Fish Rubbing Against Objects: Behavior, Cause, and Diagnosis

Depth of analysis

“The fish is scratching”: where observation turns into diagnosis

A fish suddenly brushes the side of its body against the substrate, sometimes repeating the movement several times. In the literature, this behavior is described as flashing or rubbing. It has been reported in some ectoparasitic infestations, skin diseases, and gill disorders.

The movement itself is observable. Yet a familiar explanation often comes before any investigation:

“The fish is scratching” → “It is scratching its gills” → “So it must have gill parasites.”

The first step is an everyday description of behavior. The second is an assumption about the site of irritation. The third is an assumption about its cause. Each new step requires evidence that a video of the movement alone does not provide.

The question is not whether flashing can accompany a parasitic infestation. It can. The question is: how specific is this behavior, and can its characteristics distinguish between possible causes?

Why the same behavior need not have the same cause

The original F.O.S. article proposes a thought experiment.

Imagine two groups of fish:

the first has pronounced irritation of the external surfaces, for example due to a markedly unsuitable pH;
the second has confirmed monogeneans on the gills.

Suppose both groups rub against the substrate. An observer sees only recordings of their movements, does not know the examination results, and must decide which condition is present in each group.

If a specialist is confident that they can distinguish the two, we can ask a testable question: which specific features of the movements are they using? The point of first contact? Head position? Turning angle? Speed? Repetition rate? Association with respiratory movements?

Those features would then need to be defined in advance and tested against independent video recordings for which diagnoses have been established by other methods. Ideally, the person evaluating the recordings would not know which group each fish belongs to. Only then could we discuss the diagnostic accuracy of the feature, rather than the observer’s personal confidence.

Important: this is a proposed way to test a hypothesis, not a published experiment demonstrating identical movement patterns under different conditions. The sources cited below do not establish a test capable of reliably distinguishing gill lesions from skin irritation on the basis of a single movement.

What research actually supports

Parasites and flashing

Garcia et al. (2014) described erratic movements and flashing, among other signs, in rainbow trout affected by gyrodactylid parasites. This shows that rubbing can occur during a parasitic infestation. But observing a sign in infected fish does not mean that it occurs only with that infestation or that it identifies the affected body region without further examination.

Garcia et al., 2014

Skin disease and flashing

Maddocks et al. (2016) described similar movements in rainbow trout with puffy skin disease, a condition associated with pronounced skin changes. The fish descended to the bottom, rubbed their sides against it, and rapidly righted themselves.

This is an important counterexample to the claim that “this movement necessarily means the fish is scratching its gills”: similar behavior has also been described in fish with skin disease.

Maddocks et al., 2016

Can water cause irritation without parasites?

Mettam et al. (2012) investigated sensory nerve endings in the head region of rainbow trout. They recorded responses to a range of chemical irritants, including acidic solutions and low pH.

This provides a physiological basis for considering environmental irritants independently of parasitic infestation. However, the study measured receptor activity, not differences between types of flashing. It cannot be cited as direct experimental evidence that fish exposed to low pH necessarily rub against the substrate in a particular way.

Mettam et al., 2012

Taken together, these studies support several possible explanations for the behavior, but they do not justify choosing a single cause without investigation.

Can a fish “scratch its gills”?

The gill filaments lie inside the gill chamber, beneath the operculum (gill cover). When a fish makes external contact with a rock or the substrate, it may touch the skin of its head, the operculum, or the region of the gill opening. The gill filaments themselves do not necessarily make contact with the object.

It helps to distinguish three statements:

Observation: the fish makes a sudden rubbing movement against the substrate.
Physiological hypothesis: gill irritation may have triggered this motor response.
Assumption about the purpose of the movement: the fish is trying to scratch its gill filaments specifically.

The second is entirely reasonable as a working hypothesis. The third is a much stronger claim: it assigns a specific purpose to the movement that cannot be inferred from the video alone.

Moreover, when a gill infestation is present, flashing may coexist with lesions on other external surfaces. Even confirmed gill monogeneans do not establish that every sideways contact with an aquarium decoration is directed specifically at the gills.

It is therefore more accurate to say “gill irritation may be accompanied by flashing” than to state categorically that “the fish is scratching its gills.”

Which observations are useful, and which are insufficient?

A behavioral record can document the frequency of episodes, the side of the body involved, the area of contact, the sequence of movements, environmental conditions, and changes over time. All of these may be useful for subsequent analysis.

But the level of detail in an observation is not the same as its diagnostic specificity. Even a high-quality recording cannot, by itself, establish a parasitic cause if the relationship between a particular movement and that cause has not been validated.

In practical diagnosis, behavioral observations should be considered alongside other findings and examination results:

water parameters and recent changes in husbandry conditions;
the condition of the skin, fins, and gill region, and the fish’s breathing pattern;
how widespread the behavior is among fish in the system and how the signs change over time;
when justified, microscopic examination of skin scrapings or appropriate gill samples, taking into account the risks of sampling for the fish.

Such examinations help test competing explanations. For example, finding monogeneans is diagnostic evidence in its own right; it does not become evidence merely because the fish appeared to “scratch in a characteristic way.” Conversely, failing to see parasites in a superficial sample does not, by itself, exclude gill disease or other causes.

Observation, interpretation, diagnosis

It is useful to distinguish three levels:

Level	Example wording	What remains unproven
Observation	“Repeated sudden lateral contacts with the substrate”	Why they occur
Interpretation	“The behavior may be associated with skin or gill irritation”	The source and location of the irritation
Diagnostic hypothesis	“Ectoparasitic infestation should be investigated”	The presence and identity of a causative organism

A hypothesis is not wrong simply because it has not yet been proven. The error occurs when it is recorded as an established fact.

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

“You can tell by looking at the fish” may be the starting point of an informed professional suspicion. But for that suspicion to become a diagnostic criterion, we must be able to explain exactly what we are seeing and show how reliably it is associated with the proposed cause.

References and further reading

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. Full text.
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. DOI.
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. Full text.

Other levels: F. — a simple story · F.O.S. — the original in-depth analysis.