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HAVBRUK2-Stort program for havbruksforskning

Optimising Feed Withdrawal for Safeguarding Fish Welfare

Alternative title: Optimal fôringspause for trygg fiskevelferd

Awarded: NOK 10.0 mill.

Project Manager:

Project Number:

295200

Application Type:

Project Period:

2019 - 2024

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, periods of extreme weather and farming salmon in localities that are more exposed can increase the risk that farm personnel are unable to visit their farm, or that freighters cannot deliver feed to the farm site. This may also mean that the salmon cannot be fed and are subjected to unplanned fasting. There can therefore be many reasons why farmed salmon must fast for a period, but there is limited knowledge about what fasting means for their health and welfare. We have now carried out five experiments to shed light on the effects of long-term feed withdrawal on the physiology and welfare of farmed salmon. In Experiment 1, salmon were fasted for zero to four weeks. The salmon adapted to the feed withdrawal by lowering their energy consumption and their acute stress response was unaffected after three weeks, only marginally reduced after four weeks, and after being fed again for one week their ability to handle acute stress had returned to the same level as before. Experiment 2 was similar to experiment 1, but here we investigated how fasting affects salmon's ability to swim to exhaustion. 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 fasting 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 compensatory growth, and at slaughter, there were no longer any difference. In experiment 4, appetite-regulating genes were measured after four and six weeks of fasting. The data showed that these genes were downregulated in the fasted groups, and that the salmon thus probably did not have a chronic feeling of hunger. In experiment 5, we investigated how high temperature (18 °C) affects salmon's tolerance to fasting. The data showed that the salmon managed to maintain swimming capacity but did not lower their energy consumption equally well. 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 was resumed and showed no signs of reduced welfare from having been fasted for five days. Further experiments have now been done to document how nitrification efficiency of the biofilters is affected by fasting during smolt production in RAS. The results will be available at the end of the project. During late 2021 and 2022, we applied knowledge from the first work packages and incorporated recommendations from the expert group in the experimental designs. We i) examined the effects of fasting vs full ration feeding in RAS before handling, and ii) studied the stress response of fish when fasted and handled in relation to stable high, stable optimal and fluctuating (simulated marine heatwave) temperatures. These experiments are now completed, and data is currently being analysed. We are continuing discussions with the expert advisory group to ensure the operational relevance of the project and that the results can be translated into operationally applicable advice and recommendations.

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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.

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Funding scheme:

HAVBRUK2-Stort program for havbruksforskning