“No specific factors” were established as having caused the recent death of nearly 1,100 wild Atlantic salmon in a Co Sligo river, but gill-damaging phytoplankton may have contributed, the national fisheries conservation agency has said.
Inland Fisheries Ireland (IFI) conducted “extensive” investigations into the fish kill in the lower Ballisodare river, south of Sligo town, over the summer but it could identify no more than “potential” contributory factors, an IFI spokesman said.
The investigative report suggests at least some of the salmon experienced stress at sea, perhaps linked to excessive levels of phytoplankton in Drumcliff Bay that can damage salmon gills and compromise breathing, the report said.
This stress may have been further exacerbated by low water conditions in the river, where significant numbers of salmon congregated, which could have facilitated disease outbreak, it went on.
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There was no evidence of a substantial pollution event in the affected stretch of river, the report said. However, water samples taken in mid-July were unlikely to have shown any single pollutant that contributed to mortalities observed from late June, it said.
It noted with “general concern” that ammonia levels in the effluent discharge of the Ballysadare wastewater treatment plant exceeded its licence limits in recent years. The results were clear immediately before the first mortalities were recorded near to the facility.
Between last June 28th and July 21st, 1,079 salmon deaths were recorded in the Ballisodare river, but the actual number of mortalities could be “somewhat higher”, the report said. No other fish species appears to have been affected, the IFI said.
The IFI’s deputy chief executive, Barry Fox, said the organisation is “deeply saddened at this unprecedented and tragic loss” of such a large number of wild Atlantic salmon.
The report recommends, among other measures, that there should be weekly phytoplankton sampling in the Ballisodare estuary before and during the period when salmon return.
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