When light shows the way but might become a trap: a new study by the Fisheries Research Station (Langenargen, Germany) and the University of Konstanz reveals how fish larvae perceive their environment.

How do minute fish larvae find their way in complex and dangerous environments? And what role do different light colours play in their journey? These questions have hardly been researched. A new study by the Fisheries Research Station (FFS) and the University of Konstanz, which lie on the shores of Lake Constance, has uncovered, for the first time in such detail, how young Lake Constance whitefish (Coregonus wartmanni) use light to find suitable habitats – and how sensitive this system is to environmental changes.

Shortly after hatching, larvae are drawn toward light, being particularly attracted to green and yellow wavelengths. This behaviour is crucial for survival: due to the characteristic light conditions in aquatic ecosystems, it guides them directly into shallow waters close to the shore. These ‘nurseries’ offer protection and better feeding opportunities for the larvae.

To investigate this behaviour in more detail, the researchers tested how the animals responded to different light colours in a Y-shaped maze. They also analysed how the larvae's orientation is affected by increased water temperatures, which occur more frequently now due to the climate crisis.

The results of the study show that very young larvae selectively follow light in the green-yellow range. As they grow older, this attraction gradually decreases – a mechanism that later directs juvenile fish back into deeper parts of the lake. Under elevated temperatures, however, this process is altered: larval development accelerates and orientation to light changes earlier and less precisely. This behavioural change could cause larvae to leave their protected nursery habitats prematurely.

Artificial light also influences this sensitive system. Street lighting along the shores of Lake Constance may attract fish larvae to unsuitable areas where they encounter poorer feeding conditions and are more exposed to predators. In addition, artificial light can disrupt their natural day-night cycle.

"At present, this effect is probably still of limited significance in Lake Constance", says Alexander Brinker, head of the Fisheries Research Station in Langenargen and professor at the University of Konstanz, "but as climate change progresses – particularly with more frequent heatwaves – the combination of temperature stress and light pollution could become a serious problem in the future."

At the same time, the results point to simple solutions. Because fish larvae are particularly sensitive to certain wavelengths, a targeted adjustment of light colours in the shore area could help to significantly reduce the negative effects of artificial lighting.

Key facts: Original study: Roberts, B. J., J. Baer, A. Ros, and A. Brinker. 2026. "Heatwaves and Anthropogenic Light May Disrupt Phototaxis-Based Behaviours That Regulate Early-Life Migrations of Fish." Freshwater Biology71, no. 4: e70219. Link to the study https://doi.org/10.1111/fwb.70219 Corresponding author: Apl. Professor Alexander Brinker, Fisheries Research Station (FFS) at "Landwirtschaftliches Zentrum Baden-Württemberg" Alexander.Brinker@lazbw.bwl.de