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Fish gate opening exposes feral Vasse population

15 March, 2017

Fish kills have been recorded in the Vasse Estuary for over 110 years, however the death of 3000-5000 goldfish last week was a first.

“I was relieved when I identified the dead fish as Goldfish,” Dr James Tweedley from Murdoch University’s Centre for Fish, Fisheries and Aquatic Ecosystem Research said.

“No one likes to hear of a fish kill, but it certainly is less disturbing knowing that the impacted fish are a highly invasive feral species that cause environmental damage and can contribute to algal blooms.”

Scientists from Murdoch University have been working on control programs for feral Goldfish in the Vasse River since 2003.

In 2012 their research expanded to include mapping seasonal distributions of Goldfish in the Vasse Wonnerup wetlands, tracking their movements and determining their salinity tolerances.

When James saw photos of the dead fish, he immediately recognised it as a familiar species.

“The fish look similar to Black Bream, however, Goldfish are not silver, have a more forked caudal (tail) fin and the dorsal fin has no spines.

“Juvenile Goldfish in the wild are a green-brown colour and only change to the gold we see in pet stores when they are much larger.

“Typically Goldfish live in pure freshwater water with salinity of around zero grams per litre (g/l), however, those that live in the Vasse River can survive in low salinities up to 20 g/l , which is roughly half as salty as seawater.

“This means they can live in the Vasse Estuary during spring, when salinities are low, but they die during summer when evaporation increases salinity.”

Opening the gates on the Vasse surge barrier to allow seawater into the Vasse Estuary is conducted each year to maintain minimum water levels in the estuary and wetlands.

The increase in salinity from 12 g/l to 35 g/l is therefore considered to have been the cause of death of the Goldfish.

“Estuarine fish species are much more tolerant to rapid changes in salinity, as they naturally experience such changes, and therefore opening of the gates did not impact the native fish species,” Dr Tweedley said.

“It is concerning that there were so many Goldfish in the estuary, however, without the fish kill we would not know the scale of the problem and have found a highly effective action to potentially eradicate them.”

Want to help? Never release your pet Goldfish into natural waterways as it can cause major harm to aquatic ecosystems. For more information on aquatic pest species of the Vasse Wonnerup visit: https://geocatch.asn.au/resource/vasse-feral-fish-leaflet/

“If you no longer want your Goldfish, you can take them back to the pet store or humanely euthanise them by putting them in cold water in the freezer,” Dr Tweedley said.

Dead Goldfish; note the forked caudal (tail) fin and the dorsal fin with no spines – Photo Kath Lynch

Baby Black Bream are more silvery in colour and have spines on their dorsal fin – Photo James Tweedley

People are asked to report any fish deaths to the Department of Fisheries FISHWATCH on 1800 815 507.

The Department of Health advises against recreation in waterways during a fish kill or algal bloom event, and contact with dead fish should be avoided.

The Department of Health advises against eating or handling of fish found in these circumstances, and advises against recreational activities such as swimming, wading, crabbing, shellfish collection and canoeing where algal scum or discoloration is visible in a waterway.