A SIGNIGICANT freshwater fish kill 15km west of Albany earlier this month has been attributed to a large bloom of blue-green algae affecting the waterway.
Residents in Elleker living adjacent to Marbellup Brook first notified water authorities of the bloom at the end of January when hundreds fish were found dead along a 1.4km stretch of water within the Torbay catchment area.
Phytoplankton analysis of water samples taken from Marbellup Brook by the Department of Water confirmed the bloom was cyanobacteria Nodularia spumigena, a species not uncommon to waterways in the region.
Nodularia is known to produce a liver-damaging hepatotoxin that can adversely affect birds and mammals, but evidence in the water samples showed no sign of any microalgae species toxic to fish.
Department of Water south coast regional manager Brett Ward says analysis indicated the algal bloom was in late stages of decomposition when samples were taken.
“During the decomposition phase [of an algal bloom] oxygen is taken up, which often depletes the system, as indicated by the very low oxygen levels present at the time of sampling,” he says.
“This supports the likely cause of the fish deaths being due to the low oxygen levels.”
At least 100 black bream and five mullet were officially recorded dead.
However, department staff were unable to obtain suitable fish samples for analysis due to decomposition, thus were unable to conclusively state oxygen depletion caused the fish-kill event.
Low rainfall over winter resulting in reduced stream flow, combined with recent hot weather has been attributed to the increased risk of algal blooms, which according to many Elleker residents have been a common occurrence throughout the Torbay catchment for 25 years.
The annual stagnation of Marbellup Brook also has residents concerned as provides conditions for the spread of mosquito-borne Ross River virus (RRV).
At least two recorded incidents RRV have been recorded in the area in the last seven years.
The Water Corporation has installed a temporary pumping operation in the affected area to help improve water circulation, aiming to minimise reformation of large algae blooms.