Populations of brown trout have both higher population density and are more stable where rapid-flowing habitats in rivers are large and well connected to each other. This is shown in a new study by researchers from Linnaeus University, the result of which can contribute to making restorations of brooks, streams and rivers more successful.
Brown trout (Salmo trutta) is a socioeconomically and ecologically important salmonid found throughout Sweden and in large parts of the world. The brown trout is very flexible and occurs in brooks, streams and lakes as well as in the ocean. Something that characterises all brown trout populations is that they require flowing water, both for spawning and as nursery areas for the young.
In the research article, which was recently published in the journal Proceedings of Royal Society B – Biological Sciences, the research group from Linnaeus University shows that both the size of the rapid-flowing habitat and its proximity to other rapid-flowing habitat patches have significant positive effects for the popular angling and food fish.
Human activities causes negative consequences
“Exploitation and climate change can be devastating for biodiversity. Knowledge about how living conditions for different species can be improved is therefore crucial. Unfortunately, the majority of our rivers have been severely modified by human activities during the past century, which has resulted in major negative consequences for the viability of many brown trout populations”, says Carl Tamario, doctoral student and lead author.
Not only are brown trout more abundant in larger rapid-flowing habitats – the populations also have a higher density and are more stable over time. That brown trout populations are more viable if the rapid-flowing habitats are closer to each other is seen as a consequence of an increase in the exchange of individuals and genes between subpopulations in different rapid-flowing habitat patches. This has been shown by combining data from electrofishing at the Swedish University for Agricultural Sciences with geographic and topographic analyses of the landscape.
“We also discovered that the closer to the edge of slow-flowing habitat the brown trout population was sampled, the fewer brown trouts were there. However, this only applied if pike was present on the location. Pikes are efficient predators. It is often said that pikes lurk in the reeds, but in watercourses they lurk in the calm waters, preferably in the outskirts of the rapid-flowing water”, explains Anders Forsman, professor of evolutionary ecology at Linnaeus University and one of the co-authors.
Brown trout are more vulnerable in small habitats
The results indicate that pikes make shorter excursions into the rapid-flowing water to try to catch brown trouts, which either escape or get eaten, a so-called negative edge effect. When rapid-flowing habitats are small, the threat from pikes becomes more pronounced. Dams also hinder the exchange of fish between different rapid-flowing habitats, which means that brown trouts lose the ability to migrate and that the amount of valuable habitat decreases.
“The realization that the size of rapid-flowing habitats and proximity to other rapid-flowing habitats has such great impact on the numbers and dynamics of brown trout has important implications for the planning of resource utilisation and for restoration of watercourses. When the fragmented water landscape with small isolated units is reconnected, for instance through dam removals or fish passage solutions, the negative edge effects can be reduced”, Tamario concludes.
The researchers behind the report are Carl Tamario, Petter Tibblin, Daniela Polic, and Anders Forsman, all are part of Linnaeus University’s cutting-edge research group Linnaeus University Centre for Ecology and Evolution in Microbial model Systems, EEMiS, which is part of Linnaeus Knowledge Environment: Water.
- The full article Size, connectivity and edge effects of stream habitats explain spatio-temporal variation in brown trout (Salmo trutta) density
- Linnaeus University’s cutting-edge research group EEMiS
- Linnaeus Knowledge Environment: Water