Heat stress can change the intestinal flora of salmon

That's what researchers at the Tasmanian Institute of Agriculture, at the University of Tasmania, wanted to find out.

The results were recently published in the Nature journal "Scientific Reports".

The starting point for the study is that when Atlantic salmon are exposed to elevated sea temperatures, they often stop eating and begin to excrete so-called casts, also called pseudofaeces, which often consist of shed intestinal mucosa.

In the study discussed here, it is shown that this response is closely linked to a sharp increase in bacteria in the intestine from the Vibrionaceae family.

The findings point to what they call microbial dysbiosis (bacterial imbalance) as a key factor in decreased appetite (inappetence) caused by high water temperature.

"Colonisation of the gut of Atlantic salmon by Vibrionaceae should be a main focus in studies of the microbiology associated with heat-induced inappetence and dysbiosis in Atlantic salmon," states the scientific article.

Microbiology changes with temperature

The article describes that Atlantic salmon are physiologically best adapted to temperatures between 13 and 16 °C. When temperatures rise towards 19 °C, as they can in hot summers, a combination of challenges arise: lethargy, reduced feed intake and the production of pale yellow, mucous-containing faecal masses called pseudofaeces or casts.

This study investigated how the gut microbiome changes during such a heat phase.

Salmon were kept in seawater tanks at 15 °C, before the temperature was raised to 19 °C until voluntary feed intake ceased, and then reduced again. The experiment was repeated for three consecutive years, with microbiological analyses.

The results were clear: when the fish stopped eating and began producing casts, the amount of Vibrionaceae increased dramatically – typically 1.9–3.4 log units per gram of faeces. After returning to 15°C, the levels decreased again.

Although species composition varied between experimental years, the pattern was stable, namely a significant growth of a few heat-favoured species such as Vibrio scophthalmi, Aliivibrio sifiae and A. finisterrensis.

An imbalanced gut

The article describes that the salmon gut microbiome is known to be influenced by environment, temperature and diet composition. Autochthonous bacteria such as Candidatus Mycoplasma salmoniarum are common in cold and subarctic conditions, while Vibrionaceae dominate at 10–20 °C in more temperate regions.

When the temperature rises rapidly, as in this experiment, this trend is greatly amplified, according to the article. At the same time, larger amounts of intestinal mucosa are shed, and casts become a clear indicator of intestinal stress. Whether Vibrionaceae drive cast production, or whether they simply exploit it, is still unclear.

But whatever the mechanism, the findings point to a temperature-induced dysbiosis, i.e. a destabilisation of the normal gut microbiota. This coincides with physiological stress in the kidney, liver, and osmoregulation systems, and is amplified in summer seasons characterised by gill diseases, algal blooms and low oxygen levels.

This article was created in "collaboration" with AI.