Agriculture accounts for majority of fish kills with most happening in Cork and Cavan
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Cork and Cavan have had the highest proportion of fish kills over the last half century, with agriculture the most commonly known cause, a vast new study shows.
Agriculture, eutrophication — when nutrient enrichment depletes oxygen in the water — industry and municipal activities are the most common causes of the more than 2,100 recorded fish kills in Ireland between 1969 and 2022. Rivers were the most impacted waterbody followed by lakes.
An analysis of the data shows there was a peak in fish kill reports in Ireland in the 1980s, coinciding with an intensification of agriculture here, and that Cork also had the highest percentage of fish kills attributed to either agriculture or industrial sources during the 55-year study period.
Salmonids were the main fish group affected during the period, with brown trout and salmon the main species affected. While there was a spike in fish kills in the 1980s, there has been a downward trend since 1992, the report shows.
But despite that, there have been several relatively high numbers of fish kills recorded since 2012, mainly coinciding with heatwaves and droughts.
The details are contained in the ‘Fish Kills in Ireland – History, Current Status and Recovery’ report which was compiled by Inland Fisheries Ireland (IFI) and released on Wednesday.
The study set out to digitise all available data on fish kills in Ireland from 1969 to 2022, to examine various trends for the various fish kill events and to establish a ‘geographical data layer’ to support Water Framework Directive (WFD) reporting.
The authors gathered and digitised data from 2,107 fish kill events, spanning the long study period. Their research shows that there have been four phases of reported fish kills in Ireland since 1969, with the worst years in the 1980s — particularly 1984, 1987 and 1980 respectively with a total of 347 fish kills recorded.
Fish kills were more prevalent during the summer months, when warm weather and low water levels exacerbated potential underlying problems within a channel.
The authors also used four case studies to provide information on recovery after fish kills — examining the rivers Dodder (Co. Dublin), Kiltha (Co. Cork), Vartry (Co. Wicklow) and White (Co. Louth) where fish kills occurred in 2012, 2013 and 2017.
The analysis shows that the recovery of fish populations varied from site to site and river to river, but in general, the impact of the fish kills and recovery followed a similar pattern, with total fish abundance often higher in the polluted zone than in the unaffected stretch one year after the fish kill occurred.
This was caused, in some cases, by smaller fish species — tolerant species such as stone loach, minnow and three-spined stickleback — filling the void left after the fish kill.
Certain age cohorts of brown trout and salmon were absent or present in low densities immediately, or one year after each fish kill.
The report says although there was a perceived recovery of fish populations at many of the case study sites, the fish ecological status on many sites had not improved to the required Water Framework Directive standard of ‘Good’ status.
Fish kills have occurred in every county since 1969, with agriculture the most significant reported cause over the last half century.
Agriculture accounted for 23% of known causes of fish kills, followed by eutrophication (13%), where elevated levels of nutrients in the water deplete oxygen levels. But some of the eutrophication cases could also be associated with agriculture, such as run-off from fertiliser and other sources including the release of sewage effluent, the report compiled by Inland Fisheries Ireland shows.
Industrial (12%), municipal (8%), mining (3%) and construction (2%) accounted for the other fish kills.
Overall, Co Cork had the highest percentage of fish kills, attributed to agriculture between 1969 and 2022, the study shows. This was followed by counties Monaghan, Limerick, Cavan, Tipperary, Meath and Wexford.
Co Cavan had the highest percentage of reported fish kills due to eutrophication, followed by counties Cork, Monaghan, and Laois. The lowest percentage (0.4%) was recorded in counties Dublin, Kerry, Roscommon and Sligo.
The highest percentage of fish kills attributed to municipal sources was recorded in Co Laois, followed by Cork, Cavan, Dublin, and Tipperary. Cork also had the highest percentage of fish kills associated with industrial sources, followed by Tipperary, Kerry, Dublin, Monaghan, and Cavan.
Some reported fish kills were assigned a cause due to high temperatures and drought, however, fish kills resulting from high temperatures and drought alone in Ireland are likely to be rare.
Many peaks in fish kills — in 1989 where there were 112, 1995 when there was 88, 2013 when there were 52, and 2018 when there were 40 — coincided with heatwave and drought events.
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The report authors say mortalities attributable to this cause probably happened when other pressures also existed, such as eutrophication and other forms of pollution discharge and it was potentially the increase in water temperature and low water levels that compounded the effect of the underlying water quality problem.
Official heatwave events — five consecutive days with a maximum temperature more than 25C — 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.
Fish kill events were also most prevalent during the warmest and driest months (May to September), with the lowest instances occurring during the coldest and wettest months (November to February). In contrast, there was a relatively low number of fish kills, 10, reported in 2012 when Ireland experienced one of the wettest summers on record.
The proportion of fish kills attributed to agriculture, eutrophication, industry, construction and mining was lower in the 2007-2022 period than the 1969-2006 period; however, the proportion of fish kills associated with municipal and other sources increased between both periods.
In the 1970s, the highest number of fish kills, where spatial data was available, was reported from Co Tipperary and in the 1980s from Co Cork.
In the 1990s and 2000s, Cavan had the highest, and in the 2010s it was Cork again, and since 2020, the highest number has been recorded in Cavan, followed by Cork.
Since 1969, the Erne Catchment had the highest number of reported fish kills, followed by the Lee, Cork Harbour and Youghal Bay, the Barrow, Suir, and Liffey and Dublin Bay catchments.
Across IFI’s fishery regions, the Eastern River Basin District (ERBD) had the highest number of fish kill reports during the study period, with two river sub-basin waterbodies in that district, in and around Avoca, showing the highest number of fish kill reports.
The Erne, Feale, Barrow, and Cavan were also among the top six sub-basin waterbodies with the highest number of fish kill reports.