Inland Fisheries Ireland says it's investigating what's described as a "severe algal bloom" on Lough Sillan in Shercock.
Concerns have been raised about water quality in the lake in recent days.
A spokesperson for Inland Fisheries told Northern Sound News that water levels have dropped significantly and temperatures have increased which are having "a huge impact on it".
They say no fish kill has been detected in the lake but monitoring is ongoing.