Agriculture most significant cause of fish kills over five decades, study finds

Agriculture was the most significant reported cause of fish kills in Ireland from 1969 to 2022, accounting for 23 per cent of overall causes, according to a new report.
Inland Fisheries Ireland (IFI) has published a detailed study on 2,107 fish kills that occurred in Irish rivers and lakes from 1969-2022. The report is a first comprehensive overview of related trends, locations, causes and population recoveries over the 53-year period.
The most common known cause of fish deaths after agriculture was eutrophication, which is when algal blooms and excessive plant growth deplete dissolved oxygen in the water.
Other common causes of kills were industrial, municipal, mining, construction and other activities.
[ ‘The river was effectively sterilised’: At least 5,000 fish dead in Co Cork fish killOpens in new window ]
Separately, the report shows that almost 19,000 fish died in 30 recorded fish-kill occurrences between January 2023 and July 2024. Fish mortalities included Atlantic salmon, trout, eel and lamprey of all ages.
Dr Ronan Matson, lead author on the new report, said: “Fish kills caused by pollution are heartbreaking for local communities. Many of these lethal events are needless, avoidable and criminal.
“The ripple effect of ecological damage on biodiversity is long-lasting, at a time of rapid environmental change. Best practice must always be adhered to in all works in or near watercourses.”
The Department of the Environment is carrying out a review of inland fisheries legislation. Existing sanctions are being examined, and it would include a potential update of penalties in respect of fishery, environmental and other offences.
The report found that fish kills occurred in every county since 1969. The highest percentage of fish kills attributed to municipal sources – a cause which has more than doubled since 2007 – was recorded in Co Laois, followed by Cork, Cavan, Dublin and Tipperary.
[ Investigation into Co Donegal fish kill after 300 trout and eels found dead in riverOpens in new window ]
Co Cork and Co Cavan had the highest proportion of fish kills, while Co Roscommon and Co Westmeath had the lowest in the period from 1969-2022.
Fish-kill hotspots were prevalent in the east, south and north midlands, coinciding with intensive agricultural activity and large urban centres.
Brown trout and Atlantic salmon were the main fish species impacted by fish-kill incidents, followed by roach and European eel.
Rivers were the most impacted waterbody type (81 per cent), followed by lakes (12 per cent), and artificial waterbodies such as ponds and canals (5 per cent).
[ Wild Atlantic salmon deaths in Sligo river trigger investigationOpens in new window ]
Many peaks in fish kills coincided with heatwave and drought events. Official heatwave events (defined as five consecutive days with a maximum temperature more than 25 degrees) were recorded in the summers of 1976, 1983, 1989, 1995, 2003, 2006, 2013, 2018 and 2022.
Meteorological droughts also occurred during many of these summers. In contrast, there was a relatively low number of fish kills reported in 2012 when Ireland experienced one of the wettest summers on record.