Foyle and Carlingford’s many rivers and loughs provide ideal habitats for good populations of brown trout. Unlike the anadromous form known as sea trout, the brown trout spends all its life in freshwater.
Loughs Agency runs several projects aimed at conserving, protecting and improving brown trout populations. Annual, semi-quantitative, 5-minute electrofishing surveys are conducted to study trout numbers and distribution. This information is used to monitor changing trends.
Trout size depends largely on water chemistry and on the quality of invertebrate life it sustains. In the low-nutrient, upland loughs of Donegal, for example, trout are smaller but numerous. Larger trout can be found in lowland loughs and rivers, and where limestone outcrops provide better feeding conditions.
The Lough Derg Wild Trout Conservation Project ran for a number of years and identified the key spawning habitats for this large lake trout. Genetic stock identification, or GSI, techniques were used to estimate the proportion of fish using inflowing and outflowing tributaries. As a result, habitats have been improved and potential barriers to production eased.
Brown trout are possibly the most abundant native fish in Ireland and the biodiversity of their populations is staggering. Using techniques like GSI, the fishery manager can conserve and protect this diverse resource appropriately.
It is not fully understand as to why sea trout migrate. One theory is that it is caused by a combination of genetics and a desire to gain competitive advantage through improved feeding.