ANALYSIS - The use of mussels in an Integrated Multitrophic Aquaculture system can help reduce the threat of ISA to fish. This was the result of research done by Ian Bricknell, University of Maine, which looked at how using mussels in an Integrated Multitrophic Aquaculture system can increase or decrease disease pressure, writes Lucy Towers, TheFishSite Editor.
Speaking at a State of the Seas series lecture, Mr Bricknell stated that Integrated Multitrophic Aquaculture (IMA) is where more than one species is grown in the same environment. Waste products from the main crop are recycled to provide nutrients that are used to grow other crops, to support either your main crop or provide additional income to your farm.
This system is therefore good to use as it provides the farmer with economic back up if the main crop fails, provides feed for other crops farmed and it lowers the carbon footprint. However, whereas a monoculture system only needs attention to be paid to one species, IMA requires thought and management to cover a range of species. A good health regime therefore needs to be considered as a treatment for the first crop may affect the secondary crop, Mr Bricknell said.
In regards to disease, Mr Bricknell looked specifically at an IMA system using mussels and seaweed, which is ideal for the marine conditions in Maine, USA. The aim of Mr Bricknell's work was to find out whether or not mussels used in the IMA system acted as a reservoir for disease and therefore whether they helped or hindered the disease risk to fish in the close proximity. It is known that mussels filter bacteria but, can they be used to break bacteria/disease life cycles?
Mr Bricknell stated that an integrated multitrophic fish farm, for example, if it had a pathogen event in the fish, the pathogens are shed from the fish and they interact with the mussels. "What we don’t know is whether the mussels will amplify that pathogen and cause a bigger infectious pressure in the wild or reduce that pathogen and provide a smaller infectious pressure in the wild. Equally, if a disease occurs in wild fish, it interacts with the mussels within the farm before it leaves the fish. Does that amplify that infectious pressure and put more pressure on the farm fish or does it reduce it and decrease that pressure?" said Mr Bricknell.
In order to find out the answer, Mr Bricknell looked at infectious salmon anaemia (ISA) and infectious pancreatic necrosis virus (IPN).
With ISA, Mr Bricknell found that the virus could not survive being exposed to mussels. “The mussels have taken up the virus but somehow inactivated and prevented the virus being viable, as we could not get it to grow in a fish tissue culture,” said Mr Bricknell.
It is thought that the mussels do this by stripping ISA of its lipid outer layer.
Next, Mr Bricknell looked at the tough little virus IPN. Sadly, it was found that with IPN the virus is cultured and concentrated in the feces of the mussels and then shed into the water where it can infect fish.
In conclusion, Mr Bricknell found that the risk of ISA was decreased and with IPN, the risk was increased slightly, but it is quite clear that there are benefits of IMA to disease control, not just in reducing the carbon footprint of aquaculture.