Atlantic Salmon

Atlantic Salmon

The Salmon, the ‘king of fish’, is under pressure. The number of salmon returning to Scottish waters has dropped. It is an emotive issue. Some blame fish farming for increasing the number of fish lice. Some think that climate change has a hand in the decline because salmon need cold water and the rivers and burns may be getting warmer. Warm water does not dissolve oxygen as well and salmon are active fish and need oxygen. Eutrophication may be involved. Some might say that hydro-schemes have blocked the way with inefficient fish ladders or have turned the smolt heading to the sea into sushi as they pass through the turbine blades. I don’t know what combination of these reasons, if any, or what other reasons might be responsible. I do know that there are very strong opinions.

The salmon's life cycle is complex. The eggs are laid in a crude nest in the gravel on the bed of a burn or river. This nest is called a Redd.

The eggs hatch to produce an alevin which initially is fed by an egg sac. The alevin becomes a fry which emerges from the gravel and has to find food. The next stage has vertical stripes and is called a parr. It feeds for one to three years before preparing itself for salt water at which stage it is called a smolt.

There is a distinct osmotic problem for fish moving between freshwater and seawater. As it goes into the sea its cells risk dehydration. When it goes the other way its cells risk swelling with water and bursting. Not too many species of fish can manage this drastic transition but the sea trout, the eel and in Scandinavia the charr are some other examples. The salmon may feed in the sea for one year or a number of years before using its amazing sense of taste / smell to find its spawning river. If it returns after one year it is called a grilse. If it returns as a fully developed adult ready to breed it will fight its way up the intervening waterfalls and rapids to get to the spawning grounds in the upper reaches of Scottish rivers. (Incidentally beaver dams are not a barrier to salmon. Before man exterminated the beaver it and salmon co-existed for millennia).The vast majority of adult salmon die soon after spawning.

Interestingly in certain parts of the world salmon encourage the growth of trees by providing them with nitrogen from their dead bodies. In the western part of North America where bears and eagles consume vast numbers of salmon the dropping of these animals fertilise the riverside forests. By this means nitrogen makes its way from the oceans deep into the continental landmass.

The number of Salmon reaching Loch Rannoch declined with the advent of hydroelectric dams, perhaps because the fish ladders were ill designed? Whatever the reason the Fishery Research lab at Faskally had a programme to reverse this by releasing parr into burns surrounding Loch Rannoch. They returned from time to time to electro-fish. This means electrocuting fish sufficiently to stun them but not kill them. The electrocuted fish float to the surface and can be counted - a way of surveying the fish which soon recover. In the Rannoch area the Errochty River is known to be an important salmon spawning water. There has been a debate about whether the alders lining the river should be cut back or indeed in places should more trees be planted to shade the water and keep it cool. I don’t know the answer to this debate.

What might eat the salmon in the Rannoch area? Well, Ospreys and Otters certainly; maybe mink but they now seem to be absent, unless anyone can tell me otherwise.

The Tay District Salmon Fisheries Board run a hatchery at Almondbank and here eggs are hatched in large numbers and the resulting fish are released in various waters to boost the number of salmon. The Almondbank hatchery is an interesting establishment and allows school visits. There has also been a series of school projects whereby the children are supplied with eggs to install in artificial redds near their school.

(2nd January 2020)

BBC report on the effect of warming waters on Welsh Salmon.

Climate change: Wales' salmon population 'at risk' https://www.bbc.co.uk/news/uk-wales-50716852