A scientific report from Inland Fisheries Ireland (IFI) has found that "no specific factors" could be conclusively established as the cause of death of 1,079 adult salmon in the Ballysadare River in Co Sligo.
Extensive investigations undertaken by IFI and partners identified a number of contributory factors that may have potentially been involved.
The IFI report suggests some salmon returning to the river in a short period of time may have already experienced some stress at sea which was potentially further exasperated by the low water conditions in the river.
It added that 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 report also suggests that 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.
Scientific investigations and sampling were carried out by IFI, and partners at the Marine Institute and the Department of Agriculture, Food and the Marine.
No evidence of a specific substantial pollution was present in the water samples taken from the affected stretch of the river and no other species of fish appear to have been affected.
Water sample tests were taken in mid-July and so were unlikely to have detected any single pollutant that could have potentially caused such mortalities observed from late June.
The IFI report noted separately with general concern that ammonia levels in the effluent discharge of the Ballysadare Wastewater Treatment Plant exceeded licence limits in recent years and for some periods this year.
The wastewater facility is adjacent to the location of the mortalities.
Deputy CEO of Inland Fisheries Ireland Barry Fox said they "remain deeply saddened at this unprecedented, and tragic, loss of such a large number of wild Atlantic salmon".
He added that IFI and the 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 Ballysadare 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, suggesting a provisional mortality rate for stock of 12.8%.
The catchment has reopened for angling on a catch-and-release-only basis after it was closed on 17 July.
