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Fish populations vary naturally but are also affected by anthropogenic factors such as changes in land use, climate, acidification, invasive species and eutrophication. An overall picture is important for understanding the changes that are taking place and their effects. In this study, I have linked several environmental factors, such as turbidity and chlorophyll-a, with the presence of pikeperch and perch in Lake Mjörn. I have also investigated how the size of the perch has changed between the years 2000 and 2018 for the entire population, and specifically within the size class 80 - 120 mm. Gill net surveys from 2000, 2007 and 2018 shows that the median length of pikeperch decreased from 2007 to 2018 by 39.6% and increased in average per net effort by 630% between 2000 and 2018, a clear shift in the population from few and older to more, and a larger proportion of, younger individuals. The turbidity has had a clear downward trend with a decrease of 3.9 cm per year since 1985, with a Secchi depth of less than 4 meters in the last 10 years, which is probably an important factor contributing to the increase of the pikeperch population. At the same time, the chlorophyll-a concentration has shown an upward trend of 0.174 μg/l per year. An observed decrease in macrophytes around the lake over the past 20 years may have caused some increased wind-driven mixing and a greater amount of suspended solute particles. An increased amount of nutrients from the sediment could have explained the increasing amount of phytoplankton, but sampling carried out at the inlet to and the outlet of Lake Mjörn shows that the concentrations of nutrients in the lake have decreased. The median length of perch has decreased by 52.3% from the year 2000 to 2018, and the median length of 2 summer old perch shows a decrease of 17.5% during the same period. The difference in median length between the entire perch population and the 2 summer old individuals was 47% in 2000 but only 8.3% in 2018, which indicates that the population in 2018 largely consisted of 2 - 3 year old individuals.

The main theme of this thesis is the interactions of animals with the environment and each other. The thesis was written within the framework of a biomanipulation project “Pikeperch in Himmerfjärden”. With the aim to investigate possible trophic pit-falls, give the manipulation the best possible start, and find ways to monitor the progression of the manipulation. In Paper I the diet of the invader cladoceran Cercopagis pengoi is analysed with stable isotopes; conducted prior to stocking. C.pengoi has a preference for large copepods, indicating possible competition with fish. Paper II investigates the behavioural differences between pikeperch fingerlings reared in different environments (pond vs. tank). Results suggest that fish reared in semi-natural ponds are more likely to survive directly after stocking. In Paper III and IV, the diel vertical migrations (DVM) of copepods are in focus. In Paper III the migrations of two copepod species: Acartia spp. and Eurytemora affinis are studied over season and life stage. The amplitude of migration was found to increase with ontogeny for both species, indicating evasion of visual predators. Paper IV examines the varying migratory patterns of adult female E. affinis finding that these animals migrate more actively when feeding conditions deteriorate and growth decreases. The overall conclusions of the thesis are that behavioural, not only direct trophic interactions are key when studying ecosystems. / <p>At the time of the doctoraldefense, the following papers were unpublished and had a status asfollows: Paper2: In press. Paper 4: Manuscript.</p>

Pike-perch (Sander lucioperca) is a species of fish whose breeding in recirculation systems has a great prospect in the years to come. Due to the unfavorable conditions of intensive rearing of these fish in recirculation systems, they are often exposed to stress resulting in reduced fish defenses. Thus, fish are exposed to attack of a wide range of bacteria. Although there are preventive measures to protect fish against pathogen attack, knowledge of the immune system and the immune response of the fish is crucial for further evolution of vaccination. Even though we know the mechanisms of immune response of many fish species,but the knowledge of the pike-perch's immune response is very limited.

Le sandre est un percidé largement répandu en Europe. Le fonctionnement de ses populations méridionales est cependant mal connu. L'objectif de ce travail est d'apporter des connaissances nouvelles sur l'écologie du sandre par l'étude de la population d'un réseau de canaux en Camargue (sud de la France). Différentes techniques ont été mises en œuvre pour étudier l'isolement de la population (génétique, morphométrie), l'utilisation de l'habitat (télémétrie), la biologie (traits de vie) et la dynamique dans le système. Le fonctionnement de cette population dépend plus de facteurs biotiques qu'abiotiques (contrairement à ce qui est observé sur les populations septentrionales), ainsi que de facteurs anthropiques (agriculture...)

This thesis was written within the framework of a biomanipulation project where young-of-the-year (YOY) pikeperch (Sander lucioperca) were stocked to a Baltic Sea bay to improve water quality through a top-down trophic cascade. The aim of my doctorial studies was however focused on a broader ecological question, namely predation (the main driving force in a biomanipulation). Hence, this thesis consists of four papers where we study the interactions between predator and prey using fish and zooplankton and how these interactions can be measured. In paper I we evaluated the performance of different diet analysis methods by individual based modelling and found that when having a nutritional gain perspective, mass based methods described diets best. Paper II investigated how the explorative, foraging and anti-predator behaviour of the YOY pikeperch used for stocking were affected by their rearing environment (pond vs. tank rearing). The more complex and varied environment in the semi-natural ponds seemed to promote a more flexible and active behaviour, better equipping young fish for survival in the wild. For paper III we studied the diel vertical migration in the six copepodite stages of the zooplankton Acartia spp. and Eurytemora affinis in relation to fish biomass, phytoplankton abundance and temperature. Both species migrated and in addition showed increased migration range with size within species, indicating evasion from visual predators. Paper IV addressed the movement of littoral Eurasian perch (Perca fluviatilis) via stable isotope signatures (13C and 15N) and body condition. We found clear indications of sedentarity and intra-habitat dietary differences. Interactions between predators and prey are complex and affected by both physiological and environmental characteristics as well as behavioural traits. The results in this thesis suggest that different species and even different life stages pursue different strategies to survive. / At the time of doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Submitted.  Paper 2: In press.  Paper 4: Submitted.

Nomenclature: Sensitivity to anesthetic clove oil different species of adult fish Sensitivity of breeding fishes of various species (Siberian sturgeon, Brook trout, Black carp- young breeding fishes) for anesthetic clove oil was probed by the tests performed at temperature in the period of stripping (apart from Brook trout and Black carp). The tests were carried out with 17 fish species- with cyprinids (Ide- golden form, Common dace, European Chub, Sneep, Barbel, Grass carp, Black carp, bighead carp), salmonids (Brown trout, Rainbow trout, Brook trout), thymallinae (Grayling), pikes (Northern pike), perches (Pike-perch), sturgeons (Siberian sturgeon, Beluga) and airbreathing catfishes (North African catfish). Using concentration of 0,03ml.l-1 (Brown trout, Rainbow trout, Brook trout, Grayling, Pike, Pike perch, Ide, Dace, Chub, Barbel and Sneep), 0,04ml.l-1 (Grass carp, Black carp, Bighead carp), 0,05ml.l-1 (North African catfish) and 0,07ml.l-1 (Siberian sturgeon, Beluga) the IIb. Phase has been achieved, which is sufficient for manipulation with fishes during stripping. The anesthesia subsided after time period of 10-17 minutes (12 species) at the majority of fishes and at others after interval of 20-26 minutes (5 species). The results confirmed concentrations recommended for salmodis and Grayling (0,025-0,03ml.l-1), sturgeons (0,07ml.l-1), and North African catfish (0,05ml.l-1). At the majority of cyprinids (apart from Grass carp, Black carp and Bighead carp), the concentration used was lower than recommended (0,04-0,05ml.l-1)