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Hot weather raising risk of fish deaths

06 February, 2015

Actions have been taken by the Vasse Estuary Technical Working Group (VETWG) in response to hot weather and high concentrations of a potentially fish killing algae species that raised the risk of a high fish kill in the Vasse estuary at the end of January.

As part of a pilot study looking at the best way to manage the operation of the Vasse Estuary surge barriers (floodgates) the Vasse Estuary Technical Working Group (VETWG) opened the fish gates on the floodgates on 21 January 2015. This action coincided with high concentrations of a potentially fish killing algae species. The opening of the fish gates allowed a two- way exchange of water between the Vasse Estuary and the Wonnerup Inlet to improve conditions and allow fish to move out of the estuary, thereby avoiding a possible fish kill.

The algae Prymnesium spp, commonly referred to as golden algae, are microscopic algae that are capable of producing toxins that can cause extensive fish kills. This species was thought to be the main cause of the fish kill in the Vasse estuary in February 2014.

Since last February’s fish kill the frequency of phytoplankton monitoring has increased to weekly during summer and autumn, providing the data needed to make management decisions on the Vasse Wonnerup wetlands. After opening the fish gates, the concentrations of the Prymnesium spp declined significantly between 21 and 27 of January. Factors that may have contributed to the decline are being further investigated.

Increasing salinity at the Vasse floodgates for short periods to reduce harmful algae blooms is a key research question being investigated in the seawater pilot study being undertaken by VETWG this summer. Reducing the frequency and severity of fish kills and improving water quality at the floodgates is the overall objective of the study.

Monitoring also showed that dissolved oxygen levels were low from 26 January. VETWG responded on 30 January by installing pumps until the conditions improved on 2 February. The quick response from VETWG demonstrated that the fish kill mitigation and response plan is working and the partner agencies are working effectively together.

VETWG will continue to closely monitor and respond to environmental conditions that may increase the risk of fish kills over the summer/autumn period including higher water temperatures and increased likelihood of summer storms. The high risk period for fish kills will continue over the next few months. Increased monitoring and a clear response plan have, however, have improved the chances of preventing a fish kill in the Vasse estuary.