Effects of fasting in RAS facilities
Short-term fasting is common in aquaculture before handling operations, but how this affects microbial communities and fish health is still poorly understood.
This is the starting point for the study from Nofima, in which researchers examined Atlantic salmon post-smolts in a semi-commercial recirculating aquaculture system (RAS) after five days of fasting and subsequent refeeding. They monitored water quality, system microbiome, skin mucus, distal intestine, histomorphology, serum cortisol, and operational welfare indicators.
Short fasting less researched
According to the article, how fasting affects fish health and welfare can vary with the fish's life stage, previous and current health status, and the environment the fish is exposed to.
“For example, 14 days of fasting in adult Atlantic salmon has been shown to have no effect on general stress levels or the ability to cope with handling stress. Later studies have also suggested that four or eight weeks of feed withdrawal have negligible effects on fish welfare,” the researchers write.
However, fasting also affects the gut transcriptome and microbiome in fish, but according to the article, these changes are quickly reversed after feeding is resumed.
The transcriptome includes all transcribed genes at a given time, or is thus the total amount of RNA molecules of a cell, tissue or individual.
It is pointed out that previous studies of fasting in Atlantic salmon have often focused on longer fasting periods, while the effects of short-term fasting and subsequent refeeding have been far less investigated.
"The intestinal tract is the first organ affected by fasting and refeeding, both physically and through the registration of nutrients after absorption," the authors write.
Can affect the biofilter
It is pointed out that although production using RAS offers the opportunity for a better controlled environment, there are also concerns related to negative effects, as such systems operate with a highly bioactive environment and an accumulation of microbes that can affect fish health and welfare.
"In RAS, the microbial communities include both biofilter-associated autotrophic nitrifiers and heterotrophic bacteria that break down organic matter," the authors explain.
They add that heterotrophic bacteria respond quickly to changes in available organic matter and compete with autotrophs for oxygen and space, but can also protect fish from pathogens.
“Changes in daily feed load of more than 15% can negatively affect nitrification in the biofilter. In addition, varying concentrations of organic matter in the system are likely to affect the stability of the microbial communities,” the researchers write.
Study objectives
While previous studies have mainly investigated long-term fasting and fish raised in flow-through systems, far less is known about how short-term fasting affects both host-associated and system-associated microbiomes in the RAS.
"Due to the distinctive microbial ecology and higher microbial loads that characterise RAS, the response to fasting in this environment may differ from that observed in flow-through systems."
Given the limited knowledge about short-term fasting in RAS, this study aimed to:
- describe temporal changes in water quality and microbial communities in the RAS during fasting and refeeding
- characterise corresponding patterns in the microbiome on the skin and in the distal intestine
- examine associated histomorphological changes in the intestine and skin
- investigate the effects of fasting and refeeding on serum cortisol levels and some common injury-based operational welfare indicators (OWI) in Atlantic salmon.
Reacts differently in water and biofilm
One of the clearest findings was that the microbiology in the water phase was far more dynamic than in the biofilter and biofilm. While the bacterial profiles in the biomedia and biofilm remained relatively stable through fasting and refeeding, the water microbiome changed significantly, especially after feeding was resumed. The researchers interpret this as an expression of different microbial life strategies in the RAS: more stable, resource-limited communities in the biofilter and biofilm, and more responsive communities in the water masses when nutrient availability fluctuates.
In practice, this means that short changes in feeding regime can cause rapid shifts in the microbial background of the fish, even if the biofilter itself appears robust. The study also points out that water chemistry changes co-varied with this dynamic: during fasting, the bacterial count was lower, total inorganic carbon was lower and dissolved oxygen was higher, while refeeding gave the opposite pattern, thus indicating increased microbial metabolism in the system.
The skin microbiome followed the water – the gut followed the fasting
The skin mucus showed a microbial profile that largely mirrored the aquatic environment. This in itself is interesting for RAS operators, because it highlights how tightly connected the fish's outer mucus barrier is to the production water. Changes in the water microbiome thus appeared to directly affect the skin microbiome, without the researchers simultaneously finding histomorphological changes in the skin.
In the gut, the picture was different. There, the diversity remained more stable, but the composition changed. One finding in particular stood out: fasted fish had a significantly higher relative abundance of Vibrio in the remaining gut contents. In some samples, Vibrio reached very high relative levels.
The researchers point out that this is likely related to a changed nutritional niche in the gut contents when the fish is not eating. When "cast" is formed and mucus and residual material are left in the lumen, bacteria that are good at utilizing changing carbon sources may gain a competitive advantage.
Distal intestine responded quickly – and recovered quickly
The researchers found no clear changes in the skin, but in the distal (back) intestine they saw differences during the fasting period. This included enterocyte vacuolization, the thickness of the lamina propria. At the same time, more affected tissue and increased inflammatory changes were recorded in the fasted group by day 5. After six days of refeeding, much of this had normalised or pushed back towards control levels.
Result show that the distal intestine has plasticity and responds quickly to both feed cessation and refeeding. In other words, one can interpret that feed interruption does not necessarily result in a measurable stress response or external welfare damage, but nevertheless that it affects the microbiology and tissue structure of the intestine.