The Fish Is Scratching. But Why?

Depth of analysis

The fish is scratching

A fish swims up to a rock, suddenly turns sideways, and brushes against it. A little later, it repeats the movement.

“The fish is scratching itself,” an observer might say.

Then perhaps: “There must be something wrong with its gills.” And after that: “It’s probably parasites.”

A familiar chain of reasoning. But these are three different claims — and the only thing we actually saw was the movement.

What did we actually see?

We saw a fish rub against a surface. This behavior does occur in fish with certain diseases. In scientific studies, it is commonly called flashing or rubbing: a sudden sideways contact with an object or the bottom.

But the movement itself does not tell us why the fish did it.

Perhaps its skin is irritated. Perhaps the problem involves its gills. Perhaps something in the water is affecting it. Or perhaps the cause is something else entirely. The same action does not necessarily have the same cause.

“Scratching” is a convenient word in everyday conversation. But it already suggests an explanation we have not yet tested.

Two fish, one movement

Imagine a test. In one aquarium, a fish has irritated skin because of unsuitable water conditions, such as excessively low pH. In another, a fish has parasitic worms — monogeneans — on its gills.

Now suppose both fish suddenly rub against the substrate. We are shown only video recordings, without being told what is happening in either aquarium.

Could we tell from the movement which fish is reacting to the water and which has parasites?

Perhaps their movements really do differ. But then we need to describe those differences: how the fish turns, which part of its body touches the surface, and how often the movement is repeated. Next, we would need to test whether those features actually help distinguish the causes.

If the only explanation is “you can just tell,” we have an impression, not evidence.

This is a thought experiment, not a description of a study that was actually performed. But it shows exactly what kind of evidence is missing.

Can a fish actually scratch its gills on a rock?

Here is where things get interesting.

The gills are inside the gill chamber, protected by the gill cover (operculum). When a fish rubs against a rock, it touches the rock with the outside of its body — its skin, head, or gill cover.

Gill irritation really can be associated with this behavior. But that does not mean the fish is physically rubbing its gill filaments against the rock.

So there is a difference between these two statements:

“Gill irritation may cause this movement.”

And:

“The fish is scratching its gills.”

The first suggests a possible cause. The second sounds as though we already know exactly what the fish is trying to scratch. We do not.

What should you do if a fish starts rubbing against objects?

Do not ignore the behavior. But do not diagnose a disease from a single movement either.

First, look for other changes: how the fish breathes and feeds, what its skin and fins look like, and whether other fish are behaving similarly. Check the water parameters. If there is reason to suspect parasites, examine an appropriate sample under the microscope.

Rubbing against objects is a useful signal to pay attention to the fish, not a ready-made explanation of the cause.

Studies have described this behavior in fish with parasitic infections ( Garcia et al., 2014) and in fish with diseases involving marked skin changes ( Maddocks et al., 2016). The sensitivity of fish nerve endings to chemical irritants, including low pH, has also been studied separately ( Mettam et al., 2012). The latter study supports the possibility of chemical irritation, but does not demonstrate that low pH necessarily causes this particular rubbing behavior.