Fish mortalities in rivers and lakes are reported as ‘kills’ if there is a strong suspicion that the cause of the mass mortality is pollution or otherwise unnatural.  Mortalities due to natural causes, e.g., salmon and lamprey die naturally after spawning, are not considered kills.  The primary cause of fish kills in Ireland is the deoxygenation of water brought about by anthropogenic inputs of organic matter resulting in excessive plant growth. Silt is a secondary driver of fish kills by clogging gills or smothering eggs and/or fry in spawning areas.

In the past 40 years, the number of reported fish kills to Inland Fisheries Ireland has declined from a peak of 235 in 1986-1988 to a low of 72 cases in 2007-2009.  From 1986 until 2006, agricultural activities were the most frequently reported cause of fish kills representing an average of 46.2 ± 3.7 % (± 95% CI) of cases during that reporting period, compared to 18.2 ± 2.0 % for industry and 13.1 ± 5.5 % for other causes.  Since 2007 however, other causes including oil spillages, high water temperatures, or extended periods of low water flow, were the leading driver of fish kills representing 42.6 ± 3.1% (± 95% CI) of reported cases in the 11-year reporting period (i.e., 2007 to 2018).

Further information on fish kills in Ireland and information on how report suspected fish kill can be found here:

https://www.fisheriesireland.ie/what-we-do/protection/environmental-protection/fish-kills

https://www.epa.ie/publications/monitoring--assessment/freshwater--marine/water-quality-in-ireland-2010-2015.php