http://www.salmon-trout.org/wp-content/uploads/2017/09/Loch-Maree-collapse-A-Walker-report1.pdf
COLLAPSE OF LOCH MAREE SEA TROUT
HOW CULPABLE IS SALMON FARMING?
Author: Andrew F. Walker MSc PhD
EXECUTIVE SUMMARY
In the late 1980s, concurrent collapses of sea trout angling fisheries occurred in north-western Scotland and western Ireland soon after the introduction of Atlantic salmon (Salmo salar) farming in complexes of floating mesh cages moored in parts of bays and sea lochs (loughs), sheltered from exposure to the open Atlantic. Many of the cage units were placed close to important rivers for sea trout and salmon angling. In both countries, scientific studies identified close similarities in the patterns of sharp declines in sea trout abundance, size, longevity and fecundity. Previously unknown epizootic levels of parasitic salmon lice (Leophtheirus salmonis Krøyer) were detected on sea trout in salmon farming areas, but not elsewhere.
The world-famous sea trout angling fishery at Loch Maree collapsed from 1988 and has not recovered. Intensive salmon farming began in saltwater Loch Ewe in Wester Ross, NW Scotland, in 1987 and has continued there. Previously, in 1980, extensive catch sampling and electro-fishing of mature fish in the spawning burns (>1100 fish) was undertaken to provide representative biological details of the overall sea trout stock, repeating old studies carried out in the 1920s (catch sampling and netting). The results were almost identical, confirming a slow-growing, long-lived overall population, with many large, old, multi-annual, spawners and a stable stock structure. Further annual samples obtained later in the 1980s and until 2001 had to be pooled because of limited fish abundance. There was a substantial fall in mean length at sea ages, maximum sea age (from 11 sea winters in 1980 to 5 SW by 1997-2001) and in estimated total fecundity, prompting a sea trout stock collapse.
Later research in nearby sea loch Loch Torridon identified a two-year positive relationship between louse incidence on sea trout post-smolts near the River Shieldaig and on a local salmon farm, corresponding to the main production years in their two-year rearing cycle. These findings were supported by longterm scientific studies carried out in western Ireland and in Norway, highlighting extra marine survival problems for sea trout in areas of intensive salmon farming, particularly involving sea lice. Further research raised scientific awareness of acute physiological and behavioural impacts of heavy infestations of sea lice on post-smolts and the lethality of infection levels sustained above previously ambient densities. International tagging studies consistently showed that smolts treated for protection against lice in their first weeks at sea survived and grew better than untreated control batches. However, since there are many other interacting causes of marine mortality of sea trout, it has remained scientifically elusive to quantify their impacts at a population level.
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 is no evidence of a collapse of sea trout catches or loss of large specimens in North Coast rivers, where the coastal environment is more exposed to the open Atlantic Ocean and rapid tidal flushing. In most other parts of Scotland, reported angling catches in recent years indicate a decline in numbers, but an increase in annual mean weight of sea trout. Both trends are at least partly explained by a gradual introduction of greater angling restrictions for stock conservation. A long-term study of an indicator population in a tributary of the River Earn in East Scotland found that the annual length and age distributions of the sea trout spawning there remained stable between the early 1980s and 2001, contrasting with north-western river systems.
Unfortunately, knowledge of Scottish sea trout stock dynamics remains weak and monitoring is patchy. Overall, while angling catches are assumed to be indicative of trends in abundance, they have many flaws. In consequence, potential crises are not sufficiently identified and accepted in time to take proper remedial action. Salmon farming has become a massively important industry, with current production already dwarfing wild sea trout and salmon populations and likely to grow substantially. Its technology continues to improve, but the present heavy dependence on floating cages with free water exchange with the open environment is in urgent need of change. In the meantime, a decision should be made to relocate existing salmon farms well away from river mouths, using scientifically predicted transport distances for sea lice infective stages under prevailing local hydrological conditions.