A scientific report by Inland Fisheries Ireland (IFI) has found that no specific factors could be conclusively established as the cause of death of nearly 1,100 adult salmon in the River Ballisodare, Co Sligo.

However, extensive investigations undertaken by IFI and partners have identified a number of contributory factors that may have potentially been involved.

IFI deputy CEO Barry Fox commented that the association is "deeply saddened at this unprecedented, and tragic, loss of such a large number of wild Atlantic salmon".

“IFI and the Irish Government are deeply committed to wild Atlantic salmon conservation, protection and sustainable management through its programmes in research, fisheries protection, habitat restoration and education and outreach.”

The fish counter at the Ballisodare system recorded a total of 8,451 salmon returning there from January 2024 to 31 July 2024.

A total of 1,079 mortalities were recorded from 28 June to 21 July 2024, suggesting a provisional mortality rate for stock of 12.8%.

Stress at sea

Overall, the IFI report suggests that at least a proportion of the many salmon returning to the river in a short period of time may have already experienced some stress at sea, which was potentially further exacerbated by the low water conditions present in the river.

“This stress at sea may have been caused by exposure to excessive levels of a species of phytoplankton (plankton bloom) with spines that can damage salmon gills and compromise breathing.

“The significant numbers of salmon congregating in the low water conditions in the river, some of which were already stressed, may have then facilitated the outbreak of disease and potentially contributed to the mortalities observed.

“There was no evidence of a specific substantial pollution event in the affected stretch of the river - based on water samples taken - and no other species of fish appear to have been affected,” according to the IFI report.

However, these water sample tests were taken in mid-July and were unlikely to have detected any single pollutant that could have potentially caused such mortalities observed from late June, it added.

Wastewater plant

The report separately noted that ammonia levels in the effluent discharge of the Ballysadare wastewater treatment plant exceeded licence limits in recent years and for some periods in 2024.

This facility is adjacent to the location of the mortalities.

Scientific investigations and sampling was carried out by IFI and partners at the Marine Institute (MI) and the Department of Agriculture, Food and the Marine.

The catchment has reopened for angling on a catch-and-release-only basis after it was closed on 17 July.