New paper on how sea trout use lake before they spawn

We’re excited to share that our new paper is out, where we tracked the movements of sea-run brown trout (or sea trout/anadromous brown trout) in Lake Vassbygdivatnet in western Norway to understand how they use the lake before they spawn. These trout are migratory—they travel between rivers and the ocean —but when a lake is available along their path, they often stop and spend time there. What exactly they do during that time, however, has remained a bit of a mystery.

One of the tagged trout

Using tracking technology and advanced data analysis tools, we followed the movements of individual trout over several months. Our findings revealed that trout spend a lot of time close to shore, especially in areas where cool, fresh water from mountain streams (called cascades) flows into the lake. These spots seem to serve as popular hangouts—possibly because they offer shelter, better water quality, or food.

Interestingly, the trout didn’t show the same interest in the area near a hydroelectric power station outlet, which also releases water into the lake. That’s good news, because it suggests that the power station isn't a major attraction for the fish—and more importantly, it doesn’t appear to pose a big risk of drawing them into dangerous tunnels. We also discovered differences in behavior depending on where the trout were in the lake. Near the shore, they tended to move more slowly—possibly resting or waiting. Out in the middle of the lake, they were more active, swimming in straighter, longer paths.

One of the plots from the paper that shows the positions of the trout in lake Vassbygdivatnet

Why does all this matter? It helps us to better understand if and how they use lakes as habitat. These data is also important for the management and the hydropower company by showing potential effects (or lack thereof) on the behaviour of the fish. Furthermore, the data can also help guide fishing regulations and activities, especially during the sensitive pre-spawning period when trout gather in certain lake areas.

You can read the full paper here