The survey was carried out by AFBI Fisheries Scientists in July 2021 in response to water quality issues which were causing an algae bloom to form on the lake, even during the winter months. Whilst algae blooms are naturally occurring events, the reasons they occur can be complex and due to a wide range of interdependent factors, such as excess phosphates and nitrates. Analysis of the 2020 bloom by NIEA confirmed the presence of blue/green species which can produce harmful toxins to water users. It was the east side of the lake that contained the most abundance.
This survey found that the fish community of Castlewellan Lake consists of 5 species, brown trout, European eel, gudgeon, perch and roach. Abundance metrics for each species show that in comparison to the 2011 survey results the population size has remained stable for eels and gudgeon. The perch population has increased by approximately 30 %. The stocked brown trout population has halved and rainbow trout which had been recorded in 2011 where absent from this survey. The roach population had increased significantly (CPUE 2011 = 0.026 and CPUE 2021 = 0.42).
The expansion of the roach population may be linked to increased nutrient input in the lake. Whilst not yet at a level where overstocking is a concern, it is possible that in the future roach will become the dominant species in terms of both number and biomass, particularly if nutrient levels and primary productivity increase. If this occurs it could have the potential to limit the capacity for trout stocking to avoid zooplankton grazing and algal blooms which could pose problems to management of the site as a stocked game (salmonid) fishery. Stocking at the lake can continue at the current level, with targeted numbers at the pre spawning time for roach.