The first stage in a nationwide campaign to counter salmon poaching was launched yesterday on the River Tay.

Poaching had dwindled as the numbers of fish returning to Scotland's rivers fell during the late 80s and early 1990s.

But poachers have been returning to the riverbanks as intensive efforts to conserve and increase the number of fish begin to show results.

Now a joint operation between Tayside Police and Tay District Salmon Fisheries Board, Operation Salmo, has been launched to coincide with the opening of the salmon fishing season on the River Earn but will see officers monitor activity at all of the salmon fishing areas across Tayside.

A similar scheme will be launched today on the River Tweed in a bid to protect the fish which are central to a fishing industry which employs more than 3000 people nationally and earns millions of pounds for Scotland's economy. On the Tweed alone it is estimated to be worth more than £18m.

Throughout Scotland, officers and water bailiffs will work even more closely together, carrying out joint operations where necessary. They have similar powers but it has been shown amalgamating their experience makes them more effective.

Under Operation Salmo, Tayside's wildlife crime officers and bailiffs from the fisheries board will meet regularly to discuss poaching trends. They will confer over any poaching cases under investigation to ensure the strongest possible case is submitted to the procurator-fiscal. If a poaching "hot spot" is identified, they will consider mounting a joint operation.

Alan Stewart, Tayside Police wildlife and environment officer, said: "I dealt with literally dozens of salmon poaching cases in the 1970's and 1980's. After a quiet spell when salmon stocks were low, poaching is definitely on the increase. The River Tay is a prime target for salmon anglers, and poaching on a commercial scale, in addition to adversely affecting salmon stocks, has the potential to damage the local economy."

Paddy Tomkins, Lothian and Borders chief constable, said: "Salmon poaching has been going on for centuries, but the modern-day poacher now steals on a commercial scale to sell to unscrupulous traders.

"Poaching has a significant impact on a number of factors, not only the obvious environmental ones.

"Not only does it have consequences for the salmon population and the legal fishing industry, it can seriously affect local businesses and damage rural communities.

"Poachers are often involved in other forms of crime, including housebreaking and theft, and will think nothing of travelling to other areas to commit these offences.

"The National Wildlife Crime Unit will work with police and other agencies across Scotland, gathering and sharing intelligence across borders to target mobile poachers."

He added: "Generally poachers work in groups, taking large quantities of fish and attempt to sell them illegally.

"We would urge anyone who notices anything suspicious at rivers or is approached by anyone selling fish who isn't a bona fide fishmonger, to contact police."

l At the opening of the Dee salmon season yesterday, an appeal was made for anglers, and other users of the river, to be especially vigilant against accidentally introducing the parasite gyrodactylus salaris.

If the parasite, which is native to rivers in the Baltic Sea and has spread to many other European countries, reached the Dee or any other Scottish rivers it would cause a catastrophic decline in fish numbers and would kill off the local angling industry. Proper cleaning of equipment is the primary safeguard.