When river water flows strongly, young Atlantic salmon have a much better chance of surviving their first big journey to the sea. That's the finding from a two-year study tracking 74 young salmon as they swam from the Haringvliet estuary toward the North Sea.
Researchers from Wageningen Marine Research and a company called ATKB fitted the fish with special acoustic tags and released them near Numansdorp, about 28 kilometers upstream from the Haringvliet sluices. These are large gates that control how much river water flows out to sea. Using a network of underwater receivers, the scientists followed every fish's progress.
The results showed how dramatically water levels matter. In 2024, when river discharge was high, nearly half of the tagged young salmon made it all the way to the North Sea. But in 2025, when discharge was low, only 6 percent succeeded. The difference in travel time was just as striking: at low water levels, salmon took an average of about 27 hours to find their way through the sluices, compared to just 11 hours when water was flowing freely.
"At high discharge, young salmon reach the North Sea much more easily," said Jacco van Rijssel, an ecologist at Wageningen Marine Research. "Their journey takes less time, reducing the likelihood of them being eaten."
The tags used in the study were specially designed. Besides tracking a fish's location, they send a different signal if the fish gets eaten. A built-in sensor even measures the temperature inside a predator's stomach, helping researchers tell whether the predator was a warm-blooded animal like a bird or a cold-blooded fish.
The data revealed that most predation happened while the salmon were still in the freshwater section, before they even reached the sluices. The main culprits appear to be predatory fish like pike, pike-perch, perch, asp, and catfish. Birds probably take some salmon too, though those deaths are harder to confirm because birds often digest their prey completely.
The findings offer a practical path forward. Almost all successful passages occurred when the sluices were open and actively discharging water. Van Rijssel suggests managers could improve salmon survival by opening as many of the 17 Haringvliet sluices as possible during the peak migration season.
Yet there is a challenge on the horizon. Climate change may make periods of low river discharge more common, which could make things even harder for young salmon trying to reach the sea. Researchers say the hope is that salmon have enough genetic variation to eventually adapt to these changing conditions.