Do fish have flatulence?

I WAS out fishing the other day and suddenly wondered: Do fish have flatulence? Given they don't breathe in the same way as we do and if they got full of gas they might float to the surface? And that they probably have a different digestive process (not to mention that they probably don't eat beans)? Do you think you could find an expert to answer this question?

The passage of gas from the anus is the standard definition of flatulence. I have dissected many hundreds of fish guts and not yet seen one with a gut distended by gas – that is, with the necessary predisposing condition that might conceivably lead to a fart.

The gas problems suffered by humans are largely an effect of the microbes in our intestines. These microbes perform a vital role in helping us access the nutrients from the food we eat. Gasses such as methane are a byproduct of their good works, leading to both discomfort and embarrassment for some, and a source of amusement for others.

Some fishes such as surgeon fish (e.g. Dory in Finding Nemo) also use microbes in their gut to help digest plants that they consume, and these might produce gas. However, I have seen no evidence of fish farts despite a number of decades snorkeling and diving on the reef. I do watch the rear ends of fish closely as I have an interest in what fish eat and what comes out the other end.

Other fishes to watch closely are those that eat from the seabed around coral reefs. If they swallow calcium carbonate sand then in combination with acid in the stomachs carbon dioxide should be produced. However, I reiterate, and despite close inspection, I have never seen a telltale burst of bubbles or a sheepish fish looking around for a dog to blame.

Interestingly, some fish are immune to this potential effect as they lack stomachs and food just goes straight from oesophagus to intestine. Possibly a history of embarrassing gaseous emissions in their heritage led to the selection of non-farting fishes. Alternatively, maybe those that farted were detected more swiftly by predators and their genes swiftly (and hygienically) removed from the gene pool. This argument is likely to rage for some time among the two diametrically opposed schools of thought.

If one expands the definition of fart to include the expulsion of gas from any posterior orifice, then some fish do fart. Or rather FRT. Investigators in Canada studying herring found that at night they (the herring!) expel gas from their posteriors. This produces a sound they called a Fast Repetitive Tick. I am sure the authors thought long and hard about that initialism and never ever intended it to be used as an acronym. These herring sounds are apparently involved in social communication and derived from air swallowed by the fish and then expelled from a duct opening near the anus.

In summary, and based on the original definition, my answer is NO. We might reflect that this is indeed fortunate as both methane and carbon dioxide are greenhouse gasses – and all we need is fish farts adding to our current problems.

All in all, a good question, I think.