A naturally occurring algae bloom - commonly known as "red tide" - has hit the western seaboard in recent weeks.
The microscopic planktonic species called Karenia mikimotoi leads to a discolouring of water, and large concentrations can lead to the death of shellfish such as Pacific oysters and cockles, fish that live near the sea-bed, such as plaice, and other marine organisms including crabs.
In recent days there have been reports of finfish and shellfish killings off Killybegs and Sheephaven Bay in Co Donegal and off Killary Harbour in Co Galway.
Yesterday, the Marine Institute reported a "marked decrease" in the cell concentration counts off the western seaboard. However, it warned that "all affected areas will be continuously monitored and extra phytoplankton samples will be taken".
The last serious outbreak of red tide occurred in 1995 and the current bloom is under continuous surveillance by the Marine Institute. The organism has no impact on human health but is of concern to certain sectors of the fishing and shellfish industry.
According to Dr John Joyce, spokesman for the Marine Institute, the bloom can spread where the algae is in still, sunlit water with high nutrients.
"What we want now is a continuation of dull weather. We do not want sun to trigger its re-emergence," he said.
Although the red-brown bloom can be seen on the surface of the sea, it is when it dies and drops to the seabed that it can lead to the suffocation of marine life.