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Fish behaviour and appearance affected by stress

Pike swimming in dark water. Photo.
The presence of predator fish like pike causes bodily changes in fish that fear to be eaten. Photo: Marcel Einig, Pixabay

Jerker Vinterstare at Lund University has studied the crucian carp species. In his doctoral thesis, he shows that residues of antidepressant medication in the water can cause changes in fish behaviour, making them nervous and cowardly. He also shows that when predatory fish such as northern pike are in the vicinity, the crucian carp can change their body shape, eye size and colour, but also modify their circadian rhythm.

In addition, he has discovered major differences between the ways in which males and females react to an imminent risk of being eaten. 

Become nervous

SSRI medication such as Prozac and fluoxetine is prescribed fairly often to treat mental illness. Use of this type of medication has resulted in detectable residues of antidepressants in waterways. Vinterstare’s thesis investigates among other things how residues of the signal substance serotonin, an ingredient in SSRI medication, affect crucian carp. His findings show that individuals which receive a relatively high dose become nervous and cowardly in a completely different way to those that have not been exposed to the substance. 

However, the changes caused by the ingestion of antidepressants are not limited to fish behaviour but also affect their body shape (the belly increases in size).

React differently

Stress in the form of approaching danger from predatory fish also changes the body shape of the crucian carp. When northern pike are lurking in the reeds, the carp react in part by acquiring a darker colour, and in part by becoming taller as their backs grow. But males and females react differently to predatory fish: whereas the males become significantly taller, the females barely change.

“The males appear to direct their energy towards preparing to protect themselves against their natural enemies, whereas the females dedicate their energy towards producing offspring”, says Vinterstare.

He defended his thesis in March: Defence on Demand: A physiological perspective on phenotypic plasticity in anti-predator traits.