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.

One of the key elements of animal social behaviour is the recognition of individuals on the basis of previous experience. This relationship is called familiarity and affects individual's behaviour. We know many examples of familiarity-based behaviour among fish. Many results show that familiarity is advantageous. Familiar individuals are less aggressive, forage more, make more cohesive shoals and grow faster. We tested the influence of the familiarity on the spatial distribution of juvenile wels catfish Silurus glanis from two unfamiliar groups, which were held in separate tanks. In a laboratory experiment was the spatial distribution of individuals monitored in the artificial stream by using passive integrated transponders. Shelters, which were occupied by individuals, were placed in the stream. Two groups of fish were subsequently added into the experiment under conditions of limited or unlimited shelters. These groups came from the same tank (so they were familiar) or from different tanks (so they were unfamiliar). 1080 individuals were used in this experiment and over three million individual fish positions were recorded. We discovered changing levels of activity under different conditions. When we compared activity of original and additional individuals, we measured higher level of activity in...

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.

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.