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Reasons for Decining Salmon and Sea Trout Stocks in Scotland

Sea trout runs would appear to have been in general decline in many British rivers since the late nineteen eighties. The most dramatic, and catastrophic, decline in sea trout stocks occurred on the west coasts of Scotland and Ireland, coinciding with the rapid growth of salmon farms from around the mid nineteen eighties. At one time, some of the best of our Scottish sea trout fishing was to be found on the great sea trout lochs of the north west highlands, on Lochs Maree, Shiel, Eilt, Stack, More and Hope and on the great loughs of western Ireland. Sadly, the fishing on the great sea trout lochs is not what it once was. Stocks of sea trout have collapsed in recent decades. Catches on the once famous sea trout lochs and rivers are a mere fraction of what they were. Boats on the lochs lie idle, hotel rooms empty.

A scientific study on the sea lice problem associated with the salmon farms ["Patterns of Sea Lice Infestations on Scottish West Coast Sea Trout: Survey Results 1997 - 2000" published by The Association of West Coast Fisheries Trusts] reported:

    "In areas with epizootics (outbreaks of disease affecting many fish at one time), lice can directly cause the mortality of 30% to 50 % of all migrating sea trout smolts and 48% to 86% of all wild salmon smolts..… Studies in Norway, Ireland and Scotland estimate that, in salmon farming areas, most sea lice larvae are produced from farmed salmon, due to the far greater numbers of farmed hosts relative to wild hosts. This is reflected in significantly higher lice infestations on wild sea trout in salmon farming zones compared to farm free areas in Ireland and Norway. Similarly, in Scotland the highest burdens found on sea trout occurred in the salmon farming zone of the west coast. Consequently, in Norway, western Ireland and western Scotland, lice infestations are regarded as a major factor in the decline of wild salmon and sea trout populations...."

The catastrophic impact of salmon farming on stocks of wild sea trout on the west coast of Scotland is now widely accepted. A recent report, in 2017, by Dr Andrew F. Walker MSc PhD on the Collapse of Loch Maree Sea Trout stated:

     "Nevertheless, it can be concluded that the introduction of salmon farming in Loch Ewe close to the River Ewe’s estuary played a prominent part in the changes in sea trout stock dynamics in the River Ewe system, leading to the collapse of the angling fishery in Loch Maree. The rapid change in sea trout stock structure there in the late 1980s was consistent with many other badly affected sea trout fisheries throughout the West Highlands and Islands following the development of local intensive coastal salmon farming."

There can be little doubt as to the extent and nature of the problem. While the Scottish Government, a strong supporter of the salmon farming industry, continued to turn a blind eye to the unfolding environmental catastrophe, it seemed clear to many of us that the sea lice problem associated with the salmon farms had been a major contributory factor in the decline of sea trout stocks on many west coast rivers and lochs. Salmon stocks face an additional threat from the many escaping farmed salmon entering our rivers, where they may interbreed with wild fish, weakening the genetics and future development of the wild salmon stock. It is to be hoped that stricter controls combined with improved production techniques and a growing environmental awareness on the part of salmon farmers, fishery owners and politicians might one day halt, and perhaps even reverse, this sad decline and that the sea trout, and the fishermen, might one day return to the West Highlands.

In the meantime, there had been a general, and perhaps more baffling, decline in sea trout stocks also in areas where no salmon farms existed, for example in the rivers flowing into the Moray Firth and Solway Firth. In the summer of 1989, the Solway rivers appeared to suffer a particularly dramatic collapse in sea trout runs. Indeed, the total Scottish sea trout rod catch shows a similar calamitous fall at around the same time, the 1987 total rod catch of 55,112 sea trout falling to 48,386 in 1988, 43,285 in 1989 and 28,060 in 1990, a fall of almost 50% in that three year period. The downward trend in Scottish sea trout rod catches has continued fairly steadily ever since, to a depressingly low total of 16,899 in 2019. Why, we might ask, are our sea trout stocks now in such general decline? It has become clear that, despite their amazing resilience, our sea trout face a multitude of threats to their very survival, in both their marine and freshwater habitats.

I have noted the grave problem of sea lice infestation associated with the growth of salmon farming in the north west of Scotland and in the west of Ireland, undoubtedly a major factor in the declining fortunes of our sea trout. We may cite further problems in the marine environment. Gradually rising sea temperatures, associated with climate change, are thought to affect the availability and location of many of the prey species on which salmon, sea trout and other fish species depend. Over-fishing of salmon at sea increased on the discovery of the main salmon feeding grounds in the North Atlantic, while over-fishing of particularly important prey species such as sandeels around our coasts has similarly reduced food availability, not only for fish but also for many of our seabirds.

The uncontrolled growth in populations of both grey and common seals around our coastline over the past few decades has posed a severe predatory threat, while a similar increase in the numbers of protected species of avian predators, in particular mergansers, goosanders and cormorants, poses an equally serious threat to young salmon and sea trout in the river environment. Sea trout continue to be netted around our coasts, albeit at a reduced number of locations, now concentrated along the north east coast of England.

Afforestation in the upper catchments of our rivers has led to problems of both high levels of acidification and increased run-off of rainwater, resulting in excessive erosion in some parts of the river and excessive deposition in others, both of which may have an adverse impact on sea trout and salmon spawning streams. Such large and sudden variations in river flows are likely to be exacerbated by climatic change which may alter the flow regimes of rivers, resulting, for instance, in extended drought in summer but milder, wetter winters causing big winter floods.

While many historic barriers to sea trout and salmon migration, such as dams and weirs dating from early industrial times, have been gradually removed or modified, others are now appearing in the form of mini hydro-electric schemes, tidal barriers etc. which hinder both the upward migration of salmon and sea trout and the downward migration of smolts in the springtime.

Finally, while much of the pollution of our rivers from industrial sources has been reduced dramatically in recent decades, pollution from other sources has increased. Agricultural insecticides, pesticides and fertilisers, including slurry, have found their way, in ever higher concentrations, into our rivers, with their seriously adverse impact on both fish and invertebrate life. Despite advances in sewage treatment, the growth in the size of human settlements on our river banks has been associated with a general increase in river pollution in recent times, while the increased demand for water, for both human and agricultural use, has led to increasing water abstraction and lower river flows throughout the increasingly hot and dry summer months.

See also

Trout and Salmon Fishing in Scotland

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