THE GREAT ESCAPE

As farmed salmon continue to be unleashed in Scottish waters, Michael Wigan fears the worst for the future of our native wild species and their gene pools

Michael Wigan highlights the risks posed to wild salmon by aquaculture escapees

Attention is finally turning to escaped farm salmon’s hybridisation with wild salmon and the loss of native strains. Is the steady corruption of wild salmon’s Ice Age genetics being smothered by surges of cage salmon unleashed into the wild? It has drawn less attention than more conspicuous environmental damage, or the alarming films of disease and deformity beneath salmon farms’ waterlines, or the seabed cemeteries of crustacean skeletons.

An escaped farm salmon looks recognisably the same. Its fins are rubbed, but it is identifiably a salmon. Salmon farming long ago selected strains from Norway which would put on weight fast. The industry keeps its genetic information secret, but it is known that many early farms are stocked with big fish from the same Norwegian river, reducing genetic variation. With escapements numbered in millions every year locals are worried about the security of their native stocks.

Newfoundland’s River Conne used to have salmon runs of thousands. Today a labyrinth of 25 salmon farms necklace the foreshore. In 2018 the river’s fish counter recorded under 500. Wild salmon’s dramatic decline moved the department of fisheries, following a huge escapement, to investigate the ancestry of smolts in 18 rivers adjacent to salmon farms using genetics. Over a quarter of smolts were wild/escapee hybrids and most rivers contained not hybrids but pure-bred escape strains. Not only were salmon in the wild disappearing but feral fugitives were displacing complicated evolved strains.

In Scotland different research methods show similar genetic mixing. Using a colour detector which only works on females, out of 16 grouped rivers 14 had young salmon deriving from aquaculture escapes. When 200,000 salmon escaped into Loch Eriboll in 1989 cage escapees were observed spawning with wild salmon.

Our scientists know that interaction between wild fish and escapees and farm/wild hybrids militates against native stocks. Hybrid juveniles grow faster than wild fish and compete successfully for food. Later, though, their survival is lower. They displace wild fish as parr and underperform them as smolts. The result is less salmon. If escaped farm fish continue to pollute the wild salmon gene pool scientists say it leads to ‘cumulative fitness depression’, and potentially to the endgame ‘extinction vortex’. The species disappears.

Look at it another way. Salmon types have evolved specially in different rivers. Even tributaries of bigger rivers have unique strains. Evolution engineers better survival chances.

On some Scottish rivers salmon migrate into the relative sanctuary of the river every month. Small runs in winter, bigger ones in summer. This extends and optimises their survival chances. If there is a pollution catastrophe in mid-summer it kills those fish in the river; the ones idling offshore are unperturbed. If seals are chasing them at sea, they seek refuge in rivers. The Atlantic salmon is an evolutionary miracle.

Some aquaculture fugitives head for Norway, where their progenitors hailed from. A Norwegian salmon, differently programmed, has a tighter timeframe in which to procreate. Migrators arrive from June. They are all finished river-entry by July or August. If winter is going to arrive fast and freeze flowing water, these salmon breed while there is still some heat and flow. The far north salmon has a compressed breeding cycle.

If Scottish Atlantic salmon were replaced by Scandinavian aliens the bone-chilling ‘extinction vortex’ could result from limited flexibility. The run timing would focus perilously on mid-summer, missing early and late seasons.

When hundreds of thousands of farm salmon escape to mingle in the wild it is not a neutral event, or just someone’s revenue loss. Nature takes a hit. Hardly any are recovered, unlike in Norway, where the fish farmer’s legal obligation is to find them. Smolts caged in Scotland’s freshwater lochs escape too, spreading their debilitating genes earlier on.

Feral salmon loosed in millions on the wild every year explain why containment aquaculture systems, being trialled in many places, are critical to the survival of our champion fish.

“Interaction between wild fish and escapees militates against native stocks