An investigation has been launched following reports of dead fish, blood and scales polluting a river in Fort William.
Locals spotted the emission - which they said smelled like "rotting fish" - coming from a waste pipe at the Lochyside flood defences into the River Lochy on Sunday.
Scottish Water and the Scottish Environment Protection Agency (Sepa) are looking into the matter.
Seafood provider Mowi Scotland, which has a fish processing plant in the nearby Blar Mhor industrial estate, said they had carried out their own investigation and that the contaminants did not come from their facility.
Activist Don Staniford, who campaigns with the group Abolish Salmon Farming, said the incident was "nothing new" and locals see blood in the river "all the time".
Mr Staniford, who lives in the area, told BBC News locals had seen blood-tainted water coming from the outflow pipe on Kilmallie road, near Caol beach, and shared pictures to social media.
He said they reported the incident to Sepa.
He said: "It’s an absolute horror-show out there. It’s stinking, smelling like rotten fish.
"There were so many scales very heavily concentrated around the area of the out pipe but the blood went the full length of the river.
"Locals have seen blood here before. Things like this happen all the time."
Following a visit to the site of the incident, Mr Staniford believes the dead fish in the water to be wrasse.
Wrasse fish are used to keep farmed species such as salmon free from sea lice and helps minimise the use of chemical treatments.
Mowi Scotland is the UK's largest supplier of farm-raised salmon with 48 farms producing 68,000 tonnes of salmon annually.
A spokesperson said: "Mowi has undertaken its own investigation into the incident at the River Lochy where local people reported a red discharge.
"Mowi is confident that the discharge did not come from its facility at the Blar Mhor Industrial Estate."
The company said it had checked all of its operating procedures, its effluent system and CCTV.
It said effluent is only released to the Scottish Water treatment works at Caol Point, less than two miles down the river.
The company added: "Mowi is not the only business which processes this type of material on the Blar Mhor Industrial Estate but is the only one regulated by Sepa under IPPC regulations."
A spokesperson for Scottish Water who run the waste treatment at Caol Point said the water company are working with Sepa to investigate the incident.
Sepa ask members of the public with concerns about a potential pollution incident to contact the 24-hour pollution line.