A SCIENTIFIC REPORT from Inland Fisheries Ireland (IFI) has found that there were “no specific factors” which could conclusively establish the cause of death of nearly 1,100 adult salmon in a Co Sligo river during the summer.  

However, a species of phytoplankton that can damage gills may have contributed to the fish kill. 

The dead Wild Atlantic salmon were found in the lower Ballisodare River, near Ballisodare, south of Sligo town. 

The IFI launched an investigation into the matter in July. Other agencies involved in the probe include the Marine Institute Fish Health Unit, and veterinarians from the Department of Agriculture, Food and Marine. 

According to the report, the fish counter at the Ballisodare system recorded a total of 8,451 salmon returning there from 1 January to 31 July.

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

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

This stress 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,” the report said. 

It said there was no evidence of a specific substantial pollution event in the affected stretch of the river and no other species of fish appear to have been affected.

However, the water sample tests, which were taken in mid-July, were unlikely to have shown any single pollutant that could have potentially caused the level of deaths seen from late June.

The report noted “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 in 2024.

This facility is adjacent to the location of the mortalities.

It said that excessive levels of the un-ionised form of ammonia in water in particular “is known to be toxic to fish.” But it said it was not known whether this factor contributed individually or in combination with other potential stressors to the salmon deaths. 

“We remain deeply saddened at this unprecedented, and tragic, loss of such a large number of wild Atlantic salmon,” deputy IFI CEO Barry Fox said.

“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,” he said. 

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 (DAFM).

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

The report recommended that a risk analysis should be undertaken to assess all aspects of the river environment that may have contributed to the salmon deaths, and identify where any precautionary and mitigation measures could be implemented. 

It also recommended that there should be weekly phytoplankton sampling in the Ballisodare estuary next year before and during the period from early May to late July, when salmon return to the river.