The adult and juvenile trout died after the river was polluted by "an agricultural source."A statement from the department said that the incident is being classified as High Severity.A spokesperson for the Northern Ireland Environment Agency (NIEA) said: "On Friday evening NIEA received the first report from a member of the public,via the Water Pollution Hotline, that there was pollution in the Glenavy River."Officers, acting on behalf of NIEA, travelled immediately to the site in order to carry out an investigation, determine the environmental impact and confirm the report of dead fish."The NIEA officers initially observed a number of dead trout over a distance of approximately 500 metres in the Glenavy River. Further to the NIEA investigation, the polluting discharge was confirmed as an agricultural source."The source of the pollution was identified and samples have been taken with a view to initiating prosecution proceedings. Investigations are ongoing in conjunction with DCAL Inland Fisheries."Environment Minister Mark H Durkan said that he will be "monitoring developments here closely along with NIEA officials."Glenavy Sinn Féin representative Mary Kate Quinn has condemned the polluting of the river.She said: "A few years ago this community commended Glenavy Conservation and District Angling Club on restocking the river. It saw a largest restock of 20,000 salmon fry and 40,000 trout fry."This had a positive impact on our local environment, promoted our natural resources and enhanced the local wildlife population."She said that not only had the pollution killed off some of the stock, but put the entire eco system of the river at risk."I would ask that anyone with information about this offence contact the PSNI to ensure the culprits are held accountable for their actions and are dealt with appropriately".Members of the public are asked to call the Water Pollution Hotline on 0800 807060 if they come across any water pollution incidents.Meanwhile, a separate incident is being investigated at Brantry Lough, near Dungannon, Co Tyrone involving a "significant number" of dead fish.The final number is not yet known, a statement from the Department of Culture, Arts and Leisure said."NIEA has taken water samples for analysis and DCAL staff will be taking samples of dead fish for testing by AFBI [Agri-food and Biosciences Institute]. "There is no evidence that this is a pollution incident. Initial inspection confirmed that as well as stocked brown trout, roach and perch present in the Lough have also perished."