Bigger pupils help migrating fish survive

Each year, billions of animals migrate in search of better conditions. These journeys shape ecosystems, drive evolution, and challenge individual survival.

Migration, however, is not a simple instinctual response. It demands a blend of physical traits and behavioral choices. In fish, even a subtle anatomical difference – like eye size – can influence who migrates and who stays behind.

Recent research from Lund University reveals that the European roach (Rutilus rutilus), a common freshwater fish, shows a surprising link between eye structure and migration behavior.

Specifically, roach that migrate between lakes and streams tend to have larger pupils than those who remain year-round in a single habitat. This physical feature helps them see better in murky water and boosts their ability to find food, a vital edge in the energy-intensive act of migration.

What makes a fish migrate

Roach are partial migrants. That means only some individuals in a population leave the lake each winter to enter connected streams.

The rest stay put, braving seasonal changes in place. This difference in behavior offers researchers a unique lens through which to understand how physical traits affect life strategies.

The research team, led by biologist Kaj Hulthén, tracked more than 2,000 roach across two Danish lakes – Loldrup Sø and Søgård Sø – over several years.

They photographed the fish, implanted each one with a tiny transponder, and monitored movements using underwater antennas placed at stream inlets and outlets.

“We started by taking photos of the fishes’ eyes and then followed the migration patterns of 2,000 individual roaches over several years and in two different lakes by using implanted electronic transponders,” said Hulthén, who is a biology researcher at Lund University.

“It was shown that roach that migrated had larger pupils relative to individuals that chose to stay in the lake throughout the year.”

Eye size matters for fish that migrate

Larger pupils allow more light into the eye, improving vision in low-light conditions. For roach, this visual boost helps detect tiny prey like zooplankton. Using simulations, the researchers showed that roach with larger pupils could see farther – especially in dim twilight conditions, when many fish feed.

Even a 1-4% increase in pupil diameter led to as much as a 5% increase in the volume of water a fish could visually scan.

That matters because in aquatic environments, visual range often dictates feeding success. These small gains are enough to help migratory roach gather the energy reserves they need before entering food-poor winter streams.

In twilight, detection range for 1 mm prey reached up to 20 cm – enough for practical foraging. The advantage fades under starlight but remains relevant during dusk and dawn, which are key feeding times.

Feeding now to survive later

Migrating roach face a trade-off. Streams have fewer predators but offer less food. To survive the winter, migrants must build up fat stores beforehand. Larger pupils may be a tool to gather more food before departure and to stay active longer each day by foraging in low light.

This connection between eyesight and feeding has deep evolutionary roots. Eye size reflects both opportunity and cost. Bigger eyes offer better vision, but they require more energy to build and maintain. For a migrating roach, the benefits seem to outweigh the cost.

“The fact that migrating individuals have an improved ability to detect visual objects such as zooplankton indicates strong selection for effective food searches. This is crucial to cover the energy needs that migration requires,” said Hulthén.

“These results correspond well with previous research, which showed that migrating roach with insufficient energy reserves run a higher risk of dying in the streams.”

Fish with big eyes are likely to migrate

In both study lakes, researchers found a clear pattern: roach with larger relative pupil sizes were more likely to migrate.

The correlation was strongest in Søgård Sø, where murkier water creates a dimmer visual environment. This suggests that visual demands may vary by location, shaping individual behaviour accordingly.

On average, migrant roach in Søgård Sø had pupils 4.2% larger than residents. In Loldrup Sø, the difference was smaller at 1.1%. While modest, these changes are functionally significant. Larger pupils helped fish see better, especially in the dim light that dominates northern winters.

Roach are planktivores. They feed on small, fast-moving prey that require sharp eyesight to detect. Under experimental conditions, fish given more food before winter were more likely to migrate, did so earlier, and stayed in streams longer. Those with less food died more often.

This supports the idea that strong visual skills, like those enabled by larger pupils, help fish meet their energy needs.

A migratory syndrome takes shape

The study’s findings contribute to a growing view that migration is not just a behavioral choice. It’s a full-body strategy involving vision, body shape, energy storage, and risk-taking.

Together, these traits form a “migratory syndrome” – a suite of characteristics that help fish survive the challenges of moving between habitats.

The eye, often taken for granted in ecological studies, now appears central to this survival kit. By enhancing foraging efficiency, it helps determine who has the resources to leave and who stays behind.

“Roach play an important role in the ecosystem. By understanding why some individuals migrate and other don’t, we can gain a deeper insight into how fish populations respond to environmental changes, which in turn could affect both fish stocks and the lake dynamics as a whole,” said Hulthén.

Fish eyes can change shape

Whether these differences in eye morphology are genetic or a response to the environment remains unknown.

However, past studies show that fish eyes can change shape depending on food availability and predator pressure. That means both nature and nurture could play roles in shaping eye size and which fish migrate.

Interestingly, the benefits of bigger eyes seem to matter most in moderate turbidity. In very clear water, all fish see well. In extremely dark or muddy water, vision may become useless. This window of usefulness hints at why migration-related eye differences show up more clearly in some lakes than others.

Eye size and fish migration

Roach migration has ecological ripples beyond the fish themselves. Migrants affect predators, prey, and nutrient cycles.

Understanding how physical traits like pupil size influence migration choices can help scientists predict how fish populations may respond to environmental change.

In this study, a tiny difference in pupil size revealed a key factor in migratory success. It highlights the tight link between form, function, and behavior.

Eyes, often symbols of awareness and insight, prove once again to be more than just windows to the world—they are tools of survival, helping fish decide whether to stay or go.

The study is published in the Journal of Animal Ecology.

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