Your fish has dark eyes? It might be stressed.

Some time ago in a laboratory in Brazil, a research assistant sat for six hours in front of fourteen fish aquaria, each slightly larger than a car battery. One fish, the Nile tilapia (Oreochromis niloticus L.) swam inside each aquarium. The assistant snapped close-up photos of the tilapia’s eyes every thirty minutes.

PurelineScience owes much gratitude to such patient assistants. And you would too—if you keep fish pets, buy fish from the market or handle live fish in any other way. Those photos of fish eyes can tell you if your fish is (or was, if dead) stressed.

How? It’s easy. Look at the fish eyes and check how much of the bright area around the black pupil is darkened. The more darkened its eyes are, the more stressed the fish is.

[And if the fish stares at you with eyes dipped in ink, you should be stressed. Stand back from that fish!]

Stressed fishes do not stress alone—their owners are concerned, and the eyes of fishery operators might turn red too from sleepless nights worrying about bad harvest. Aquaculture needs effective methods—easy, cheap and non-invasive—to examine stress levels in fishes, but current methods like checking fish steroids, swimming activity and ventilation do not suffice.

The degree of eye-darkening reliably indicates social stress in fish: hierarchy is strong within fish schools, and suppressed individuals often have darkened eyes. If eye-darkening could also indicate other forms of physiological stress, eye-darkening could just be the diagnosis the industry is looking for.

A team of scientists in Brazil looked for the solution in the eyes of the aquaculture favourite, the Nile tilapia. They compared between the eyes of normal tilapia and those that were stressed by either being confined for thirty minutes in a tight space where the fish cannot turn or brought out of the water and exposed to air for two minutes.

Normal fish sport eyes that were slightly darkened. When juvenile tilapias were tightly confined, eye-darkening increased by six-fold within minutes but appeared normal fifteen minutes later; exposure to air also darkened almost half of the eyes and remained so after two hours.

Adult tilapia showed almost complete eye-darkening in response to confinement and air exposure stress; the effects however receded quickly and eyes became normal after half an hour, except in females exposed to air where their eyes took three-times longer to look normal again.

That a fish is freaked-out when exposed in air—more so than the pangs of claustrophobia—is not surprising, but the immediate eye-darkening deserves scrutiny. The quick response to non-social stress indicates that eye-darkening reveals more than just social stress. Although we do not know yet how stress leads to eye-darkening, the scientists suggest that the physiological pathways might be similar to skin-darkening in lizards. The link between eye-darkening and stress levels need to be examined in other fish species too.

We love dogs for they could tell us so much with their eyes. Our fish in our badly-ventilated aquaria would love us too if we could learn to read their eyes, especially when those orbs turn into black distress signals, screaming “I’m suffocating! Help!!”

You might want to check the eyes of this goldfish.

You might want to check the eyes of this goldfish.


Freitas, R.H.A., Negrão, C.A., Felício, A.K.C., and Volpato, G.L. (2014). Eye darkening as a reliable, easy and inexpensive indicator of stress in fish. Zoology.


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