A two-year study has concluded that extremely high levels of phosphorus from farming sources is a main factor behind the deterioration of Carrowmore lake, a prime salmon and sea trout fishery in Co Mayo.
Water quality problems have severely disrupted salmon angling on Carrowmore in recent years, resulting in major losses to local tourism.
Bryan Kennedy, fisheries catchment manager with the North Western Regional Fisheries Board, carried out the study in conjunction with the Environmental Protection Agency (EPA) and with support from Mayo County Council.
The report classifies Carrowmore lake as strongly eutrophic - rich in nutrients and so supporting a dense plant population, the decomposition of which kills animal life by depriving it of oxygen.
It says discharges of phosphorus into the lake are extremely elevated and need to be reduced by at least 45 per cent if the water quality is to be improved.
Land-use practices, principally agriculture and forestry, were found to be the main contributors to the enrichment problem. A study of the lake sediments also confirmed that phosphorus contaminations in the lakebed had increased significantly, especially over the last 10 years.
Samplings of inflowing streams revealed that the highest concentrations of phosphorus are to be found downstream of pasture land.
The report says that in addition to run-off of phosphorus from the land, farmyards may be contributing to the problem.Elevated phosphorus levels were also evident downstream of catchments containing forestry.
It notes that a forest in the Glencullin river sub-catchment area was fertilised from the air shortly before a massive flood in the area in July 1997. It says this may have resulted in a substantial quantity of fertiliser being washed into the lake.
It expresses concern about overgrazing of sheep, and refers to the possible impact of domestic septic tanks.