Natural Resources Wales have identified the cause of an agricultural pollution incident which took place on 11 July on the Afon Wygyr which has impacted a section of the watercourse and bathing water at Traeth Cemaes.
NRW officers have taken water samples and completed ecological impact assessments.
Several hundred fish including trout and flounders have died in the affected reach.
Once their investigation is complete, they will decide what further regulatory action needs to be taken.
Isle of Anglesey County Council is advising against bathing at Cemaes Beach until Tuesday, 18 July and signage has been put in place to warn members of the public.
The council’s beach wardens will be advising visitors against swimming over the weekend.
People can report an environmental incident to NRW’s freephone 24-hour hotline on 03000 65 3000.