Recirculation Aquaculture System reduces infectious gill pathology

Epitheliocystis is a gill disease affecting Atlantic salmon in both freshwater and marine environments. It is characterized by the presence of “cysts” in the epithelial cells and is caused by obligate intracellular bacterial pathogens of the phylum Chlamydiae and is frequently described in multifactorial ‘complex gill disease’. In extreme cases, clinical signs of epitheliocystis may include fish gasping at the water surface, decreased feed consumption, excessive production of mucous, weak swimming and lethargy. 

In aquaculture systems, where fish are held at higher density and water quality may be impacted, mortalities and lowered productivity have been reported due to this disease. Although epitheliocystis is not an issue for the salmon industry in Australia, the disease has been widely reported in northern hemisphere production areas where wild salmonids act as reservoirs. Traditional flow-through (FT) freshwater hatcheries which receive water from natural lakes or streams may be particularly susceptible to natural infection. Newer ‘recirculation aquaculture’ (RAS) technologies, which reuse continually filtered and UV treated water may reduce the risk of pathogens entering farmed fish populations. 

Experiments were carried out with the Marine Institute (Ireland) at the Newport Research Facility (Co. Mayo). Eggs from Irish and Icelandic strains were hatched in a flow through natural water system, then distributed to FT and RAS systems with a common freshwater intake. Sequential sampling occurred from three weeks prior to marine transfer until five weeks post transfer. 

A significant interaction between hatchery system and fish strain on the prevalence and intensity of gill epitheliocystis was found both using histological and molecular methods. Our results indicate that pathogen infection can be reduced by use of RAS technologies as fish from FT had higher prevalence and intensity than RAS reared fish. We saw evidence of reduced epitheliocystis susceptibility in Icelandic fish which may indicate a genetic effect. Although epitheliocystis intensity (lesions per gill filament) rapidly reduced following marine transfer, pathology was still prevalent at five weeks post marine transfer.

More details of this work can be found in the Journal of Fish Diseases.

CSIRO Aquaculture contact

Petra Quezada – PhD candidate

Dr Richard Taylor– Senior Experimental Scientist