been sent infringement notices or summonsed to attend court."
MPI encourages people to report any suspicious fishing activity to 0800 476 224. All calls are treated as confidential.