By Richard Baynes

SCIENTISTS and fisheries bosses are braced for the arrival of hordes of alien salmon in Scottish rivers after an “explosive invasion” of the fish in Scotland two years ago, possibly driven by climate change.

Anglers have been put on alert to report sightings of pink or humpback salmon, which could harm native river life. Experts are on standby in case the fish are found to be carrying disease.

If they arrive it is expected to be later this month or next. Anyone who catches one has been told to kill it and send it to fisheries managers. Anglers are also warned to look out for spawning fish, which could mean populations becoming established.

Only a handful of humpback salmon, native to Pacific North America, were found in Scottish waters until 2017. That year 139 of them were seen, with some spotted laying eggs in the Spey, Ness and Aberdeenshire Dee.

The fish have a two-year life-cycle and come back from the sea in odd years, so scientists have been waiting until this year to see if the invasion happens again, and if pink salmon have a breeding toehold here.

If they get established scientists say they would be difficult to shift, and their nest-building could damage the rare lamprey.

They are highly aggressive when defending nests, so could attack native fish, and could also possibly compete for food, and interfere with native salmon migration and the life cycle of very rare freshwater pearl mussels. Pink salmon die after spawning, and another concern is the of effect masses of dead fish on rivers.

Chris Conroy, director of the Ness District Salmon Fishery board, filmed pink salmon spawning in 2017 after anglers caught two in the River Ness.

He said: “We have footage of them chasing trout, they are very territorial. They just shouldn’t be here.”

Writing in the Journal of Fish Biology, experts from Scottish Natural Heritage, Marine Scotland and Fisheries Management Scotland described the 2017 invasion and set out concerns over Oncorhynchus gorbuscha – the scientific name for the fish.

The authors – John Armstrong, Colin Bean, and Alan Wells – say the fish originate from stocks introduced to western Russia, which got into Norwegian rivers and then crossed the North Sea.

They list just 13 pink salmon detected between 1960 and 2015 in Scotland, with the maximum in any year being five in 1973, and just one in 2015.

They say: “Against this sporadic appearance, the 2017 reports represent an unusual and explosive invasion of O. gorbuscha to Scotland.

“A total of 139 ... were recorded across a broad range of rivers”

No explanation has been found for 2017’s invasion, but the scientists say: “Of greatest concern is that, if progressive change in climate is a root cause, then invasions of O. gorbuscha to Scottish waters will probably continue. At this stage, however, the magnitude and frequency of such occurrence cannot be predicted.”

They believe a breeding population of pink salmon is unlikely as things stand, as they spawn early here and migrate to sea in the autumn when food is scarce, but if they begin spawning later in the year it might mean spring migration, boosting survival chances and the likelihood of the fish breeding here.

Most deliberate introductions of the salmon have proven tricky, but the team says the introduction in western Russia was “very successful”: that could mean invaders in Scotland are genetically selected to be good invaders.

The scientists say rooting out established pink would be difficult. Digging up nests was deemed “too resource intensive” and risky, and netting them would damage other fish including native salmon.

Chris Conroy added: “If their numbers build up and they’re at the same feeding grounds at sea they could compete with our salmon for food. It’s an unknown – they’re not native and any non-native shouldn’t be here ... they could potentially be carrying disease. We don’t know what the risk is, so I class that as being of concern.”

Jo Long, senior conservation policy officer from the Scottish Environmental Protection Agency, said: “One bad year doesn’t indicate a trend of any kind, so the real focus is on gathering information and trying to assess whether this is a sign of things to come.

“We’re much more organised this year to gather the information to help answer these questions

“We’ve got everyone on the alert in the angling community so we can get much better data and see if this is something we should be worried about.”

She added: “To date there have been no diseased fish of those that have been reported. If there were signs of diseases we would be looking at those fish in detail and we’ve got plans to do that if that should happen.”