A juvenile brown trout in the experimental flume.

Johan Watz, postdoc at Karlstad University, recently published the scientific article “Stress responses of juvenile brown trout under winter conditions in a laboratory stream” in the journal Hydrobiologia. In the abstract he writes: “Winter can be a challenging period for fish in northern temperate rivers and streams, particularly in those that are channelized, structurally simple or regulated by, for instance, hydropower. In these systems, dynamic sub-surface ice formation commonly occurs and stable periods with ice cover may be short. Under these adverse conditions, access to shelters has been shown to be an important factor that influences overwinter survival, and exclusion from shelters by anchor ice may cause stress. Here, stress responses of juvenile brown trout under simulated winter conditions in an artificial stream were studied. Trout were subjected to three treatments in which the trout (1) were excluded from an instream wood shelter, simulating the effects of anchor ice, (2) had access to the shelter or (3) had surface ice cover in addition to the shelter. There was a positive correlation between ventilation frequency and plasma cortisol concentration. Trout without access to shelter had 30% higher ventilation frequency than trout with instream shelter and surface ice, but no differences in cortisol concentration or stress colour were found between the treatments. River regulation that reduces surface ice and increases anchor ice formation may lead to increased stress and consequently reduce overwinter survival rates.”. 

Access the paper here: Stress responses of juvenile brown trout under winter conditions in a laboratory stream.

A PhD-position in the field of global climate change, as it relates to the ecology of stream fishes in winter, is now open for applicants at Karlstad University. The position is a full time position for 4 years within the River Ecology and Management (NRRV) group at the Department of Environmental and Life Sciences. Read more about the position on

As advertised previously, NRRV also has two openings for full-time post-doctoral research fellows. One position is in the field of stream-riparian ecology with focus on the reciprocal interactions and linkages between aquatic and terrestrial habitats. The other position is on river connectivity with focus on rehabilitation, management and development strategies. Read full post-doc position announcements here: Post-doctoral Research Fellow in Aquatic-Terrestrial Linkages and Post-doctoral Research Fellow in Ecology of River Connectivity.


Johan Watz contributed the cover photo.

The paperIce cover affects the growth of a stream-dwelling fish”  by Johan Watz, Eva Bergman, John Piccolo and Larry Greenberg was published as the cover article in the May issue of the scientific journal Oecologia. The journal presents its issue and the paper:

“Winter conditions are believed to play an important role in the population dynamics of northern temperate stream fi sh, challenging the ability of fi sh to physiologically and behaviourally adapt. In this issue, Watz et al. show that brown trout (Salmo trutta) that spent the winter under ice cover grew more and used a broader range of habitats than trout in uncovered stream sections. These results indicate that the presence of surface ice may function as overhead cover against terrestrial piscivores and improve the energetic status of stream fish during winter.”

Read the abstract and access the paper here. If you don’t have access to the journal’s content, email any of the authors.

Ice and brown trout

Posted by Daniel Nyqvist | Vinterekologi

The stream aquaria with ice cover.

The Lab and Stream, a project that “strives to bridge the gap between anglers and academics”, has written about NRRV:s lab research on winter ecoology of brown trout:

“I think that one of the reasons that people are fascinated with fish is that they are hard to observe. Fish live in a world that is difficult to visit and largely mysterious to us air breathers, even though that world might be only a few feet underneath your boat or dock. While the underwater world might seem like an alternate dimension, the under ice world is even more distant from people’s understanding. Anyone that has stood on a frozen lake has wondered what is going on underneath them, but under ice behavior is notoriously hard to study because it’s cold and dark down there.

A cool new study by researchers from Sweden and Norway has shed some light on what trout are doing under the ice. They built stream channels in the lab that had a window on one side (to observe the fish) and added ice cover to the top of the channels to simulate a frozen river environment. In each channel they added 4 brown trout and observed their feeding and swimming behaviour…”

Read the full article at

Johan Watz led the experiment described and focoused his PhD on the winter ecololgy of salmonids. Read his thesis here. Links to the papers included are available in this blog post.

Johan_spikUnder den gångna veckan har Johan Watz spikat sin avhandling “Salmonid behaviour under winter conditions”. Kappan kan läsas online här. Avhandlingens publicerade artiklar är följande:

Paper II: Effects of ice cover on the diel behaviour and ventilation rate of juvenile brown trout

Paper III: Day and night drift-feeding by juvenile salmonids at low water temperatures

Paper IV: Prey capture rates of two species of salmonids (Salmo trutta and Thymallus thymallus) in an artificial stream: effects of temperature on their functional response

Paper V: Ice cover alters the behavior and stress level of brown trout Salmo trutta

Avhandlingens försvar kommer att gå av stapeln den 22 Januari 2016 kl. 10:15 i Nyquistsalen (9C 203) på Karlstads Universitet. Opponent är Richard Cunjak från University of New Brunswick, Kanada. Alla intresserade är välkomna.