The thick shelled river mussel belongs to a highly endangered group of organisms worldwide. A parasitic larval stage is included in the mussel's life cycle where its glochidia larvae attach to the gills of a host fish. The glochidia infection can affect the ability of fish populations to form schools, a behavior that is fundamental to the survival of many fish species. As the mussel is involved in conservation projects where the reintroduction of the glochidia larva is included, one should know more about how the parasite affects its host to be able to make well-founded decisions regarding possible reintroduction. In this study, a group of glochidia-infected minnows is compared to a control group where it was analyzed if the infected fish obtained a higher nearest neighbor distance (NND), a larger average school area (ASA) and a lower movement of the schools (MOS), two and four weeks after being infected. The minnows were infected in vitro where their schooling behavior was recorded with a video camera of which the footage material where analyzed. Collected data were analyzed with 2-way ANOVAS, where the results showed that there is a relationship between infection and changed schooling behavior with respect to the density and area of the fishes in general. NND and ASA increased in the infected group, but not in the non-infected group, at two and four weeks. If the goal through conservation measures is to restore the original balance of the parasite-host interactions of the thick shelled river mussel and the minnow, more research on how mussel parasites affect its fish hosts should be done. / Den tjockskaliga målarmusslan tillhör en starkt hotad organismgrupp över hela världen. Ett parasitiskt larvstadium ingår i musslans livscykel där dess glochidielarver fäster vid gälarna hos en värdfisk. Denna glochidieinfektion kan påverka fiskpopulationens förmåga att bilda stim, ett beteende som är grundläggande för många fiskarters överlevnad. Då musslan är med i bevarandeprojekt där återintroduktion av glochidielarven ingår bör kunskapen fördjupas gällande hur parasiten påverkar sin värd för att kunna ta välgrundade beslut inför eventuella åtgärder. I den här studien jämfördes glochidieinfekterade elritsor med en kontrollgrupp där det undersöks om de infekterade fiskarna får en högre nearest neighbour distance (NND), en större average school area (ASA) och en lägre movement of the schools (MOS) två och fyra veckor efter att de infekterats. Elritsorna infekteras in vitro där deras stimbeteende spelades in med en videokamera för att sedan bildanalyseras. Insamlade data analyserades med 2-vägs-ANOVOR, där resultaten visade att det finns samband mellan infektion och förändrat stimbeteende med avseende på fiskarnas täthet och area generellt. NND och ASA ökade hos de infekterade, men inte för de icke-infekterade under två och fyra veckor. Om målet genom bevarandeåtgärder är att återställa den ursprungliga balansen i parasit-värdinteraktionen för den tjockskaliga målarmusslan och elritsan, bör det undersökas vidare hur musselparasiter påverkar sina värdfiskar.

The main subject of this experiment was to explain the behaving of the minnow during its reproduction, the preferation of the spawning substratum, the length of the incubation of eggs, the evolution and the growth of the juvenile and the fry. Brood fish were taken by elektrical unit in the Blanice river, in the area between the railway bridge and the weir in the village of Blanice. These were put into the experimental pond of 75 square metres. There were two spawning processes. The first took place on 10 June , the fry hatched after ten days and in the next ten days it could be found all over the pond looking for food. In the selection of the substratum the gravel (of the size of 2-5cm) was prefered at all. The standard length of the fry at the end of the process of breeding (after a hundred days) was 39{--}48 mm (the amount of 800 individuals). The second spawn took place on 11 August. The fry hatched after eight days and the next week it was hidding in the substratum and consuming yolk sack. The standard length at the end of the breeding was 18{--}26mm. The ammount of the breeded fry was 350 individuals.