Action: Establish fallowing to reduce parasites/disease
A study in Scotland recorded lower lice numbers on Atlantic salmon in cages using a fallowing system. Another study in Australia found no difference in mortality from Amoebic Gill Disease in cages where a fallowing system had been used.
Husbandry practice on salmon farms is important in reducing the spread of disease within and beyond the farm. Fallowing has been practiced in terrestrial agriculture for centuries to reduce the incidence of disease. Many salmon farms use fallowing as a technique for disease control and some to reduce the build-up of sediments below cages. Chemical and biological remediation of benthos near salmon farms has been proven. A two month fallowing cycle is mandatory to break parasite cycles on salmon farms.
Supporting evidence from individual studies
A study of four fish farms on the west coast of Scotland between 1990 and 1992 (Bron et al. 1993) found that fallowing was effective in controlling lice numbers on Atlantic salmon. The salmon at site 1 were heavily infected with lice throughout the period of the study and therefore required regular treatment. Lice numbers were low in new smolts at sites 2-4 (where fallowing took place) and treatment wasn’t required as often as for site 1 (where there was no fallowing). Treatment for lice wasn’t required as often for salmon at sites 2 and 3 (long fallow periods of at least 16 weeks) compared to site 4 (fallow period of around 9 weeks) suggesting longer periods of fallowing were more effective in controlling lice numbers. The four farms had fallowing periods of 0, 17, 16 and 9 weeks respectively (hereby sites 1, 2, 3 and 4 respectively). Sites 2-4 contained only one intake of salmon at a time whereas site 1 contained salmon of multiple year classes. Samples of salmon were taken from each farm every two weeks for 20 months and lice numbers (of species L.salmonis and C.elongatus) were counted.
A replicated study from 2000- 2002 in Tasmania, Australia (Douglas-Helders et al. 2004) found no difference in mortality from amoebic gill disease (AGD) in Atlantic Salmon using the method of fallowing cages. Cumulative mortality at the end of the trials was 2.06% for the rotated cages and 2.88% for the stationary cages which was not significantly different in both years of the experiment. The fallowing period in different cages ranged from 4 to 97 days and the experiment was repeated in 2 growing seasons (December to March 2000/2001 and December to April 2001/2002). Average biomass per pen was 15,026 kg and 20,304.4 kg for the stationary treatment groups in the two seasons respectively. Average biomass for the rotated pens was 17,115 kg and 21,000.9 kg per pen. Signs of clinical disease were assessed monthly using the routine Tasmanian salmon farmers gill assessment method by examining at least 20 fish for the presence of AGD.
- Bron J.E., Sommerville C., Wooten R. & Rae G.H. (1993) Fallowing of marine Atlantic salmon, Salmo salar l., farms as a method for the control of sea lice, Lepeophtheirus salmonis (Kroyer, 1837). Journal of Fish Diseases, 16, 487- 493
- Douglas-Helders G.M., Weir I.J., O'Brien D.P., Carson J. & Nowak B.F. (2004) Effects of husbandry on prevalence of amoebic gill disease and performance of reared Atlantic salmon (Salmo salar L.). Aquaculture, 241, 21-30