Following the retreat of the ice sheets from northern Britain some 10,000 years ago, a massive block of ice was left stranded just north of the Firth of Forth in what is now south east Scotland. As the climate slowly warmed, the ice melted, and its water cut a river into the Forth at Largo Bay. The lake that now fills the depression made by this gigantic block of ice has become the most famous trout water in the world – Loch Leven.
Since the middle of the 19th century, the Loch Leven strain of brown trout has been transported to many regions that were formerly far beyond the range of any trout species: Argentina, the Indian subcontinent, the Falkland Islands, Australia and New Zealand. In North America it now lives alongside the native rainbow and cutthroat trout. And in fisheries throughout Europe, trout farmers take pride in the knowledge that their brown trout originate from the Loch Leven strain.
Presumably Loch Leven was colonised soon after the ice melted, by trout from the sea running through the Forth and the River Leven. Over thousands of years, the trout population became specially adapted to life in the loch. They acquired a coloration that presumably camouflages them from predators better than any other. They became adapted to feeding on the invertebrates with which the Loch abounds, especially crustaceans such as the freshwater shrimp and planktonic Daphnia, which are rich incarotenolds and produce the appetising redness of the trout flesh.
The quantities of trout caught on rod and line, all on fly, from Loch Leven have in the past quite immense for the size of the water. In 1929 it is recorded that 53,605 trout were taken at an average of 15 trout per acre. Considering that the average weight is close to the pound mark since records began in the 1870s, it is clear that the Loch can be a very productive fishery.
These days, catch records are not comparable as the majority of trout are caught & released but recent seasons have seen a discernible recovery in catches following several decades of decline. The factors behind that decline most probably relate to the deterioration in water quality that accompanied among other things increased population within the catchment area and more intensive agricultural practices. Measures introduced since Scum Saturday (13th June 1992) when a blue-green algal bloom created national headlines, have seen water quality improve dramatically as levels of phosphates / nitrates going into the Loch have fallen over 60% from pre-1992 levels.
In former centuries, Loch Leven was about four miles long and three miles wide. But in December 1830 a drainage scheme was completed that dropped the water level of the Loch by up to 9 feet and reduced its area by almost a quarter. The scheme also involved cutting a new channel for the out-flowing River Leven and creating sluices to control the flow of water from the Loch.
The appearance of the Loch before the drainage can be gauged by the visitor at the old churchyard of Kinross. Originally the water lapped at the foot of the churchyard wall. On Castle Island, when Mary, Queen of Scots was imprisoned there in the 1560s, the Loch reached the battlements. Today the Loch reveals seven islands, but prior to the drainage there were but 4: St Serf’s, Castle, the Reed Bower and Roy’s Folly. Most of the Loch is now very shallow, with the exceptions of two 60-foot holes to the east of Scart Island and around the western and southern sides of St Serfs. Before 1830, the large area known as “The Shallows” was more than twice its present depth.
This massive alteration has had major effects on the fish populations of Loch Leven. Salmon, and possibly sea trout, ran the old River Leven and they are now gone. So too is the charr which, presumably, could not tolerate the shallower water. The pike too also almost became extinct here, but not because of the drainage: it was exterminated to protect the trout stocks (in 1903, around 14,000 pike were removed by netting). However recent seasons are showing signs that the pike population is on the rise again, so too are Perch, both of which is encouraging as it confirms the Loch is returning to rude health.