Japanese Wakame seaweed, slipper limpets and Chinese mitten crabs are three of the eight invasive species, covered by the code, now found in UK waters that can dominate native species and 'smother' them out of existence.

The state-of-the-art mussel dredgers, Mare Gratia and Valente, have to transport around 4,000 tonnes of young mussels each year into the Strait to sustain the industry.

Valente's owner Kim Mould said the fishery, now in its 46th year, is totally dependent on imported seed mussel to maintain its high level of productivity.

"We are now working within a marine conservation area and the introduction of this code of conduct will not only afford our mussels protection from invasive species but also serve to protect the wider environment," said Mr Mould.

Dr Kate Smith of Countryside Council for Wales believes the foreign invaders are slowly moving north from southern seas due to climate change and an increase in sea temperature.

The code of conduct should ensure that all the boats and lorries importing young 'mussel seed' into the area know of the risk posed to Britain's biggest mussel fishery which lands £10 million worth of mussels each year.

Dr Smith welcomed the proposals from the fishermen and developed a Memorandum of Understanding which was signed by all four directors of the Ltd companies involved in the initiative, Myti Mussel, Deepdock, Extra Mussel and Ogwen Mussels.

"This is a voluntary code of practice and if the fishermen stick to it then we are happy for the import of seed mussel to continue," said Dr Smith.

"The invasive species can be present in an area for a long time and then their population will just explode, and when they become established they can be hard to get rid of," she said.

Last year the fishermen found slipper limpets amongst a haul of mussels imported from the English Channel and carried out emergency measures to remove the species from the Strait.

"The slipper limpet is moving slowly north because of climate change and this is a dominant shellfish which can smother the local species such as mussels and oysters," said Dr Smith.

"The Code provides a clear, practical framework within which decisions about movements of mussels into the Menai Strait can be made," she said, adding that risk assessments would be carried out on certain areas before seed mussel could be taken from them.

James Wilson, who works on the Mare Gratia and developed the code with Dr Smith, said it was essential to reduce the risks of importing potentially destructive species into the area.

"The accidental introduction of slipper limpets into the Strait last year was a wake up call for us and this will be a much more robust way of preventing any further introductions," said Mr Wilson.

"When there's evidence that one of these species occurs in a seed mussel area then we will do addtitional work to reduce the risk of transferring them into the Menai Strait," he said.

"It's now up to CCW to have the final say if we can bring in mussels from areas that have invasive species because that is one of the conditions of the code of conduct."