Interpretation.

“The fish is scratching”

A fish rubbing against rocks, substrate, or the bottom is, by itself, an objectively observable behavior. In the literature this behavior is usually described as flashing or rubbing: the fish suddenly contacts a surface with its side, rolls its body along it, and may repeat the movement several times. Such behavior has indeed been described in ectoparasitic infections and other lesions of the skin and gills (Garcia et al., 2014) (Maddocks et al., 2016). At this level, everything is simple: we see an action. The problem begins afterward — when a cause is automatically assigned to that action.

“The fish is scratching.”

Then:

“Probably its gills.”

And then:

“So it must be parasites.”

But how well can we actually determine the cause of such behavior from observation alone?

Imagine a simple experiment

The examiner already knows the true cause of the behavior. There are two groups. In the first, the fish have pronounced skin irritation caused by sharply unsuitable pH. In the second, there are large numbers of monogeneans on the gills. In both groups, the fish rub against the substrate and decorations. The pH example is not hypothetical: chemonociceptive receptors responding to low pH and other chemical irritants have been demonstrated in rainbow trout (Mettam et al., 2012). And with monogeneans, flashing is indeed described as a clinical sign (Garcia et al., 2014). The observer is shown only the behavior and asked:

“In which case is the skin irritated, and in which case are the gills irritated?”

And here an uncomfortable question appears:

“How much better would the rate of correct answers be than random guessing?”

If it were significantly better, then there must be specific behavioral features by which one cause can genuinely be distinguished from the other. Those features could then be described, tested, and used. But if we cannot name such features, then the statement:

“It is scratching its gills”

is not an observation, but an interpretation.

What features are we actually using?

Can you confidently determine what exactly is causing the rubbing without a scraping, necropsy, or microscopy?

Can you describe:

“how rubbing caused by skin irritation differs from an attempt to scratch the gills?”

Which part of the body touches the surface first? How does the trajectory of the movement change? Is there a characteristic turn of the head? Does the frequency differ? Is the movement linked to respiration? Is there a specific sequence of actions? If such differences exist, they can be studied.

If the answer is reduced to:

“You can just tell by looking at the fish,” then this is no longer a diagnostic criterion…it is the observer’s impression.

And an even more uncomfortable question

Are we even sure that a fish can scratch the gills themselves? The gills are located beneath the operculum or inside the branchial cavity and do not directly contact the substrate or a rock. During external rubbing, the fish can mechanically affect the head region, the skin, the operculum, and the margins of the gill opening. But the movement itself does not show that the fish is acting specifically on the gill lamellae. Gill irritation may indeed trigger such a motor response. This is entirely plausible and consistent with descriptions of severe monogenean gill infestations, in which flashing is observed together with respiratory disturbances (Garcia et al., 2014). But the literature usually records the presence of this behavior as a clinical sign rather than describing a specific kinematic pattern that would allow one to say confidently from the movement alone: “the fish is scratching its gills.”

But that is already a different statement:

“Gill irritation can cause flashing.”

It is not the same as:

“The fish is scratching its gills.”

The first describes an association between irritation and behavior. The second assumes a specific purpose of the movement. And that purpose is precisely what we do not know.

Where does observation end?

One can say:

“The fish repeatedly makes abrupt lateral contact with the substrate.”

One can say:

“This behavior is often described as flashing and associated with irritation of the skin or gills” (Garcia et al., 2014) (Maddocks et al., 2016).

But the phrases:

“It is itchy.” “It is scratching its gills.” “It is monogeneans.”

— already belong to different levels of interpretation.

And each subsequent level requires additional evidence.

We observe behavior. The cause of that behavior still has to be demonstrated.

References

Garcia, R.L., Hansen, A.G., Chan, M.M., Sanders, G.E., 2014. Gyrodactylid Ectoparasites in a Population of Rainbow Trout (Oncorhynchus mykiss). Journal of the American Association for Laboratory Animal Science, 53(1), 92–97.
A clinical case of gyrodactylid infestation in rainbow trout. The authors describe erratic swimming and flashing; they also note that gyrodactylids affect the superficial layers of the skin and gills and can cause irritation, epithelial hyperplasia, mucus hypersecretion, and pruritus, which is inferred from flashing.
https://pmc.ncbi.nlm.nih.gov/articles/PMC3894654/

Mettam, J.J., McCrohan, C.R., Sneddon, L.U., 2012. Characterisation of chemosensory trigeminal receptors in the rainbow trout, Oncorhynchus mykiss: responses to chemical irritants and carbon dioxide. Journal of Experimental Biology, 215(4), 685–693. https://doi.org/10.1242/jeb.060350
An experimental study of chemically sensitive receptors in rainbow trout. Responses to potentially damaging chemical stimuli, including low pH, acetic acid, and citric acid, were demonstrated. This confirms the physiological possibility of pronounced skin irritation without the involvement of parasites.
https://pubmed.ncbi.nlm.nih.gov/22279076/

Maddocks, C.E. et al., 2016. Puffy Skin Disease Is an Emerging Transmissible Condition in Rainbow Trout Oncorhynchus mykiss Walbaum. PLOS ONE, 11(7), e0158151.
In an experiment involving a disease with pronounced skin lesions in trout, flashing was observed: the fish moved down toward the bottom, rubbed their sides against the surface, and rapidly righted themselves. This is a good example of similar behavior arising from skin pathology and not, by itself, indicating a gill localization.
https://pmc.ncbi.nlm.nih.gov/articles/PMC4938586/