<p>Abstract</p><p>This study takes up a local issue in the village of Lekhyttan, where Johan Molin is concerned about the freshwater pearl mussels (Margaritifera margaritifera) situation. He is a landowner and wonders what the situation is for the mussels and what he himself possibly can do, to hopefully improve their situation.</p><p>This paper looks at the different aspects on why the mussels are distributed as they are in the stream and trying to understand why new recruitment is poor in the Lekhyttan Stream.</p><p>Dividing the area into two zones, east (that has a population of mussels) and west (which has none), makes it easier to analyse which type of habitat they prefer. Why the mussels can be found in one zone and not in the other can be due to several different factors.</p><p>The factors investigated were: 1) bottom structure, 2) trout (Salmo trutta) - spawning area and mature trout habitat as the trout play an important role in the mussels lifecycle,</p><p>3) surrounding vegetation, 4) predation from signal crayfish (Pacifastacus leniusculus) and 5) water quality checked with a pH and iron test – from the inflows of ditches and pipes.</p><p>There was a significant difference between the two zones in the bottom sediment structure: the west zone had a high percentage of stone, whilst the east zone consisted of finer materials and had far more suitable habitat areas for the trout (both in spawning as well as for larger trout). Tests from the inflowing water in the west zone had water with low and high pH levels, as well as high iron contents. These inflows need to be fixed with a pond and reed filtration bed, as well as a recommendation on cutting back the number of crayfish that can be a potential threat to juvenile mussels. Following these recommendations will hopefully improve the situation locally for the Lekhyttan freshwater pearl mussel and maybe even lead to new recruitment.</p>

Abstract This study takes up a local issue in the village of Lekhyttan, where Johan Molin is concerned about the freshwater pearl mussels (Margaritifera margaritifera) situation. He is a landowner and wonders what the situation is for the mussels and what he himself possibly can do, to hopefully improve their situation. This paper looks at the different aspects on why the mussels are distributed as they are in the stream and trying to understand why new recruitment is poor in the Lekhyttan Stream. Dividing the area into two zones, east (that has a population of mussels) and west (which has none), makes it easier to analyse which type of habitat they prefer. Why the mussels can be found in one zone and not in the other can be due to several different factors. The factors investigated were: 1) bottom structure, 2) trout (Salmo trutta) - spawning area and mature trout habitat as the trout play an important role in the mussels lifecycle, 3) surrounding vegetation, 4) predation from signal crayfish (Pacifastacus leniusculus) and 5) water quality checked with a pH and iron test – from the inflows of ditches and pipes. There was a significant difference between the two zones in the bottom sediment structure: the west zone had a high percentage of stone, whilst the east zone consisted of finer materials and had far more suitable habitat areas for the trout (both in spawning as well as for larger trout). Tests from the inflowing water in the west zone had water with low and high pH levels, as well as high iron contents. These inflows need to be fixed with a pond and reed filtration bed, as well as a recommendation on cutting back the number of crayfish that can be a potential threat to juvenile mussels. Following these recommendations will hopefully improve the situation locally for the Lekhyttan freshwater pearl mussel and maybe even lead to new recruitment.

The freshwater mussel family Unionoida lives a complex life with its host animals and the freshwater mussel family’s existence is threatened worldwide. One of these species, the pearl mussel (Margaritifera margaritifera), is a “responsibility species” for Scandinavia and a lot of work is ongoing to save the species. In Scandinavia there are still existing populations, but in many waters recruitment of juvenile mussels is completely lacking or insufficient. To support recruitment and also reintroduce the mussel into suitable watercourses, more knowledge about its complex life cycle and how it affects its host brown trout (Salmo trutta), is required. Attempts have been made to introduce gravid mussels or already infected trout in order to try to rejuvenate or to reintroduce mussels in some rivers. In this study, 293 trout individuals were captured from three watercourses in western Sweden with no or inadequate recruitment of juvenile pearl mussels. The brown trout were treated with mussel infection by being kept in containers with the presence of gravid mussels and compared to a control group where no mussels were present. Growth, condition factor and habitat selection were investigated and checked after treatment by scanning the trout with a mobile scanner and recaptured for control. The analyzes showed a significantly lower growth on those trout treated with mussel infection during the time they were stored in the containers. After a month in freedom in the streams, there was no differences in growth or condition factor. Habitat choice studies showed that trout with a higher degree of infection chose calmer water with a bottom layer of finer sediment. The study showed that this method could be a simple way of increasing the reproduction success of pearl mussel. The method requires relatively little work effort and small disturbance to host fish or the aquatic environment. / Familjen sötvattensmusslor har ett larvstadie som parasit och lever ett komplext liv tillsammans med sina värddjur och sötvattensmusslornas existens är hotade världen över. Flodpärlmusslan (Margaritifera margaritifera) är en hotad art vars populationer i Skandinavien, där den är en ansvarsart, har en utebliven eller otillräcklig föryngring. Mer kunskap behövs om dess livscykel och hur den påverkar sin värd, öringen (Salmo trutta), för att hitta bra metoder som kan stödja musslans rekrytering. Försök har gjorts med att plantera in gravida musslor eller redan infekterad öring för att på det viset försöka få till stånd en föryngring av musslor eller att återinföra öring i vissa vatten. I denna studie undersöktes om det fanns någon skillnad med avseende på tillväxt och konditionsfaktor mellan infekterad eller inte infekterad öring och om infektionsgraden påverkade deras val av habitat. Tvåhundranittiotre öringar elfiskades ur tre vattendrag i Västsverige med ingen eller otillräcklig föryngring av flodpärlmussla. Öringarna behandlades genom att infekteras i sumpar med närvaro av gravida musslor och jämfördes med en kontrollgrupp, utan musslor i sumparna. Tillväxt, konditionsfaktor och habitatval undersöktes och kontrollerades efter behandling genom att öringen scannades med mobil scanner och elfiskades åter 30 dagar efter att de släppts. Tillväxten på de öringar som behandlades med musselinfektion var signifikant lägre än de som inte behandlades med musselinfektion under den tid de förvarades i sumpar. Efter en månad i frihet fanns ingen skillnad i tillväxt mellan de infekterade och inte infekterade öringar. Ingen skillnad i konditionsfaktor kunde påvisas innan de placerades i buren, när de togs ur buren eller efter 30 dagar i frihet i bäckarna. Öring med högre infektionsgrad valde lugnare vatten med högre andel fint substrat. Resultatet visade att den infekterade öringen påverkades negativt med avseende på tillväxt när den utsattes för stress men också att den hade förmåga att återhämta sig snabbt. Habitatvalet gav också det indikationer på att den mer infekterade öringen drog sig till en mindre krävande miljö. Studien visade att metoden kan rekommenderas och kan vara en enkel metod att öka reproduktionsframgången för flodpärlmusslan som kräver liten arbetsinsats, är relativt kostnadseffektiv och ger en liten störning på värdfisken och vattendragets miljö.

The effect of climate on the fluvial system has long been investigated due the significant impact it can have on a river’s hydrological regime and fluvial processes. In recent years this interest has increased as global changes in climate are expected to bring more frequent high magnitude flood events globally and to North West Europe in particular. Despite the knowledge that the frequency and magnitude of floods is to increase, less is known about the geomorphological implications of this for river channels and where channel instability is likely to occur at both the river network and national scale. This is certainly the case in Scotland where increased flooding is expected and large floods have been abundant over the last two decades. To manage Scottish river catchments effectively in the future, in terms of hazard mitigation and nature conservation, river managers need to be able to predict not only how climate will impact flood magnitude and frequency in Scotland but the effect these changes will have on the internal dynamics of river channels in terms of erosion, sediment transport and deposition, and morphological dynamics. Such knowledge will ensure adequate measures are implemented to reduce fluvial risks to humans and to maintain and preserve valuable river habitats and linked species. In this thesis, several novel methods incorporating field, laboratory and GIS-based analysis, have been investigated as a means of predicting how climate change will affect channel stability in Scottish rivers and the implications of this for river management and river ecology. This includes (i) analysing the potential change in the frequency of geomorphologically-active flood flows with climate change; (ii) the use of stream power thresholds to predict changes in channel stability on a national scale with climate change; and (iii) using a Digital River Network developed using geospatial data to predict changes in the rate of bedload transfer and channel stability with climate change. Studies were undertaken on 13 different rivers across Scotland from north to south and east to west. vii As a case study of ecological implications, the thesis also examines how changes in habitat and stability of freshwater pearl mussels (Margaritifera margaritifera) may be altered by increased flooding. Predictions of the frequency of geomorphic activity, channel stability, rate of bedload transfer, and the stability of freshwater pearl mussel habitat with climate change are discussed along with the methods used to obtain these outcomes. The results all suggest an increase in the frequency and rate at which bedload is transferred through the river system and an increased frequency of flood flows resulting in greater channel instability. Morphological responses vary spatially with some river reaches experiencing greater increased erosion and transport potential than others. Climate change effects on the freshwater pearl mussel are: increased occasions of disturbance and transport downstream and the importance of specific populations in more stable environments for ensuring population recovery post flooding is highlighted. It is hoped that the methodologies developed for predicting changes in channel stability with climate change will provide useful screening tools to regulatory agencies which can be developed further to assist management decisions in the future which aim to reduce fluvial hazards and maintain good quality river environments for the species that inhabit it. The approaches used in this study allow for the identification of areas at high risk of morphological and ecological change, and the pro-active planning and management of sediment-related river management issues and nature conservation.

<p>The freshwater pearl mussel, Margaritifera margaritifera, is a species in decline throughout its entire range in Europe and North America. In Sweden, the species has disappeared from one third of the watercourses it was found in a century ago, and recruitment occurs in only one third of the approximately 400 populations of mussels remaining. Changes to a watercourse induced by forestry are seen to be among the causes of decline, though there is little direct evidence. This study used GIS to determine various aspects of land cover in the catchment areas of 25 mussel populations in Western Värmland, Sweden. The road network, percentage of different land covers, and forest age were examined at four scales: whole catchment, subcatchment (up to nearest sediment trap), 250m and 100m streamside corridors. Streams containing populations of recruiting mussels had significantly fewer road crossings than streams with non-recruiting populations, perhaps due to increased sedimentation of streams in connection to road crossings. Recruiting mussels were found in catchments with more forest of age class 4-40 years, and significance increased as the scale of analysis was reduced. Almost all other age classes were insignificant. Reasons for these relationships are proposed, which include the larger proportion of birch in young forest (which provide more easily degradable material than conifer needles); and the higher acidity of soils in conifer forests, which increases with forest age and can be detrimental to mussels.</p> / <p>Flodpärlmusslan, Margaritifera margaritifera, visar en kraftig nedgång i hela sitt utbredning-område i Europa och Nordamerika. I Sverige har arten försvunnit från en tredjedel av alla vattendrag som den fanns i för hundra år sedan, och rekrytering sker bara i en tredjedel av de cirka 400 återstående populationerna. Förändringar i vattendrag, som en följd av skogsbruk, ses som en orsak till nedgången, dock finns det föga bevis. GIS användes för att undersöka olika aspekter av markslag i avrinningsområden för 25 musselpopulationer i västra Värmland. Vägnätet, procentandel av olika markslagtyper och skogsålder undersöktes på fyra nivåer: hela avrinningsområdet, avrinningsområdet uppströms till närmaste sedimentfälla och med 250m respektive 100m zoner runt bäcken. Vattendrag med rekryterande populationer hade signifikant färre vägkorsningar än vattendrag utan rekrytering, kanske på grund av ökad sedimentation i vattendrag där vägar korsar dem. Rekryterande populationer fanns i avrinningsområden med mer skog i åldersklass 4-40 år, och detta samband blev mer signifikant i och med lägre analysnivå. De flesta andra åldersklasserna uppvisade ingen signifikans. Förklaring till dessa samband föreslås bland annat vara större inslag av björk i ungskog, vilket ger ett lättare nedbrytbart material än barrträd, och den surare marken som förekommer med barrskog, vilken förvärras med skogsålder och därigenom kan vara skadlig för unga musslor.</p>

<p>The freshwater pearl mussel (<em>Margaritifera margaritifera</em>) has declined dramatically throughout its range and is faced with recruitment problems in most of the streams where populations still persist. Human activities, such as forestry and agriculture, are thought to be the main reason for these problems. In this study, spatial information on landscape features along 38 streams with known recruitment status were analyzed in an effort to determine if GIS-tools could be used to identify streams with successful recruitment. Differences in the distribution of various landscape features, measured in stream corridors of 50 and 150 m, and differences in several water chemical factors between streams with and without recruitment were investigated. The distribution of landscape features was also compared with host fish (brown trout) density and any statistically significant water chemical factor.  Both mussel recruitment and trout density were found to be negatively related to clear-cuts, and mussel recruitment was also negatively related to high water color, which has been shown to be correlated with high nutrient content, one of several adverse effects of clear-cutting close to streams. Recruitment was expected to be negatively affected by roads, but no such relationship could be found. Instead, mussel recruitment was found to be positively related to the number of road crossings per kilometer, but the strength of this relationship was questionable. In addition, even though it was somewhat unclear, high proportions of lakes and ponds were found to be positive for both recruitment and high trout density. The results of the study indicate that GIS-tools can be used to find landscape features that affect recruitment of freshwater pearl mussel and they support the belief that forestry activities are an important cause for the decline of the species in Sweden. In addition, the results indicate that leaving protective zones of forest between streams and clear-cuts can be a possible conservation method for the freshwater pearl mussel.</p> / <p>Flodpärlmusslan (<em>Margaritifera margaritifera</em>) har minskat kraftigt i hela sitt utbredningsområde och i de flesta vattendrag där populationer finns kvar sker ingen rekrytering. Mänskliga aktiviteter, exempelvis skogsbruk och jordbruk, anses vara huvudorsaken för dessa problem. Syftet med den här studien var att undersöka om GIS-verktyg kan användas för att identifiera vattendrag med fungerande rekrytering. För att uppnå det analyserades geografisk information om landskapet längs 38 vattendrag med känd rekryteringsstatus. Skillnader i fördelningen av olika landskapsföreteelser, uppmätt i 50 och 150 meters buffertzoner, och skillnader i flera vattenkemiska faktorer mellan vattendrag med eller utan rekrytering undersöktes. Dessutom undersöktes även om det fanns något förhållande mellan landskapsföreteelser och tätheten av värdfisk (öring) och mellan landskapsföreteelser och signifikanta vattenkemiska faktorer. Både rekrytering av flodpärlmusslor och öringstäthet var negativt relaterad till kalhyggen. Rekrytering var också negativt relaterad till hög vattenfärg, som har visats vara korrelerat med högt näringsinnehåll, en av flera effekter kalhyggen nära vattendrag har. Rekrytering förväntades påverkas negativt av vägar, men inget negativt förhållande hittades. Istället hittades ett positivt förhållande mellan rekrytering och antalet vägkorsningar per kilometer, men styrkan av det förhållandet var ifrågasättbar. Dessutom var, om än något otydligt, andelen sjö och damm positivt för både rekrytering och hög öringtäthet. Studien indikerar att GIS-verktyg kan användas för att hitta landskapsföreteelser som påverkar rekryteringen av flodpärlmusslor och den stödjer bedömningen att skogsbruk är en av de viktigaste anledningarna till artens nedgång i Sverige. Dessutom indikerar resultaten att skyddszoner mellan kalhyggen och vattendrag kan vara en möjlig skyddsåtgärd för flodpärlmusslan.</p>

The freshwater pearl mussel, Margaritifera margaritifera, is a species in decline throughout its entire range in Europe and North America. In Sweden, the species has disappeared from one third of the watercourses it was found in a century ago, and recruitment occurs in only one third of the approximately 400 populations of mussels remaining. Changes to a watercourse induced by forestry are seen to be among the causes of decline, though there is little direct evidence. This study used GIS to determine various aspects of land cover in the catchment areas of 25 mussel populations in Western Värmland, Sweden. The road network, percentage of different land covers, and forest age were examined at four scales: whole catchment, subcatchment (up to nearest sediment trap), 250m and 100m streamside corridors. Streams containing populations of recruiting mussels had significantly fewer road crossings than streams with non-recruiting populations, perhaps due to increased sedimentation of streams in connection to road crossings. Recruiting mussels were found in catchments with more forest of age class 4-40 years, and significance increased as the scale of analysis was reduced. Almost all other age classes were insignificant. Reasons for these relationships are proposed, which include the larger proportion of birch in young forest (which provide more easily degradable material than conifer needles); and the higher acidity of soils in conifer forests, which increases with forest age and can be detrimental to mussels. / Flodpärlmusslan, Margaritifera margaritifera, visar en kraftig nedgång i hela sitt utbredning-område i Europa och Nordamerika. I Sverige har arten försvunnit från en tredjedel av alla vattendrag som den fanns i för hundra år sedan, och rekrytering sker bara i en tredjedel av de cirka 400 återstående populationerna. Förändringar i vattendrag, som en följd av skogsbruk, ses som en orsak till nedgången, dock finns det föga bevis. GIS användes för att undersöka olika aspekter av markslag i avrinningsområden för 25 musselpopulationer i västra Värmland. Vägnätet, procentandel av olika markslagtyper och skogsålder undersöktes på fyra nivåer: hela avrinningsområdet, avrinningsområdet uppströms till närmaste sedimentfälla och med 250m respektive 100m zoner runt bäcken. Vattendrag med rekryterande populationer hade signifikant färre vägkorsningar än vattendrag utan rekrytering, kanske på grund av ökad sedimentation i vattendrag där vägar korsar dem. Rekryterande populationer fanns i avrinningsområden med mer skog i åldersklass 4-40 år, och detta samband blev mer signifikant i och med lägre analysnivå. De flesta andra åldersklasserna uppvisade ingen signifikans. Förklaring till dessa samband föreslås bland annat vara större inslag av björk i ungskog, vilket ger ett lättare nedbrytbart material än barrträd, och den surare marken som förekommer med barrskog, vilken förvärras med skogsålder och därigenom kan vara skadlig för unga musslor.

The freshwater pearl mussel (Margaritifera margaritifera) has declined dramatically throughout its range and is faced with recruitment problems in most of the streams where populations still persist. Human activities, such as forestry and agriculture, are thought to be the main reason for these problems. In this study, spatial information on landscape features along 38 streams with known recruitment status were analyzed in an effort to determine if GIS-tools could be used to identify streams with successful recruitment. Differences in the distribution of various landscape features, measured in stream corridors of 50 and 150 m, and differences in several water chemical factors between streams with and without recruitment were investigated. The distribution of landscape features was also compared with host fish (brown trout) density and any statistically significant water chemical factor.  Both mussel recruitment and trout density were found to be negatively related to clear-cuts, and mussel recruitment was also negatively related to high water color, which has been shown to be correlated with high nutrient content, one of several adverse effects of clear-cutting close to streams. Recruitment was expected to be negatively affected by roads, but no such relationship could be found. Instead, mussel recruitment was found to be positively related to the number of road crossings per kilometer, but the strength of this relationship was questionable. In addition, even though it was somewhat unclear, high proportions of lakes and ponds were found to be positive for both recruitment and high trout density. The results of the study indicate that GIS-tools can be used to find landscape features that affect recruitment of freshwater pearl mussel and they support the belief that forestry activities are an important cause for the decline of the species in Sweden. In addition, the results indicate that leaving protective zones of forest between streams and clear-cuts can be a possible conservation method for the freshwater pearl mussel. / Flodpärlmusslan (Margaritifera margaritifera) har minskat kraftigt i hela sitt utbredningsområde och i de flesta vattendrag där populationer finns kvar sker ingen rekrytering. Mänskliga aktiviteter, exempelvis skogsbruk och jordbruk, anses vara huvudorsaken för dessa problem. Syftet med den här studien var att undersöka om GIS-verktyg kan användas för att identifiera vattendrag med fungerande rekrytering. För att uppnå det analyserades geografisk information om landskapet längs 38 vattendrag med känd rekryteringsstatus. Skillnader i fördelningen av olika landskapsföreteelser, uppmätt i 50 och 150 meters buffertzoner, och skillnader i flera vattenkemiska faktorer mellan vattendrag med eller utan rekrytering undersöktes. Dessutom undersöktes även om det fanns något förhållande mellan landskapsföreteelser och tätheten av värdfisk (öring) och mellan landskapsföreteelser och signifikanta vattenkemiska faktorer. Både rekrytering av flodpärlmusslor och öringstäthet var negativt relaterad till kalhyggen. Rekrytering var också negativt relaterad till hög vattenfärg, som har visats vara korrelerat med högt näringsinnehåll, en av flera effekter kalhyggen nära vattendrag har. Rekrytering förväntades påverkas negativt av vägar, men inget negativt förhållande hittades. Istället hittades ett positivt förhållande mellan rekrytering och antalet vägkorsningar per kilometer, men styrkan av det förhållandet var ifrågasättbar. Dessutom var, om än något otydligt, andelen sjö och damm positivt för både rekrytering och hög öringtäthet. Studien indikerar att GIS-verktyg kan användas för att hitta landskapsföreteelser som påverkar rekryteringen av flodpärlmusslor och den stödjer bedömningen att skogsbruk är en av de viktigaste anledningarna till artens nedgång i Sverige. Dessutom indikerar resultaten att skyddszoner mellan kalhyggen och vattendrag kan vara en möjlig skyddsåtgärd för flodpärlmusslan.

Parasiter kan påverka sina värdar på många olika sätt. Flodpärlmusslans (Margaritifera margaritifera L.) glochidielarver lever inkapslade som parasiter på gälarna hos öring (Salmo trutta L.) i flera månader innan de blir frilevande. Genom experiment i ett strömakvarium har denna studie undersökt hypoteserna att öring med inkapslade gälparasiter tar färre byten, och simmar kortare avstånd när de födosöker än öring utan gälparasiter.   Resultaten visade att infekterad öring tog signifikant färre byten/minut än oinfekterad öring både i början och i slutet av en födosöksperiod under vilken individerna konfronterades med tvåhundra poteniella byten. Vilket visar att infekterad öring har en lägre furageringshastighet är oinfekterad öring. Däremot påvisades ingen skillnad mellan öring med och utan parasiter med avseende på antal byten tagna i förhållande till avstånd från bytet.    Den lägre furageringshastigheten som påvisades hos de infekterade öringarna tyder på att infektion av flodpärlmusslans larver kan ha negativa effekter på fiskarnas tillväxt, och därmed överlevnad. Detta kan bero på faktorer som minskad aktivitet, ökad ventilation och förändringar i beteende hos de infekterade fiskarna, som är en effekt av parasitinfektionen. / Parasites can impact their hosts in many different ways. The larval glochida of freshwater pearl mussels (Margaritifera Margaritifera L.) infect the gills of brown trout (Salmo trutta L.), where they spend several months before becoming free-living. Using stream aquarium experiments, this study investigated the hypotheses that infected trout would capture fewer prey, and would swim less distance to capture prey, than would uninfected trout.   The results showed that infected brown trout caught significantly fewer prey/minute both at the beginning and at the end of a foraging period during which individuals were fed two hundred potential prey. This suggests that infected brown trout have a lower foraging rate than uninfected brown trout. However, no significant difference between infected and uninfected fish was found in terms of the number of prey taken in relation to distance to the prey.   The lower foraging rate for infected brown trout suggests that infection of the freshwater pearl mussel’s glochidia may have negative effects on their growth rate, and consequently survival. This may be due to factors such as reduced activity, increased ventilation and changes in behavior, which are effects from the parasite infection.

Flodpärlmusslan (Margaritifera margaritifera. L) har en obligatorisk fas i sin komplexa livcykel där den lever som parasit på antingen öringens (Salmo trutta, L) eller laxens (Salmo salar, L) gälar. För värdfisken skulle detta kunna innebära en ökad energikostnad när den födosöker i rinnande vatten. Öringens furagering består i att den finner en utgångspunkt (eng. focal point) i strömmen utifrån vilken den fångar sina byten. I denna studie undersöks det om en infektion av flodpärlmusslans glochidielarver på öring påverkar öringens födointag och hantering av födan. Sextio årsungar av öring infekterades med glochidielarver av känd koncentration, och deras födosökning mättes över en 10-veckors period för att bedöma effekten av den växande mussellarven i jämförelse med en icke-infekterad (kontrollgrupp) grupp fiskar. Individuella födoförsök, tio försök per mätningstillfällena, utfördes i ett strömakvarium där en enskild öring matades med drivande föda i fem minuter. Bytet släpptes ned i strömmen med två sekunders intervall, antalet byten räknades och furageringsbeteendet observerades. Födoförsöken genomfördes med två veckors intervall. Vid försöken hittades inga signifikanta skillnader med avseende på: 1) antalet fångade byten, 2) proportionen av fångade byten av försökets första tre minuter, eller 3) de rumsliga aspekterna av öringens utgångspunkt. Vidare uppmättes ingen signifikant effekt av tid på furageringsframgången på den infekterade gruppen jämfört med kontrollgruppen. Däremot var det ett högre antal infekterade individer som spottade ut byten jämfört med kontrollgruppen. Skillnaden i utspottade byten pekar på att infektionen har en negativ påverkan på fiskarnas furageringsförmåga. Dock visade försöken inga resultat som stödjer tidigare studier där signifikant skillnad erhållits med avseende på födointag. Förklaring till detta kan finnas i metodskillnader mellan denna och tidigare studier, inklusive skillnad i typ av byte, strömakvarier, ursprungsström för musslorna och öringen och infektionsgrad hos de infekterade fiskarna samt kontrollgruppen. / The freshwater pearl mussel (Margaritifera margaritifera, L) has a mandatory phase of its complex life cycle where it lives as a parasite on the gills of either trout (Salmo trutta, L) or salmon (Salmo salar, L). For the host fish this could mean an increased cost when it forages in running water. The trout´s foraging consists of finding a focal point in the stream from where it catches its prey. This study examines whether an infection of the freshwater pearl mussel´s glochidium larva on the gills of brown trout affect trout foraging. Sixty juvenile (0+) brown trout were infected with glochidium larva of a known concentration, and trout foraging behaviour was measured over a 10-week period to assess the effects of the growing mussel larvae of the fish compared to non-infected (control) fish.  Individual feeding trials, ten replicates for each session, were conducted in a stream aquarium where a single trout was fed drifting prey for five minutes. The prey were released into the stream every two seconds, the number of prey captured was counted, and the foraging behavior was characterized. Feeding trials were conducted at two-week intervals. No significant differences between the infected and control groups were found for: 1) the number of prey eaten, 2) the proportion of prey eaten during the first three minutes of the trial, or 3) location of the focal points. Furthermore, there was no significant effect of time on foraging success of the infected group compared to the control group. However, there was a higher number of infected individuals who spat out the prey compared with the control group. The difference in rejected prey suggests that the infection has a negative impact on the fish’s foraging ability. However, the results of this experiment do not support previous studies, where a significant difference was obtained with respect to food intake. This might be explained by differences in experimental protocol between this and previous studies, including differences in prey type, stream aquaria, stream origins of mussels and trout, and the infection status of infected and control fish.