This has led to illegal harvesting and sale of shellfish on the black market, which can be a health hazard.

In practice, though, the rules are often ignored and illegal harvesting tolerated.

Sales rose dramatically over a 10-year period and then declined, a result of illegal commercial harvesting and water redistribution on nursery rivers.

The Department of Commerce says "illegal harvesting contributes to depletion of valuable resources."

Population size has declined due to overharvesting, illegal harvesting, predation by recovering sea otters, and disease.

The illegal harvesting is the subject of an episode of Vanguard.

Unfortunately, this demand has led to illegal and premature harvesting by commercial operations seeking to exceed the quantities allowed by government licensing.

There are populations in more than 50 rivers, mainly in the Highlands, although illegal harvesting has seriously affected their survival.

Perlemoen-diving has been a recreational activity for many years, but stocks are currently being threatened by illegal commercial harvesting.

These uses have made the species vulnerable to illegal harvesting.