Farmed salmon are usually fasted in the days leading up to major handling operations. This is to prevent the build-up of excess amounts of faeces in the water during handling, to decrease their demand for oxygen and increase their ability to cope with stress (high consumption of oxygen and excretion of carbon dioxide can lead to too little oxygen available to the fish and acidic water). In addition, more extreme weather and more exposed salmon farm localities increase the risk that personnel in periods are not able to visit their farm, that freighters cannot deliver new feed to the farm and that the water current or weather conditions on the farm are such that the salmon cannot be fed.
We have now carried out three experiments to shed light on the effects of long-term feeding stop on the welfare of farmed salmon. In Experiment 1, salmon were fasted for zero to four weeks. The salmon adapted to feed withdrawal by lowering their energy consumption, but their ability for maximum oxygen uptake was not reduced. Their total aerobic capacity had therefore increased, and thus also their ability to handle acute stress. Experiment 2 was similar to experiment 1, but here we investigated how fasting affects salmon's ability to swim in strong currents. The data showed that salmon that had not been fed for four weeks had as good swimming capacity as the other groups. The length of the feed withdrawal period had no significant effect on blood values or their ability to recover after swimming to exhaustion. In Experiment 3, salmon of approx. 1 kg were fasted for eight weeks and then fed again and farmed according to standard procedures. When compared with a parallel control group that were not fasted, the fasted salmon were 50% smaller after eight weeks of feed withdrawal, but in the time that followed, the fasted group showed good compensatory growth, and at slaughter, there were no longer any differences in size between the two groups. Together with experiments 1 and 2, experiment 3 shows that even after several weeks of feed withdrawal does not result in poor welfare for farmed salmon.
For large slaughter-sized salmon, we have investigated the timing of emptying of the gut and the effects of fasting on fish welfare during transportation. A new finding from these trials is that emptying of the gut is not only dependent on fish size and temperature, but also on the behaviour of the fish. Salmon that stood in low current during the fasting period emptied their intestine significantly later than salmon that actively swam in the current. This may explain some of the differences in reported gut emptying rates between different studies.
There is little knowledge about the effects from fasting on water quality and the biofilter community in recirculating aquaculture systems (RAS). These systems rely on bacteria to remove potentially toxic nitrogen compounds (originating from protein metabolism, fish faeces and uneaten feed) from the water. Fasting, therefore, has the potential to disturb the equilibrium and stability of the bacterial population and thereby lead to suboptimal water quality. In experiments, we have now documented increased levels of nitrite nitrogen and nitrate nitrogen in the water after five days of fasting post-smolt salmon. However, the changes in water quality were transient and water quality was back at pre-fasting levels one week after feeding was resumed. The salmon had good appetite when feeding resumed and showed no signs of reduced welfare from having been fasted for five days.
In the last year of the project, we will test the knowledge we have produced in the first work packages by subjecting salmon to the potential additive and interactive effects of fasting and crowding under flow-through, RAS, and exposed farming conditions. An expert advisory group oversees the operational relevance of the project and ensures that the results can be translated into operationally applicable advice and recommendations for safeguarding fish welfare in new and emerging rearing systems and under existing and emerging handling operations.
Fasting, also termed feed withdrawal, is a standard procedure in Atlantic salmon aquaculture and occurs before the majority of handling operations, both in existing and emerging aquaculture systems. However, there is surprisingly little knowledge available and no clear science-based recommendations on how long Atlantic salmon should be fasted to minimise any potential negative impacts on fish welfare. In this project, we will investigate the fundamental effects of fasting on salmon’s ability to cope with stressful handling operations and its effects on water quality in transport tanks and in recirculating aquaculture systems (RAS). To achieve this goal, we will utilize a wide range of approaches, including using swim tunnel respirometers to accurately describe changes in the metabolic capacity of salmon during fasting at different temperatures at both the individual and group level. We will also generate sufficient replicates to monitor their recovery capabilities. For large slaughter sized salmon, we will investigate the timing of emptying of the gut and the effects of fasting on fish welfare and health during transportation and also slaughter quality. Fasting in RAS is a complex challenge as the farmer has to manage both the welfare of the fish and the state of the biofilters. Investigating the effects of fasting on the microbial community in RAS (in addition to fish welfare) is therefore an essential part of this project. Finally, we will test the fundamental knowledge we generate in the earlier phases of the project by subjecting fish to the potential additive and interactive effects of fasting and crowding under flow through, RAS and exposed farming conditions and also at varying production temperatures. An expert advisory group will oversee the operational relevance of the FASTWELL project and ensure its outputs can be translated into direct advice and recommendations for safeguarding fish welfare in new and emerging rearing systems and operations.