Ontogenetic variation in phototactic responses of juvenile chum salmon (Oncorhynchus keta) under different light intensities

Fish that live in rivers face a constant challenge: how to move safely through a world that is often blocked by human-made structures. Dams and weirs, built to hold water for power or irrigation, act as walls that stop fish from swimming upstream. To help them pass, engineers build special channels called fishways. But while these channels help fish go up, the path down is often more dangerous. The massive pipes that feed water into turbines can act like giant vacuum cleaners, sucking in young fish and killing them. To solve this, scientists have been exploring whether light can guide these young fish away from danger. Many animals, including fish, have an instinctive reaction to light called phototaxis. This is simply the tendency to swim toward a light source or to swim away from it. For decades, researchers have wondered if they could use this instinct to steer young salmon away from dangerous intakes and toward safe exits, but the answer depends heavily on how the fish react to different brightness levels and how their behavior changes as they grow from tiny fry into larger juveniles.

In a recent study, a researcher named Keishi Matsuda set out to test exactly how young chum salmon, a species vital to fisheries in Japan, respond to light. The goal was to see if these fish, at the size they are when released into the wild, would swim toward a white light or avoid it, and whether this behavior changed as they grew. The experiment took place in a controlled indoor tank at a research station in Nikko. The tank was designed to mimic a river with a gentle flow, and the walls were covered to block out any outside light. Inside, the researcher placed a bank of white LED lights at one end. He then introduced one hundred young salmon, each about the size of a small finger, into the tank. These fish were not yet fully grown; they were in a stage just before they would naturally begin their journey downstream to the ocean.

The researcher did not just turn the lights on and off. Instead, he created a careful sequence of brightness levels, starting from total darkness and slowly increasing the intensity to levels far brighter than a typical room. The fish were allowed to swim freely, and cameras recorded where they went. The team counted how many fish gathered near the lights at each brightness level. They compared these results with data from a previous study that used the exact same method but with slightly older, larger salmon known as smolts. This allowed for a direct comparison of how the fish behaved at two different stages of their early life.

The results revealed that the fish did not simply swim toward the light or away from it in a straight line. Instead, their reaction changed depending on how bright the light was. When the light was dim, the young salmon tended to swim closer to the source, showing a mild attraction. As the light got brighter, more fish gathered near it, reaching a peak where the most fish were observed in the illuminated area. However, once the light became very bright—specifically when the brightness exceeded about 1,000 lux, which is roughly the intensity of a bright overcast day or a well-lit office—the behavior flipped. The fish began to avoid the light, swimming away from the source and staying in the darker parts of the tank. This pattern held true for both the tiny fry and the larger smolts. While the older fish generally showed a slightly stronger tendency to gather near the light than the younger ones, the overall shape of the response was the same for both groups.

This finding suggests that the instinct to avoid bright light is present very early in the life of the chum salmon, long before they are ready to migrate to the ocean. The study indicates that these young fish likely avoid strongly illuminated areas to hide from predators or to stay in safer, darker parts of the river. The research also clarifies a practical limitation for conservation efforts. Because the fish do not show a strong attraction to white light at the lower intensities that might be used to guide them, using such lights to pull fry toward a safe exit might not work well. However, the study suggests that very bright white lights could be useful for a different purpose: keeping fish away from dangerous intake pipes. By creating a zone of intense brightness, managers might be able to deter both young fry and older smolts from entering the machinery that could harm them.

The study was conducted in a tank, so it did not account for the complex currents, shadows, or presence of predators found in a real river. It also only tested white light, leaving open the question of whether different colors might have different effects. Nevertheless, the results provide a clear picture of how these fish perceive their world. They show that the behavior of chum salmon is not a simple switch between seeking and avoiding light, but a nuanced response that peaks at a middle ground and turns into avoidance when the light becomes too intense. This understanding helps scientists and engineers design better systems to protect these fish as they navigate the rivers of their youth.