Intermittent fasting in ongrowing Atlantic salmon (Salmo salar) reared in sea cages: Effects on feed intake, appetite control and growth

Abstract

Farmed Atlantic salmon are normally fed at a daily basis and fasted before operational procedures and harvest. While the biological mechanisms and effects of short and long-term fasting are largely known, there is sparse information of how salmon respond to intermittent fasting. Less frequent meals are considered a measure to save feed under e.g., periods with food shortage or flesh quality improvement. This study investigates how large Atlantic salmon reared in sea cages adapt from being fed to satiation every day to restricted feeding with feeding to satiation every third day in an intermittent fasting regime (lasting for 43 days) followed by a period of refeeding (37 days with feeding to satiation every day). The experiment was conducted during spring 2023 and in research scale sea cages (12x12 m) at natural light and ~10 °C; ~33 ppt. Duplicate cage groups were either the intermittent fasting regime or the daily feeding (control). The response was measured at fish group level as daily appetite and periodic growth at cage level. In addition, individual fish was periodically sampled 7 hours after onset of the meal for gut fullness index and appetite assessment, and physiological indicators related to appetite control and energy storage (hepatosomatic- and gonadosomatic index).

During the first 30 days of intermittent fasting there was a progressive increase in feed intake and the total fed amount for the fasting period was estimated to 65% of the quantity fed in the control. Growth rate and biological feed conversion rate was respectively 52% and 141% of control levels, supporting that the intermitted fasted fish used a higher proportion of their feed intake on bodily maintenance. That was also showed by the fact that intermittently fasted group had 11% lower hepatosomatic index than the control after 43 days of intermittent fasting. During refeeding the fed amount for the previously fasted fish was ~120% of control level the initial 7 days and then remained elevated at ~110% for the subsequent 30 days of the study. The stomach fullness index increased in line with the fed amount during the intermittent fasting period and decreased during refeeding period, indicating adaption to the different feeding regimes. The gonadosomatic index showed early sign of maturation, however no difference was found between the treatment groups.

This study shows that intermittent fasting by feeding to satiation every third day will save feed, but hamper fish growth and feed utilization which the fish will require a considerable time to compensate lost growth from. In terms of fish welfare, no apparent negative effects from the fasting regime were found, suggesting the current regime as viable during periods of food shortage or flesh quality adjustment prior to harvest.