Salmon Stressors

Over the last few centuries the environments the Atlantic Salmon inhabits have changed - mainly because of MAN and his interaction with the environment. Climate Change is a major factor - there is no doubt that temperatures are rising throughout the world. Here in Scotland, recent summers have been hot and dry causing river temperatures to rise with consequent stress to salmon. We are also now experiencing more dramatic storms with heavy, incessant rain which is causing major spates. At the wrong time of year, ie spawning time, these spates could wipe out eggs, forcing a rethink of mitigation for these events and how hatcheries might help.....

Sea temperatures are also rising which are having effects on the marine environment. These effects may well slow growth rates influencing when fish return to our rivers. Salmon SOS cannot do much about issues at sea - we must leave that to Governments and the larger charity organisations. But we can do something closer to home, on our rivers and tributaries:

BARRIERS:

The industrial revolution saw water powering paper and cotton mills and eventually hydroelectric schemes. To harness the power of water, dams, weirs and lades were built. Many still remain today and are major barriers to fish migration – mitigation of the effects of barriers were introduced by incorporating fish ladders but many barriers are impassable to salmon, with no access to potential spawning grounds.

Barriers sometimes combine with abstraction to cause severe issues for migrating salmon – both for smolts leaving the river and adults returning to spawn. The Ericht is a classic example where the upper weir was built to divert water down the lade to power the mills. The lade now feeds a hydro turbine and a fish farm – over abstraction in low water means the lade takes most of the water, leaving the river below the weir virtually impassable to salmon causing them to hold in deeper pools, in warming water and exposing them to disease and death.

This shows you the size of fish that Cormorants enjoy!

PREDATION:

The very nature of the salmon’s life cycle means that it is subject to predation at every stage –from early life as fry, to parr and smolts and adult salmon returning to the river to spawn. They are particularly vulnerable as smolts as they gather in thousands to migrate. River predators include brown trout, pike, otters and piscivorous birds. Cormorants and goosanders/mergansers are the main bird culprits.

Flocks of cormorants numbering over 100 have been seen on the Tay and the same number of goosanders have been seen massing around the mouths of Tay Tributaries, waiting to pick up smolts on their migration.

Cormorants need approximately 480g of fish a day and goosanders around 380g. A flock of 75 goosanders preying on migrating smolts which weigh around 100g are not long in causing considerable damage – 300 smolts/day.

ABSTRACTION:

Water is taken from the rivers of Scotland mainly for agriculture, and for hydro electricity generation.

Intensive agriculture demands water for growing crops and abstraction for this has increased with the advent of drier, hotter summers.

Abstraction in these hotter, drier conditions causes the river levels to drop considerably to the point where access to the upper spawning grounds is not possible and returning salmon must hold in deeper pools where possible, waiting for rising levels after rain.

POLLUTION:

Pollution can be a major problem in rivers when run off from intensive agriculture sprays and fertiliser makes its way into rivers. Timing of application and proximity to waterways is strictly controlled.

Less well controlled and now more of a problem with increasing human populations, is when water treatment plants cannot cope with large quantities of water entering from town and city sewers after heavy and prolonged rain storms – raw sewage will cause damage and even death to immature and mature salmon alike.

Fish farm in a west coast sea loch - probably right on a migration route!

AQUACULTURE:

Salmon farming on Scotland’s West Coast may not, at first glance, be a cause for concern over the decline in Atlantic Salmon.

However, whilst the companies exploit the general public’s view of Scotland’s pristine river and coastal waters and the vision of leaping wild salmon, the truth is very different.

Below the open cages of these factory farms, lies a morass of organic waste excess feed and chemicals which is destroying life on the sea bed.

More important, these farms are magnets for vast clouds of sea lice, an ecto-parasite which attaches itself to salmon and feeds off mucus, skin and blood, eventually killing the host fish. Vast clouds of sea lice have been detected up to 30Km from the cages and this could extend to 75Km in certain weather conditions. Smolts leaving our rivers, and returning adults, must travel through these clouds and will certainly pick up quantities of lice – and any more than 10 will almost certainly kill a juvenile fish.

Signal crayfish - introduced by man and impossible to get rid of

INVASIVE SPECIES:

Species introduced outside of their native range (e.g., North American signal crayfish, American mink, pink salmon) can have direct (e.g., predation, competitive exclusion) and indirect (e.g., habitat alteration) negative effects on Atlantic salmon populations. Non-native plants (e.g., giant hogweed, Japanese knotweed) may have impacts on salmon by their effect on river bank erosion.

EXPLOITATION:
Salmon suffer direct and indirect mortality through legal and illegal forms of fishing, including rod-and-line, coastal and in-river net fisheries. Voluntary catch and release measures, changes to annual close times to protect vulnerable spring stocks and, since 2016, statutory prohibitions on the killing of salmon in coastal waters and certain inland waters have reduced fisheries-related mortality in recent years. One area which needs close attention is the by-catch of wild salmon at sea by enormous pelagic fishing vessels.

DISEASE AND OTHERS:

Salmon can be host to a wide range of pathogens and parasites that can affect growth and survival. Diseases can be bacterial (e.g., Furunculosis) and viral (e.g., infectious salmon anaemia). Red Vent Syndrome (RVS) caused by the parasite, Anisakis, has been highlighted as a cause for concern in recent years.

STOCKING:

Stocking carried out under "Best Practice" can bring significant benefits to rivers. However, if best practice is not followed, stocking can bring about serious issues for the wild salmon stocks. This is why stocking is so contentious. Recent stocking exercises in Scotland have proven that stocking works and, therefore, MUST be a consideration to help recover stocks.