Salmon: There's trouble upstream

An epic traveller and spectacular high-jumper, the Atlantic salmon is one of Europe's most enduring creatures. For thousands of years they have roamed free across the Atlantic Ocean and spawned in the rivers and streams that flow out into the North Atlantic. Yet this remarkable animal – given the regal nickname of the "king of fish" – faces an uncertain future.

Over the past 40 years overall European stocks of Atlantic salmon have experienced a multi-decadal decline from around eight million in the early 1970s to around three million today, resulting in the lowest population levels observed since regular monitoring began in the 1970s. While stock levels in a handful of individual rivers (eg, the Tyne) have increased in recent years, due to intense and localised conservation efforts, Atlantic salmon populations in general are contracting.

Decades of freshwater pollution, habitat destruction, rampant over-fishing and reckless marine salmon farming have taken their toll.

An increasing body of evidence shows that rising sea surface temperatures and a corresponding decline in survival of salmon at sea may pose the most serious threat yet to this once abundant species. During the industrial revolution water quality in many rivers hit an historic low. Choked by sewage and severe industrial pollution, Atlantic salmon became locally extinct in rivers where they had previously flourished, such as the Rhine and the Thames. Water quality has since greatly improved and there has been some cause for celebration as salmon have recolonised some industrialised rivers. However, populations are a fraction of what they once were and salmon remain locally extinct in many of Europe's rivers. More recent freshwater issues continue to have a damaging impact. Problems range from acid rain and excessive water removal to chemical spills, agricultural pollution and siltation. Impassable barriers to migration, such as dams and weirs, are a problem, too, as they prevent adults from reaching headwater streams and good spawning habitat.

Atlantic salmon have also suffered in the marine environment. In the 1950s it was discovered that salmon from rivers in the USA, Canada and Europe, congregated in large numbers off the coast of Greenland and the Faroe Isles. A lucrative, commercial fishing operation sprang into action and unbridled over-fishing, peaking in the 70s and 80s, dealt a hammer-blow to the species.

Thousands of miles of drift-nets and long-lines were strung across migratory passages and vast shoals, destined for rivers all over Europe, were massacred. Annual catches initially soared, but then plummeted from four million to just 700,000 between 1979 and 1990. This intense and unsustainable over-fishing threatened the salmon with extinction across much of its range, prompting Icelandic angler and businessman, Orri Vigfússon, to found the North Atlantic Salmon Fund (NASF) in 1989 – an international coalition of conservation groups that work together to restore stocks of wild Atlantic salmon to their historic abundance.

Having raised in excess of £17.5 million, NASF has successfully "retired" scores of drift-net licences in the North Atlantic, significantly boosting wild stocks. Vigfússon's goal is clear: "My objective is to restore the abundance of wild salmon that formerly existed on both sides of the North Atlantic. This can only be achieved by safeguarding the fish at sea." Unsustainable drift-net fishing is still prevalent in many parts of Europe, including here in the UK, and NASF and other organisations continue to work tirelessly to keep the industry at bay.

Since the 1970s the overall catches of Atlantic salmon in the North Atlantic have decreased by as much as 80 per cent. Management measures taken to regulate the exploitation of wild stocks have dramatically reduced catches, and played a vital role in the conservation, but they have not stemmed the overall decline observed throughout their geographic range. There are many possible explanations for the continued decline, a key one of which is the impact salmon farming has on wild stocks.

There is little doubt, apart from within those groups who benefit from the industry, that salmon farming has had a disastrous impact on wild stocks, and continues to do so. Over the past 30 years, this unsustainable, environmentally damaging and poorly regulated industry has significantly contributed to the contraction of wild populations.

Salmon farms are frequently overrun by diseases and parasite outbreaks and high densities of penned salmon seriously pollute the surrounding environment. The parasitic sea louse is particularly harmful, reaching plague proportions around salmon farms and latching on to juvenile wild salmon as they pass farm cages on their seaward migration. Sea lice feed on the mucus, tissue and blood of their host, compromising their immune system and causing secondary infection which can have fatal consequences. A staggering number of salmon escape the cages. Despite pressure from conservation groups to improve the containment, around two million escape into the Atlantic every year and up to 40 per cent of all free-swimming salmon, in some areas of the North Atlantic, are thought to be escaped farmed fish. Escapees spread diseases and interbreed with wild stocks, diluting their genetic integrity.

Given all this, it is extraordinary that Scotland's First Minister Alex Salmond recently signed a trade deal with China that will see a substantial increase in Scottish salmon farming over the coming years. He was widely quoted in the media as saying: "Even if 1 per cent of the people of China decide to eat Scottish salmon, then we'll have to double production in Scotland."

While feeding the world, securing jobs and developing the economy is rightly high on every government agenda, there can be no doubt that this trade deal will have a serious knock-on effect, at a time when we should be doing everything we can to aid wild stocks. Are we to infer that the Scottish Government has given up on wild salmon?

Producing 1kg of farmed salmon requires 3kg of feed, generated from species of other wild fish. This is unsustainable and makes little sense when 80 per cent of the world's fish stocks are already severely depleted. A positive solution would be to develop more sustainable methods of aquaculture, such as rearing species that feed on primary producers like shellfish and herbivorous fish. The great irony is that farmed salmon is marketed using the iconic image of a leaping wild salmon, yet each sale of farmed salmon indirectly erodes wild stocks. The words of American ecologist Aldo Leopold spring to mind: "Harmony with land [and the natural world] is like harmony with a friend; you cannot cherish his right hand and chop off his left." Although salmon farming has had a negative impact on wild stocks, it is fair to say that other marine factors have also contributed to the decline of wild salmon. It is widely accepted that marine survival of adult salmon has fallen by more than 50 per cent over the past 30 years. There are numerous explanations for the decline, including continued over-exploitation of wild stocks at sea, the negative impact of farming and industrial over-fishing at the base of the marine food chain. It is highly likely that a combination of factors is to blame, but an increasing amount of research suggests rising sea surface temperatures may be playing a decisive role.

Over the past three decades there has been widespread sea surface warming in the North East Atlantic, including in key foraging areas exploited by Atlantic salmon. Marine scientists have found a strong correlation between rising sea surface temperature and the observed reduction in marine survival of Atlantic salmon. Research also shows that growth condition of adult salmon has fallen as sea surface temperature has risen, and there is a strong link between growth of juvenile salmon in their first year and survival at sea.

Atlantic salmon are opportunist predators that feed on a range of prey, including crustaceans and various species of fish, at the ocean surface. Scientists fear that rising sea surface temperature may be limiting food availability by redistributing zooplankton and restructuring the North Atlantic ecosystem.

There is also some evidence that ocean warming may influence the migratory routes and the extent to which they are killed by predators. Research continues in this area, but if marine survival continues to drop away, Atlantic salmon could face localised extinction in river systems, with those in southern Europe expected to be worst affected.

Many organisations have contributed to conservation efforts, but more needs to be done and fast. Governments throughout Europe urgently need to work harder to further improve the freshwater environment, halt the continued exploitation, increase regulation of salmon farming, and curb carbon emissions before it is too late.

Wild Atlantic salmon are of great economic importance, but they are also one of Europe's most celebrated creatures. They need to be conserved not just for economic gain, but for their value to the natural world. Those people lucky enough to have laid eyes on a wild Atlantic salmon leaping high into a foaming waterfall will know that the world will not be the same without the king of fish.

Robert MacDougall-Davis is a biologist and writer specialising in fish ecology