Sweden’s lakes are changing rapidly. Climate change is bringing milder winters and increased precipitation. The result? The water becomes both warmer and browner. As rain and meltwater flow through forests and land, organic material (humus) is carried into the lakes. It darkens the water and reduces light penetration.
For a long time, researchers have assumed that this would strongly affect predatory fish like pike. They hunt by sight—poorer visibility should mean poorer hunting. But new research from Karlstad University shows that reality is far more complex.
– We were actually surprised. The pike ate just as much in almost coffee-brown water as in crystal-clear water, says Viktor Nilsson, senior lecturer in biology.
Coffee-brown water is no problem
In controlled laboratory experiments, the researchers examined how pike respond to different temperatures, visibility conditions, and amounts of prey fish (such as roach). This type of relationship is called a functional response—how a predator’s food intake changes depending on prey availability—and is central to understanding how entire ecosystems function.
These experiments are unusual, not least because they are logistically difficult to carry out with predatory fish like pike. The results showed that browner water had almost no effect on the pike’s food intake. Despite significantly reduced visibility, the pike caught just as many roach. The researchers interpret this as reduced visibility affecting both parties: the pike sees worse, but so does the roach. When prey fish have a harder time detecting an approaching predator, the effects may cancel each other out.
Why this differs from previous studies in turbid water remains an open question.
When more prey leads to fewer catches
The biggest surprise, however, came in cold and clear water—typical Swedish winter conditions.
Normally, a simple principle applies: the more prey there is, the more the predator eats. And at first, the pike followed this rule. But when the number of roach became very high, something unexpected happened in cold, clear water. The number of captured prey suddenly decreased.
– We observed a so-called dome-shaped functional response, a phenomenon previously documented in some insects and spiders, but never in predatory freshwater fish, says Viktor Nilsson.
The explanation is believed to lie in the behavior of the prey fish. In late autumn, young roach often gather in dense shoals in sheltered areas. At the same time, many larger individuals leave the lake and overwinter in streams. In dense groups, the roach may make it difficult for the pike to isolate and attack a single individual.
– It seems that the roach can actually “turn the tables” by grouping together. Their collective behavior may be so effective that more prey paradoxically leads to fewer captures, says Viktor Nilsson.
Serengeti, but underwater
Roach and other cyprinid fish make up a large portion of the biomass in Swedish lakes. When they gather in large shoals or move between lakes and watercourses, it can involve enormous seasonal migrations—almost like the buffalo of the Serengeti, but beneath the surface.
The study shows that winter is not an ecological “dead period,” but a time of intense and complex interactions between predators, prey, and the environment.
– As the climate changes, these winter processes may become more important for how future lake ecosystems function than we previously understood, says Viktor Nilsson.
It is an important reminder that nature rarely follows our simple assumptions.
- Read the article in Journal of Animal Ecology, “Effects of temperature and browning on the functional response of a freshwater top predator,” here.
