In Sweden, the Wels catfish (Silurus glanis) is a protected species and exist in only three known water systems in the country. With spawning habitats being destroyed by human activities, it is important to know more about the Swedish catfish’s reproductive behaviour and habitat preferences in order to protect the species. In this report, catfishes tagged with transmitters was tracked using radio telemetry, day and night for two weeks in Lake Båven during the spawning period. Habitat data on the depth, bottom hardness and vegetation was collected in known and potential spawning areas in the lake using an echo sounder. Habitat preference analysis indicated depth and bottom hardness as important factors when catfishes chose spawning habitat.  The analysis did not indicate vegetation to be important. Habitat data from spawning positions of tagged catfishes was used to identify other sites suitable for reproduction in nearby areas. Home range during spawning was calculated for two catfishes and was found to be 1.05 and 1.35 ha. Based on this information and the identified sites suitable for spawning, the total available spawning habitat in the study area was found to be enough for 12 to 15 catfishes. This is relatively few, and indicates that additional spawning sites would benefit the population.

The aim of this thesis was to examine the influence of nitrite on fish. Influence of nitrite was evaluated on the basis of acute and sub-chronic toxicity tests results on rainbow trout (Oncorhynchus mykiss). The effects of nitrite were also observed in wels catfish {--} albino (Silurus glanis). On the basis of the results of acute toxicity tests, values of lethal concentration of nitrite for rainbow trout (Oncorhynchus mykiss) (96hLC50 = 11.2 mg.l-1 NO2-) and for wels catfish {--} albino (Silurus glanis) (96hLC50 = 15.8 mg.l-1 NO2-) were calculated. Using results of acute toxicity test for rainbow trout (Oncorhynchus mykiss) and with respect to legislative requirements, concentrations of nitrite for sub-chronic toxicity test were selected. The results of sub-chronic toxicity test showed that nitrite concentration of 3 mg.l-1 NO2- during 28-day exposition caused 65 % fish mortality. This concentration also caused growth rate decrease compared to control. Growth rate among fish exposed to concentrations lower then 3 mg.l-1 NO2- was comparable to control. Among fish exposed to nitrite concentration of 0.6 mg.l-1 NO2- and higher nitrite accumulation in muscle and in blood plasma was observed. Haematological examination showed statistically significant decrease of haematocrit value and concentration of haemoglobin and increase of the number of leukocytes in experimental fish. Other measured haematological parameters (Ery, MCV, MCH and MCHC) were comparable with control.

The work is focused on the nutrition and farming technology influences on the changes of the quality of common carp (Cyprinus carpio L.) and european catfish (Silurus glanis L.) meat. For the first experiment were used 6 groups of fish (common carp). Five of those were fed on cereals (maize, wheat, triticale, rye and barley), the sixth group was left on natural food by way of the control group. For the second experiment were used two groups of fish (european catfish). The first group was bred by the classical technology in fresh water ponds using natural food (dietary fish) by way of the control group. The second group was bred intensively in warm water in controlled conditions using feed compouned. In both of groups were defined following characteristics: weight yield, sensoric analysis, content of nitrogen compounds, fat content and spectrum of fatty acids in fat. It was ascertained that the rearing technology affects the results in all monitored characteristics. The biggest attention was payed to the results of fatty acids spectrum. While in the test using common carp was proved a higher proportion of PUFA (inclusive EPA and DHA) in the control group, in the test using catfish was this proportion higher in intensively reared group.

Knowledge of Wels Catfish (Silurus glanis) at its northernmost distribution area is limited on even basic behavioral characteristics, such as habitat preferences and activity patterns. It has thus proven difficult to aid the species in any predictable way. In a telemetry study on Wels Catfish, I examined habitat preferences, activity patters and site fidelity of ten adult individualsat their most northern range. Fish were surgically equipped with acoustic transmitters, frequently recording the position of each individual in a small Swedish lake. Wels Catfishpreferred deeper areas than randomly expected during the entire year, except for late spring and early summer. This indicates that spawning took place at shallow, vegetated areas in May-June 2019. Fish were highly active during summer, utilizing the entire lake. Behavior shifted during winter as fish became bottom dwelling with minimal activity, small home ranges and likelyminimal feeding. Wels Catfish had a nocturnal activity pattern during the entire year, but especially during summer. Fish preferred shallower areas during night than during day, indicating that deeper areas were used for resting and shallower areas for foraging. Lastly, home ranges were much larger than expected during summer, implying that S. glanis might be less territorial in small lakes than previously believed. In summary, conservation efforts should focus on protecting and restoring shallow areas with lots of vegetation and food items, adjacent to deep areas with underwater structures. These areas are important for the survival of S. glanisas they fulfill requirements for feeding, reproduction, resting, and hibernation.

Swedish populations of Wels catfish (Silurus glanis) experienced severe declines during the 19th and 20th centuries. The main causes for the decline were loss of suitable spawning habitat and fragmentation of populations. Currently, three native and two reintroduced populations remain in Sweden. Thanks to national protection and progress in restoring habitats, population sizes have increased over the past three decades. Previous studies that used microsatellite loci have found that genetic diversity and effective population sizes in Sweden are low, while population differentiation is high. A study that used whole genome sequencing (WGS) confirmed these results for native Swedish populations (those found in the Båven, Emån, and Möckeln water systems). The current project uses the same WGS methods and expands on the previous study by considering samples from non-Swedish populations (river Garonne, France; river Ebro, Spain; hatchery, Czech Republic), as well as improving read-depth coverage and sampling from the introduced Swedish population in the Helge å water system. Both a genome-wide SNP-set and full mitochondrial sequences were used to assess genetic diversity within each population, and differentiation among them. Genetic diversity in Swedish populations is lower than in non-Swedish populations. Native Swedish populations are strongly differentiated from one another. The introduced Helge å population is strongly differentiated from Emån and Möckeln, but less so from Båven. Despite Helge å individuals having heritage that can be predominantly traced to Båven, there are clear signs of admixture with the other two native populations. Swedish populations are all strongly differentiated from the non-Swedish populations. Altogether, evidence of admixture and slightly greater genetic diversity than native Swedish populations in Helge å can at the surface be seen as promising signs. However, it remains uncertain whether these improvements are durable over multiple generations. Considering the poor genetic status of Sweden taken as a whole, and the questionable nature of the improvements seen in Helge å, the long-term viability of Swedish catfish populations remains uncertain.

L'objectiu d'aquesta tesi és aportar les primeres dades sobre l'ecologia del silur (Silurus glanis) introduït a la Península Ibèrica i estimar els seu impacte ecològic sobre la biota nativa. Es van mostrejar les comunitats de peixos de 14 embassaments catalans. El silur es troba actualment introduït a quatre conques Ibèriques: a la de l'Ebre fa uns 30 anys, a les del Ter i Tajo fa uns pocs anys i recentment al Llobregat. Hem demostrat la utilitat de comparar estadis d'invasió recents i avançats per mirar d'entendre els canvis ecològics causats per espècies invasores: les introduccions recents presenten silurs més joves, de menor mida i amb taxes de creixement superiors a les de les poblacions introduïdes anteriorment i també respecte les poblacions natives; a més, depreden majoritàriament sobre peixos, en contra de les poblacions més antigues que s'alimenten sobretot de cranc. Finalment, aportem les primeres dades publicades de telemetria del silur. / The aim of this thesis is to provide the first data on the ecology of European Catfish (Silurus glanis) introduced to the Iberian Peninsula and to appraise its ecological impact on native biota. We sampled the fish assemblage of 14 Catalan reservoirs (Spain). The European catfish is nowadays introduced in four Iberian river basins: in the Ebro river basin for 30 years, in the Ter and Tajo river basin for a few years and we report the first record for the Llobregat river basin. We demonstrate the utility of comparing early and late invasion stages to understand the ecological changes caused by invasive species: recent introductions of catfish had smaller and younger catfish with growth rates higher than old and native populations and preying on fish instead of crayfish for old introductions. We also report the first published telemetry data for catfish.

The aim of this thesis was to assess the effects of long-term nitrite exposure on mortality, growth rate and blood biochemistry in rainbow trout (Oncorhynchus mykiss). Furthermore, acute toxicity tests with nitrite were performed on rainbow trout (Oncorhynchus mykiss) and sheatfish (Silurus glanis). The theoretical part focuses on aquatic toxicology including toxicity evaluation of substances and preparations for water organisms. Detailed information on nitrite are summarized there, namely information on nitrite occurrence and sources in aquatic environment and the mechanism of their uptake and toxic influence on fish including factors influencing their toxicity. The practical part consists of methodology and results of acute toxicity tests on rainbow trout and sheatfish and sub-chronic test on rainbow trout. According to the acute toxicity tests results, 96hLC50 values were estimated at 11.2 mg.l-1 NO2- for rainbow trout and 15.3 mg.l-1 NO2- for sheatfish. Sub-chronic exposure of rainbow trout to nitrite concentrations ranging from 0.01 to 3.0 mg.l-1 lasting 28 days caused an increase of glucose concentration and a decrease of potassium concentration in the blood plasma among all experimental fish compared to control. Elevated nitrite levels were found in the plasma of the fish exposed to concentrations of 0.6 mg.l-1 NO2- and greater. The plasma nitrite levels did not reach those applied in any experimental group in the present study. At highest nitrite concentration (3.0 mg.l-1 NO2-), 42 % growth inhibition and 65 % mortality among fish was noticed. On the basis of growth rate inhibition data, the values of NOEC and LOEC were estimated at 0.01 mg.l-1 and 0.2 mg.l-1 NO2-, respectively.