Increased salinity levels likely cause of Vasse estuary fish incident

The investigation into a fish kill incident which resulted in the death of 3000 to 5000 goldfish in a Vasse estuary lagoon on Friday has been closed. 

The City of Busselton worked to remove thousands of dead goldfish from a Vasse estuary lagoon on Friday. The fish kill incident was likely caused by an increase in salinity when seawater was let in through the floodgates.

The City of Busselton worked to remove thousands of dead goldfish from a Vasse estuary lagoon on Friday. The fish kill incident was likely caused by an increase in salinity when seawater was let in through the floodgates.

The Department of Water found that in the recent kill of feral goldfish species, it was likely caused by an increase in salinity when seawater was let in through gates on the surge barrier.

The department reported the increase was detected from last Thursday, whereby salinity rose from 12 grams per litre (parts per thousand) to 35 grams per litre (parts per thousand).  

Department of Water district manager Dr Kath Lynch reported last Friday that the dead fish were mostly identified as goldfish and no other species had been observed to be affected. 

“There will be no pathology as the samples were too decomposed and not suitable,” the department reported.

“Given the dead fish were exclusively goldfish our scientists are confident the salinity contributed to their deaths.” 

Ms Lynch said while the numbers of dead fish were significant, the incident didn't involve native fish populations and was a positive from an environmental management perspective. 

"Goldfish are a feral fish species and large numbers are a concern to the ecology of the system," she said.

"It is likely that letting in seawater through the gates on the surge barrier contributed to these fish deaths as goldfish are freshwater species and intolerant to elevated salinity. 

"This is the first time this action has resulted in fish deaths as native fish are estuarine species and tolerant to changes in salinity as normally occurs when water levels are low in the estuary.

"It is likely that the higher water levels and longer periods of freshwater in the system favoured the goldfish population, before seawater was able to mix back into the estuary."

Ms Lynch said fish kills had been recorded at the Vasse estuary prior to the installation of floodgates and it was normal practice to open and close the gates to allow seawater back into the estuary to maintain minimum water levels over summer periods. 

"Using the gates in this way allows passage of fish between the freshwater side of the gates and the seawater side of the gates when the water quality is poor and can also improve oxygen levels in the freshwater side in these instances." 

The floodgates were installed in 1908 to stop flooding of adjacent, low-lying agricultural land with salty seawater and the current barriers were replaced in 2004.