Johan Watz, David Aldvén, Antonis Apostolos Brouziotis, Niclas Carlsson, Eirini Karathanou, Kristine Lund‐Bjørnås, Gustav Lundqvist, Martin Österling, John J. Piccolo and Olle Calles recently published the paper “Social behaviour of European grayling before and after flow peaks in restored and unrestored habitats” in the journal River Research and Applications.

In the abstract of the paper the authors write:

“Cost‐effective implementation of fish‐friendly hydropower flow operation and habitat restoration measures require an understanding of their effects on fitness‐related behaviours of stream fish. Here, we investigated how changes in flow and bottom structure influence the social behaviour of European grayling, using large experimental flumes (700 L s−1), with and without added boulders (i.e., restored and unrestored habitat). Grayling increased their distance to nearest neighbour at the start of flow ramping up and after a flow peak compared to stable base flow. At the start of ramping up the flow, grayling made less position changes (movements >1 m) than at stable base flow and after a flow peak. In the unrestored habitat, the proportion of time grayling spent actively swimming was lower before a flow peak than it was both at the start of ramping up the flow and after the peak, an effect not found in the restored habitat. In addition, we compared two static flows, and habitat restoration mediated their effect on distance to nearest neighbour. Grayling in the restored habitat were positioned closer to each other in the low (~10 cm s−1) than in the intermediate static flow (~40 cm s−1), whereas in the unrestored habitat, grayling showed the opposite pattern. Moreover, grayling reduced their number of position changes in the intermediate static flow, which was reflected by a reduction in active swimming. Stomach analysis after the trials revealed that foraging success was higher in variable than in the stable flow treatment. These results show that flow magnitude, flow changes and instream structure play important roles in the behaviour of stream fishes.”

The paper is available open access on the journal website.

Johan Watz (left), together with Eirini Renata Karathanou and Antonis Apostolos Brouziotis.

Johan Watz, Assistant Professor at Karlstad University, recently published two papers on juvenile salmonid ecology:


Temperature‐dependent competition between juvenile salmonids in small streams

By Johan Watz, Yasuhiko Otsuki, Kenta Nagatsuka, Koh Hasegawa & Itsuro Koizumi, published in the journal Freshwater Biology.

In the abstract, the authors write:

Johan Watz, doing field work during his PostDoc in Japan.

1) Biotic interactions affect species distributions, and environmental factors that influence these interactions can play a key role when range shifts in response to environmental change are modelled.

2) In a field experiment using enclosures, we studied the effects of the thermal habitat on intra‐ versus inter‐specific competition of juvenile Dolly Varden Salvelinus malma and white‐spotted charr Salvelinus leucomaenis, as measured by differences in specific growth rates during summer in allopatric and sympatric treatments. Previous laboratory experiments have shown mixed results regarding the importance of temperature‐dependent competitive abilities as a main driver for spatial segregation in stream fishes, and no study so far has confirmed its existence in natural streams.

3) Under natural conditions in areas where the two species occur in sympatry, Dolly Varden dominate spring‐fed tributaries (cold, stable thermal regime), whereas both species often coexist in non‐spring‐fed tributaries (warm, unstable thermal regime). Enclosures (charr density = 6 per m2) were placed in non‐spring‐fed (10–14°C) and spring‐fed (7–8°C) tributaries.

A forest stream on Hokkaido, northern Japan.

4) In enclosures placed in non‐spring‐fed tributaries, Dolly Varden grew 0.81% per day in allopatry and had negative growth (−0.33% per day) in sympatry, whereas growth rates were similar in allopatry and sympatry in spring‐fed tributaries (0.68 and 0.58% per day). White‐spotted charr grew better in sympatry than in allopatry in both thermal habitats. In non‐spring‐fed tributaries, they grew 0.17 and 0.79% per day and in spring‐fed tributaries 0.46 and 0.75% per day in allopatry and sympatry, respectively.

5) The negative effect of inter‐specific competition from white‐spotted charr on Dolly Varden thus depended on the thermal habitat. However, there was no strong evidence of a temperature‐dependent effect of intra‐ and inter‐specific competition on white‐spotted charr growth.

6) Multiple factors may shape species distribution patterns, and we show that temperature may mediate competitive outcomes and thus coexistence in stream fish. These effects of temperature will be important to incorporate into mechanistic and dynamic species distribution models.


Read more about the Koizumi lab at Hokkaido University (where Johan did his PostDoc) on their website!


Structural complexity in the hatchery rearing environment affects activity, resting metabolic rate and post‐release behaviour in brown trout Salmo trutta

By Johan Watz, published in the Journal of Fish Biology

In the abstract, Johan writes:

The effects of structural enrichment in the hatchery rearing environment of brown trout Salmo trutta was linked to post‐release performance. Enrichment resulted in reduced swimming activity scored in an open field test and reduced movement in a natural river after release. Also, enrichment increased resting metabolic rates, which correlated positively with overwinter growth.


Contact the author to access the papers.


The left photographs show Dolly Varden (a) and white‐spotted charr (b). The right photographs show enclosures in a non‐spring‐fed (c) and a spring‐fed (d) tributary.


The structurally enriched (left) and barren (right) tanks used in the study on how structural complexity in the hatchery environment affects juvenile brown trout.


River Rottnan in winter.

Two papers in Animal Conservation

Posted by Karl Filipsson | Papers

Two papers from NRRV were recently published in the journal Animal Conservation. The first paper presents a field study on how sedimentation affects brown trout (Salmo trutta) fry emergence in relation to freshwater pearl mussel (Margaritifera margaritifera) recruitment. The second paper presents a combined field and laboratory study on passage solutions for upstream-migrating eels (Anguilla anguilla).


Sedimentation affects emergence rate of host fish fry in unionoid mussel streams

Martin Österling


In the abstract, the author writes:

Freshwater pearl mussel, Margaritifera margaritifera

“Free-living, sympatric sedentary life stages of hosts and parasites are often adapted to similar environmental conditions. When the environment where these life stages occur is disturbed, both species can decline, causing strong negative effects on the parasitic species. For the highly threatened unionoid mussels with their larval parasitic life stage on fish, habitat degradation may simultaneously affect the conditions for the sedentary host fish eggs and the juvenile mussels in the sediment. This study provides novel information on the effect of sedimentation on the emergence rate of yolk sac fry, and its relation to mussel recruitment in two drainage basins, and is exemplified by the brown trout Salmo trutta, host fish for the threatened freshwater pearl mussel Margaritifera margaritifera. The results imply that turbidity and sedimentation can reduce the survival of trout eggs and yolk sac fry emergence rate regardless of trout strain and drainage basin. The results further suggest that low yolk sac fry emergence rates reduce the potential for mussel infestation and recruitment. The results indicate a year round negative effect of sedimentation, having strong and combined direct and indirect effects on juvenile mussel recruitment. Conservation measures that reduce anthropogenic sediment transportation into streams are a key factor for the conservation of mussels and their host fish.”

Access the paper here, or contact the author.


Climbing the ladder: an evaluation of three different anguillid eel climbing substrata and placement of upstream passage solutions at migration barriers

Johan Watz, Anders Nilsson, Erik Degerman, Carl Tamario and Olle Calles


European eel, Anguilla anguilla. Photo: Jörgen Wiklund

In the abstract, the authors write:

“Conservation programmes for endangered, long-lived and migratory species often have to target multiple life stages. The bottlenecks associated with the survival of juvenile anguillid eels migrating into inland waters, the survival and growth of the freshwater life stage, as well as the recruitment and survival of silver eels, migrating back to the ocean to spawn, must be resolved. In this study, we focus on the efficiency of passage solutions for upstream-migrating juveniles. Such solutions can consist of inclined ramps lined with wetted climbing substrata. We evaluated different commonly used substrata in a controlled experiment, recorded eel behaviour at the entrance of the ramp with infrared videography and validated the experimental results at a hydropower dam, where we also investigated the effects of ramp placement on performance. In the experiment on eel substratum selection, 40% of the eels passed in lanes with studded substratum, whereas only 21 and 5% passed using open weave and bristle substrata respectively. Video analysis revealed that the studded substratum attracted more approaches and initiated climbs than the other substrata, but once a climb had been initiated, passage success rates did not differ between substrata. Eels using the studded substratum climbed 26% faster than those using the bristle substratum and almost four times as fast as those climbing in the open weave. The superior performance of the studded substratum was supported by data from the field validation. Moreover, ramps positioned by the bank with low water velocities caught the most eels, but proximity to the dam had no effect on performance. To strengthen the European eel population, more juveniles need to reach their freshwater feeding grounds. A critical step to achieve this increase is to equip upstream passage solutions with suitable substrata and to optimize ramp placement at migration obstacles.”

Access the paper here, or contact any of the authors.

River Rottnan in winter

Johan Watz, Olle Calles, Niclas Carlsson, Teemu Collin, Ari Huusko, Jörgen Johnsson, Anders Nilsson, Johnny Norrgård and Daniel Nyqvist recently published the paper “Wood addition in the hatchery and river environments affects post-release performance of overwintering brown trout” in the journal Freshwater Biology.

In the abstract, the authors write:

“1. Habitat structural complexity affects the behaviour and physiology of individuals, and responses to the  environment can be immediate or influence performance later in life through delayed effects.

2. Here, we investigated how structural enrichment, both pre-release in the hatchery rearing environment and post-release in the wild, influenced winter growth and site fidelity of brown trout stocked into side channels of a regulated river.

3. Experiencing structural enrichment in the rearing environment during 3 months in autumn had no pre-release effect on growth, but a delayed positive effect after release during the subsequent winter. Moreover, trout recaptured in wood-treated sections of the side channels had grown more than trout recaptured in control sections. Wood enrichment in the side channels also increased overwinter site fidelity.

Johan Watz at the field site.

4. These results show that adding structure during a relatively short period may alter growth trajectories, and adding wood to side channels is a cost-effective method to enhance winter habitat carrying capacity for  juvenile salmonids in regulated rivers.”

Access the paper here.

Teemu Collin tracking trout at the field site.


Dead wood in a side channel of the river.


River Rottnan.

A Dolly Varden trout (Salvelinus malma).

On Tuesday 16 October, Johan Watz from Karlstad University will give a seminar titled: “Report from a postdoctoral research stay in Sapporo and results from a field experiment: Condition-specific competition between two Japanese charr species”. The seminar will start at 13:15 in room 5F416 at Karlstad University. Everyone who wants to are welcome to attend the seminar.

Forskare i NRRV har under hösten påbörjat ett projekt inom ett för forskargruppen helt nytt ämnesområde. Det handlar om rörelsemönster hos den spanska skogssnigeln, även kallad mördarsnigel. Mördarsnigeln är en ovälkommen gäst i mångas trädgårdar, där den kan orsaka stor skada genom att äta rent i odlingar och blomsterrabatter. Det är en främmande art i Sverige, som mest troligt har kommit hit när ägg följt med plantor, jord, krukor och annan trädgårdsutrustning.

Frågor om sniglarnas spridningsmönster och koloniseringsförmåga när de väl har kommit till ett område skulle kunna besvaras med hjälp av studien. Till en början diskuterades sniglarna under en fikarast på universitetet, sedan följde att Johan Watz (projektledare) med flera på Karlstads universitet sökte och fick pengar av Kungliga Skogs- och Lantbruksakademien för att genomföra studien.

Nu har ca 50 mördarsniglar i en trädgård i Karlstad märkts med små chip (så kallade pit-tags, samma typ av märkning som används i forskningen om fisk), så att sniglarna kan pejlas och deras rörelsemönster och levnadsvanor studeras.

Tidningen Värmlands folkblad (VF) skrev om projektet måndag 1 oktober, som på flera platser i Karlstad tog plats på löpsedlarna.

Även SVT uppmärksammade studien 3 oktober, och reportaget når du genom att trycka här.

Johan Watz, Anders Nilsson and Olle Calles from Karlstads Universitet, and Jonas Elghagen from Elghagen FIskevård, recently published the scientific note “Evaluation of a novel mobile floating trap for collecting migrating juvenile eels, Anguilla anguilla, in rivers” in the journal Fisheries Management and Ecology.

In the abstract, the authors write: “To improve the situation for the threatened European eel in regulated rivers, better methods need to be developed that more efficiently collect and transport juvenile eels past dams. In this study, a novel mobile, floating eel trap is described, and the results from an evaluation of the trap in two Swedish regulated rivers are presented. The mobile trap was designed to reduce the length of the climbing distance while maximizing the width of the entrance. The mobile trap caught more juvenile eels than a stationary eel ladder, serving as control. Furthermore, the mobility of the floating trap enables adaptive placement and thus offers managers the possibility to search for the spatial optimum for trapping efficiency.” Access the paper here, or e-mail any of the authors

The mobile floating trap next to the stationary eel ladder used as control. Photo from the Watz et al. 2017.

The mobile floating trap without cover. Juvenile eels climb the short and wide ramps (black) and slide (on the small metallic shutes) towards the left (in the photo) were they are collected. Photo: from the papers supplementary material.

The trap in the tailrace of a hydropower plant in River Lagan.

A sample of juvenile eels caught in the study, here held in a 10 L bucket.

The book, “Brown Trout: Biology, Ecology and Management”, edit by Javier Lobón-Cerviá and Nuria Sanz was recently published. The book is described as “a comprehensive guide to the most current research, history, genetics and ecology of the brown trout including challenging environmental problems”. John Piccolo and Johan Watz, both researchers at Karlstad University, have written the chapter Foraging Behaviour of Brown Trout: A Model Species For Linking Individual Ecology to Population Dynamics? They summarize their chapter as follows:

“Within the discipline of stream fish ecology, population-, community-, and even ecosystem-level patterns and processes have assumed an increasingly larger role in recent decades. It might be argued, however, that research on the behaviour of individual organisms ought still to play a major role in ecology; it is upon the individual, after all, that natural selection acts. Thus, one might reasonably expect that observing an individual fish’s behaviour should lead to robust conclusions about the fitness costs and benefits that animals must trade-off in order to achieve reproductive success. And ultimately, it is those individuals that achieve the greatest direct fitness that ought to, on average, drive the population-level processes that have attracted so much of stream fish ecologists’ attention in recent years. In linking behavioral- to population-level ecology, we are in luck when it comes to the brown trout – there is no stream fish species whose population ecology is better quantified, nor are there many species that have received more attention from behavioural ecologists. Thus we might consider the brown trout as a model species for developing the ecological understanding of how natural selection (e.g. individual, fitness-based decisions) acts to regulate stream fish populations. To forward this concept, a further development of a quantitative approach to foraging behaviour is warranted. In this chapter we review and synthesize the literature on brown trout foraging experiments with an eye towards identifying the knowledge gaps that remain to be filled in order for ecologists to quantify the fitness costs and benefits of foraging behaviour.”

The book is available (but expensive) here. For access to the specific book chapter, email John Piccolo or Johan Watz.


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.

Johan Watz, postdoc vid Karlstads Universitet, forskar om öringens vinterekologi. Just nu pågår fältarbete i Rottnan, Värmland. Johan berättar: “Vinterförhållanden i rinnande vatten kan påverka hur mycket fisk som kan leva i älven och hur många smolt som produceras. I ett projekt tillsammans med Fortum och Bergvik Skog undersöker vi hur öringungar klarar vintern i sidofåror till Rottnan. Platser som har berikats med struktur, i form av träd som fällts i vattnet, jämförs med platser utan trädberikning. Ettåriga PIT-märkta öringar spåras genom isen, och med stationära loggerstationer. Öringarnas förflyttningar, överlevnad och tillväxt studeras. Projektet kommer fortgå fram till islossningen.”


Johan Watz, mitt i Rottnan. “Fältarbete i -20C kan vara så här roligt.”


Lisse de Groot, Erasmuspraktikant från Nederländerna, vid en damm av bottenis.


En loggerstation (vid den svarta plastpåsen) detekterar fiskar som simmar förbi.


Teemu Collin och Niclas Carlsson (studenter vid Karlstads Universitet) pejlar öring.

Verkar detta intressant? Just nu söker Karlstads Universitet en doktorand i just fiskars vinterekologi: PhD position in Global climate change and winter ecology