Feed withdrawal is a widespread practice in Atlantic salmon (Salmo salar) aquaculture to empty the gut prior to major farming operations. Moreover, emerging production practises such as RAS and offshore farm sites may also occasionally subject fish to prolonged fasting periods. However, such extended fasting periods may conflict with ethical and legal obligations to farm animals. Presently, science-based recommendations on responsible fasting times that consider fish welfare are lacking for Atlantic salmon as well as for other finfish aquaculture species.
We have investigated some of the physiological, behavioural, growth, and welfare related effects of fasting periods in a series of three studies on growing Atlantic salmon post-smolts in seawater . The purpose of this work was do define useable welfare guidelines for allowable fasting periods in Atlantic salmon aquaculture by identifying thresholds for significant impairments in performance traits.
In the first study, we measured metabolic rates in responses to increasing fasting periods of up to 4-weeks and after 1-week of subsequent refeeding in fish of ~575 g and ~38 cm at 12°C . The standard metabolic rate decreased stepwise after 1 and 3 weeks, showing that Atlantic salmon gradually adapt a mode of energy saving when resources are limited. The increase in metabolic rates following acute stress was slightly reduced after 4-weeks, indicating that stress responses first become impaired at this point. Following refeeding metabolic rate traits reverted to control levels , showing that metabolic adjustments to prolonged fasting were rapidly reversed when regaining access to feed.
In the second study, we measured the critical swimming speed in fish of ~250 g and ~29 cm at 12°C that had been fasting for up to 4-weeks. In addition, we also measured blood parameters before, at fatigue, and after 3 and 24 hours of recovery from the swim test. The 4-week fasting period reduced condition factors from 1.03 to 0.89. However, the critical swimming speed remained statistically unaffected at ~3.5 body lengths s-1 . Exhaustive exercise caused large osmotic and ionic disturbances, and large increases in plasma lactate and cortisol. During subsequent recovery, the changes in osmolality and plasma ions took the longest to correct, suggesting that these parameters may be considered the most challenging stressors during strenuous exercise in seawater. However, only minor effects of fasting period on blood parameters in relation to the swim challenge were detected which included a repressed response in red blood cell recruitment and reduced cortisol response at fatigue. Nevertheless, Atlantic salmon maintained their full swimming capacity and their ability to respond and recover adequately to acute challenges following extended periods of food deprivation.
In the third study, Atlantic salmon of ~1200 g and ~46 cm were fasted for 8 weeks at 12°C and subsequently refed for 5 weeks, whereafter they were transferred to triplicate sea cages in a common garden setup with a control group until harvest size of ~ 6100 g and ~73 cm. At the end of the fasting period fish had lost 7.3% mass and the condition factor had decreased from 1.2 to 1.0. Furthermore, fasted fish were 544 g lighter and 3.8 cm shorter than fed controls, corresponding to a size difference of 50%. Following periods of refeeding, fasted fish eventually showed compensatory growth and at harvest weight and length were statistically like controls. At harvest, males were larger than females, and immature fish were larger than maturing fish. The proportion of maturing fish was 25% higher in the continuously fed control treatments. After the 8-week fasting period, fish welfare was scored based on the salmon welfare index model. Only minor deviations were found and at similar regularities between fasted and control fish, showing that prolonged fasting did not cause detrimental welfare conditions. To assess potential long-term impacts on welfare status, vertebral deformities in the spinal column were quantified with radiology after harvest. Frequency of skeletal deformities were low and similar between treatments. Hence, Atlantic salmon are highly flexible with regards to growth patterns in response to food availability, and a prolonged fasting period neither caused reduced welfare in the short or in the long term.
Based on these three studies we conclude that farmed Atlantic salmon are well adapted to cope with prolonged fasting periods without suffering poor welfare or other detrimental and persisting consequences to physiological and growth performances. When poor welfare is observed in fish that have been fasting for long periods, the underlying cause will most likely instead by suboptimal environmental conditions or infectious diseases, not fasting on its own. As such, formulating welfare guidelines for allowable fasting periods for farmed Atlantic salmon may therefore ultimately be redundant since the required time to initiate severe starvation takes much longer than any realistically encountered fasting period in modern aquaculture practices.