Shimna River ASSI

The area, which includes the Shimna River and one of its main tributaries the Trassey River, is of special scientific interest because of the physical features of the river and its associated riverine flora and fauna.

The Shimna River rises on the slopes of Ott Mountain and flows into the Irish Sea at Newcastle. It is one of the best examples in Northern Ireland of an upland, oligotrophic (base-poor) river. Apart from the presence of Fofanny Dam, the river is in a highly natural state due to limited human impact. It is of particular note for the naturalness of the river channel, which exhibits all the physical attributes of in-channel features, flow and riverbed types typical of unaltered upland rivers. [picture of a stone loach] A short, fast flowing spate river, the Shimna River is characterised by sequences of riffles, runs and pools where its gradient is shallow and the river beds are composed of cobbles, with scattered boulders and sandy margins but where the gradient is steep and the bed composed of bedrock and boulder the flow is more dramatic with rapids, cascades and water falls.

The aquatic plants reflect the nutrient-poor and highly acidic character of the water and are dominated by mosses and liverworts. In the upper reaches the Shimna River and its tributary, the Trassey River, compressed flapwort and filamentous green algae dominate the channel, with water earwort, flagellate feather-moss and bulbous rush appearing in the channel further down stream. As the Trassey meets the Shimna there is a marked increase in diversity but again mosses and liverworts predominate. Here, and as far down river as the outskirts of Newcastle, long-beaked water feather-moss, Alpine water-moss, greater water-moss and claw brook-moss dominate the channel, and rusty feather-moss, fox-tail feather-moss, yellow fringe-moss and the liverwort overleaf pellia dominate the boulder tops and wet margins.

Another very common bank side moss along the bottom half is the rare Holt's mouse-tail moss for which there are only two other records for Northern Ireland, while at the junction of the Shimna and Trassy Rivers Portuguese feather-moss can be found at its only known location in Northern Ireland.

The Shimna River provides excellent habitat for spawning salmonids, with populations of Atlantic salmon, brown trout and sea trout present. Other species inhabiting the system include minnow, stone loach, 3-spined stickleback and eel.

Marginal semi-natural vegetation along the Shimna River is limited, except at its headwater, and is generally confined to a narrow belt of woodland. This woodland is mainly confined to the riverbank and adjacent slopes. The woodland is generally acidic (calcifugous) in type with a variable structure and composition. Some of the commoner trees and shrubs include downy birch, hazel, goat willow and hawthorn. However, in a few locations, impressive mature sessile oak line the river. The field layer is a mixture of bramble and ferns while the ground cover supports carpets of bluebell, wood anemone and greater wood-rush.