There is no evidence to suggest a disease found on one fish in the River Spey is widespread, according to the country’s top salmon body.

Concerns have been raised since a fish on the River Spey was found to be infected by Ulcerative Dermal Necrosis (UDN) - a disease which last affected Scottish salmon stocks in the 1970s.

Dr Alan Wells, policy and planning director for the association of Salmon Fishery Boards, said the find could be a “one-off case” and that steps to monitor the situation had been put in place.

Mr Wells said: “I certainly think it’s too early to worry but it is something we are taking seriously. All district salmon boards have been notified.

“[They have] only diagnosed one salmon on the River Spey and we’re getting advice from experts and putting monitoring processes into place.

“I think it’s important to note that while a lot died in [in the 1970s] there was no evidence that UDN had an impact on the overall salmon population. Infected fish can go on to recover and produce juveniles.”

He added that there was no evidence that the disease was passed to any eggs or juvenile salmon.

“It could be a one-off case. There’s no evidence to suggest it is widespread at the moment,” he said.

Mr Wells said there was doubt as to whether the disease was even infectious and that there were “conflicting” opinions about how likely UDN was to transmit between fish.

Director of the Spey Fishery Board and Spey Foundation, Roger Knight said: “The situation is that we have had one fish that has been diagnosed suffering from UDN. This was one of two fish that was submitted for analysis by Marine Scotland Science.

“We’re concerned about any disease that enters the river system.”

Little is known about the disease and it is unclear if UDN itself is even lethal.

“UDN appears not to be [deadly]. It’s not the UDN that generates the mortality, it tends to be a secondary infection – a fungal infection,” he said.

“The UDN is like an ulcer, an open wound or open sore and if that becomes infected it can be that that kills the fish, not the UDN. There’s a significant amount we still don’t know about UDN.”

He said it appeared that fish picked up the disease in the sea rather than contracting it in the river system and it was not known if it was viral or a bacterial infection.

The River Spey has experience two significant outbreaks in the past in the late 1960s to early 1970s and in the 1880s.

Mr Knight said anglers should not be alarmed and they should fish as normal but he advised them to disinfect their gear more regularly - especially their nets and waders if they come into contact with an infected fish.

Extra bio security measures have also been put in place along the Spey.

A Scottish Government spokesman said: “Marine Scotland Science has diagnosed UDN in an Atlantic Salmon sample from the river Spey. UDN is a naturally occurring condition in wild salmon that the fish contract out at sea, therefore limiting any preventative measures.

“At this stage it is unknown what the extent of the spread of UDN is. There is not any confirmation that other Scottish rivers are effected, however Marine Scotland is cooperating with District Salmon Fishery Boards, as we monitor incidents.”