Scientists help fight native oyster parasite

01 August 2016

While the common pacific oyster remains under threat from a viral disease, Agriculture Victoria scientists are taking the extra step to investigate a problematic parasite effecting native flat oysters.

Aquatic Health Veterinarian Dr Tracey Bradley is currently leading the research at Agriculture Victoria laboratories in Queenscliff and oyster farms in Port Phillip Bay.

"Bonamia is an oyster-specific parasite found within the blood cells of native Australian flat oysters.  It kills large numbers of oysters by draining their energy until they can no longer keep their shells together and are exposed to predators," Dr Bradley said.

"The disease is not harmful to humans but is the greatest potential risk to the long-term sustainability and the export potential for native Australian flat oyster farming in Port Phillip Bay."

"Infected oysters can survive with the disease and be successfully harvested for safe consumption."

Dr Bradley said its unknown what causes some oysters to be overwhelmed and killed by the parasite.

"The only way to detect if oysters are infected with the parasite is to take tissue samples for testing, this process kills the oyster," Dr Bradley said.

"When the parasite is detected in an oyster population it's hard to predict which oysters will live with the parasite and survive through to when they are ready to be harvested."

Two current research experiments are set to improve disease management practices for oyster farmers by identifying factors in the oyster's environment which switch oysters from being simply infected and living with the parasite to dying in their hundreds and thousands.

"We test and monitor carrier oysters in a range of different on farm conditions such as heavily and lightly stocked cages, deep and shallow stocked cages and different age classes," Dr Bradley said.

"We are also conducting lab experiments to determine how temperature, rough treatment and starvation affect the carrier oysters while monitoring which oysters die and whether it was the parasite that killed them."

"Results from this research aim to provide farmers with a better knowledge on how heavily to stock their cages, what age to harvest and any other management factors that will assist in making sure the oysters live happily to harvest time."

Other parts of the project are looking at how best to kill the parasite and how the parasite is transmitted.

"We know the parasite is most active in the warmer months of the year and it would appear that temperature is an important factor in stressing oysters and causing them to die from the parasite. This is particularly concerning as climate change appears to be causing an increase in our marine water temperatures," Dr Bradley said.

Dr Bradley said the project could help develop a sustainable flat oyster industry in Port Phillip Bay.

"Many of the world's fisheries are either fully exploited or overexploited and as the global consumption of seafood increases so will the need to protect and develop our aquaculture."

"There is an excellent potential market for the popular native flat oyster and our research aims to improve Victorian oyster farmer's ability to grow the constant, good quality supply required."

This project is led by Agriculture Victoria in collaboration with CSIRO and the South Australian Government. The project is funded by the Fisheries Research and Development Corporation and set to conclude in 2017.

Background

Port Phillip and Western Port Bays have historically been the home to large populations of Australia's native flat oyster.

Farming of native oysters was trialled in the Victorian bays in the late 1980s but was abandoned in the early 1990s after Bonamia destroyed both farmed and wild oysters.

A group of farmers again ventured into native oyster production in Port Phillip and Western Port Bays in 2010 and have since been working with Agriculture Victoria to establish if their oysters are infected with Bonamia and how best to prevent their oysters from dying.

In 2013, some of the oysters tested gave positive results for the parasite. However, the level of parasite was too low to affect their health.

In March 2015, approximately 75 per cent of oysters in parts of the bay were found to have been killed by Bonamia.