By Asbjørn Bergheim*
The extent of the reported outbreaks fluctuates from a single site to epizootic outbreaks with significant mortalities at most farms in a region. In Ireland and Scotland, amoebic gill disease was among the major causes of losses in salmon farms during 2011-12, where 12 out of 26 sites suffered from such outbreaks.
The parasite is characterized as the most significant amoeba in fish and the disease syndrome is normally indicated “AGD.” According to Marine Scotland (2012), AGD-attacks indicate lethargic fish, which breathe rapidly and dwell at the surface. A closer look at the affected gills shows patches of white, swollen tissue and mucus (photos provide by Jannicke Wiik-Nielsen). So-called hyperplasia, coalesced gill lamella, seems to coincide with such outbreaks. Harmed gill tissue impairs the respiratory capacity and leads to asphyxia and death in severe cases.
According to experts on AGD, salinity is found to be the single most decisive environmental factor, but long-term outbreaks are always associated with salinity coupled with temperature. Long-term infections in salmonids only occur at high salinity (> 32 ppt) during warm periods at water temperature above 12-15 ºC. Among other associated contributing factors are high fish density and fouling of the cage nets.
Losses to AGD as high as 70 % have occasionally been reported (Marine Scotland, www.gaaia.org/gill-diseases). This report also claims that low, but ongoing mortalities can last for up to three months. The highest indicated loss ever was 86% in a Norwegian salmon farm on the west coast of Norway about ten years ago! Untreated cages in Tasmania with AGD-outbreaks reached mortalities of 50 % (trout and Atlantic salmon) in the late 1980’s, while reported total losses of 5-20 % of the stock are more typical at present.
Mortalities, reduced growth and treatment costs connected to AGD represent high costs for the industry. In Tasmania, The Fish Site reports a present annual loss of US$173,465 million (AUS$230 million), while the newspaper Herald Scotland announced that the company Grieg Seafood lost one-third of its total harvest in the country a couple of years ago representing some US$37.79 million (£30 million) in lost revenue.
For a long period, Norwegian cage farms seemed to be protected against AGD. Rather low water temperature compared to that in other salmon producing countries, especially along the northern coast, was thought to be a major reason. Nevertheless, the salmon industry had to reconsider this assumption in autumn 2006 when four farms distributed along the western coast suffered losses due to AGD. The attacked farms were distantly distributed over a coastal stretch of more than 500 km and the experts at The Norwegian Veterinary Institute assumed that the “exceptionally high sea water temperature over a long period previous to and during the disease outbreaks favoured an amoeba already present in the marine environment.” During the actual period, the sea temperature at the surface was 3.5 ºC above the average level before the outbreaks (i.e. 17.5 ºC in August).
As indicated, the amoeba is dependent on high salinity and AGD infested fish are successfully treated with freshwater. Bathing in freshwater for 2-3 hours will remove the majority of the amoebae, but the treatment should normally be repeated in order to avoid re-infestation. In 2-4 weeks after the treatment, the infestation may be reestablished if the source of infection remains. A common procedure in Tasmania is 13 bath treatments in the 15 months of on-growing until harvest. Experts indicate that the salinity through treatment should be below 3 ppt. and soft fresh water with low concentrations of calcium and magnesium is most efficient. In Scotland and Ireland, usage of hydrogen peroxide as a remedy against AGD has become usual over the last years. Freshwater treatment is performed in well-boats or in floating tarpaulin enclosures and injection of oxygen to avoid severe oxygen drop during the treatment is vital.
Dr. AsbjØrn Bergheim is a senior researcher in the Dept. of Marine Environment at the International Research Institute of Stavanger. His fields of interest within aquaculture are primarily water quality vs. technology and management in tanks, cages and ponds, among others.