Mittuniversitetet

1. Global climate change is predicted to raise water temperatures and alter flow regimes in northern river systems. Climate-related factors might have profound impacts on survival, reproduction and distribution of freshwater species such as red-listed noble crayfish (Astacus astacus) in its northern limit of distribution.2. In this study, noble crayfish capture data over 27 years from the River Ljungan, Sweden, were examined. Time series of catch per unit effort (CPUE) were analysed in relation to the North Atlantic Oscillation (NAO) index, regional weather factors and water flow. CPUE was assumed to reflect differences in population size. Two models were constructed to explore the relative impact of different climate factors and density dependence on variability of catch sizes.3. The most parsimonious model for CPUE time series, explaining 72% of the variance in CPUE, included density-dependent population dynamics of the crayfish and climate or weather factors. The specific effect from density dependence in the model was 37%, while climate/weather factors contributed with 35% of the variation. The most important climate/weather factors are variations in NAO index and water flow. Temperature did not improve the model fit to capture data.4. The best model was evaluated using independent data sets that gave correlations between model predictions and data ranging from 0.44 to 0.53. The density dependence shows a time lag of 1 year, while climate variables show time lags from 2 to 6 years in relation to CPUE, indicating effects on different cohorts of the crayfish population.5. Both density dependence and climatic factors play a significant role in population fluctuations of noble crayfish. A 6-year time lag for NAO index is puzzling but indicates that some as yet unidentified factors related to NAO might act on the juvenile stages of the population. Water flow shows a 2-year lag to the CPUE, and high flow in the river may affect adult survival. The reasons for fluctuation of crayfish catches in response to climate need to be identified, and fishing quotas should consider the different cohort sizes because of variation in environment. Reintroduction programmes for crayfish need to consider effects of climate change when designing management strategies.

Catch per unit effort (CPUE) is used as a standardized trapping method by local fishermen and in monitoring studies. In this study, CPUE was compared with population estimates made with a capture-recapture method based on the passive integrated transponder (PIT-tag) marking of individuals. The results show a stronger positive correlation between the estimated population sizes from the capture-recapture method with an estimated CPUE effort of 120 traps. The fishermen used 15 traps, and even this effort showed a fair correlation with the mark-recapture estimates. This indicates that the standardized way of trapping with 15 traps can be used to evaluate reintroduction programs and monitor crayfish populations

Free-flowing water is thought to be of importance for reproduction and body growth of noble crayfish (Astacus astacus (Linné)) in the northern edge of its distribution area.

A hydroelectric power plant with a bypass tunnel was built in the river Ljungan in 1976.  This reduced the mean water flow in the old river bed from about 60m³/s to 3m³/s. The sites with the largest reduction of water flow had the largest decline in catch per unit effort of crayfish.  At one site the catches were reduced by >60%, but the catch success varied between locations.  In all sites the regulation seemed to have an effect, even though catch per unit effort was possibly affected by a number of other factors.

KEY WORDS: Catch yield, noble crayfish (Astacus astacus), water flow, water level regulation

We found that concentrations of nitrogen (N) in the stomach of voles varied between forest sites. Concentrations of nitrogen as well as the ratio of nitrogen to carbon (C) in stomach content were not different between the sexes or with season. Body mass varied between sites and between seasons but were unrelated to N concentration in spite that the body mass of voles in the spring season were on average 4.8 g heavier than in the autumn A positive relationship was found between N and C concentrations in the stomach content implying the diet quality could be related to this ratio. N concentrations in the stomach per unit body mass were positively and significantly related to catch per unit effort of voles. We suggest that sites with high N concentrations in stomach content and with higher NC-ratio imply better habitats that support higher population densities of voles.

Animal-borne diseases are likely to be affected by climate change. It is of interest to analyse sensitivity of reservoir species to climate change and how changes in their ecology would affect transmission of zoonoti diseases to man. Elevated temperature in northern hemisphere may have direct effects on reservoir species by greater survival and hence generally higher population densities. Further, warmer climate may make certain key habitats more beneficial for replication and possibly greater transfer of disease. On the other hand, animal responses to climate change may vary among species and changes in their ecology causing increased variability in population size or dependence of timing with certain food resources may adversely affect reservoir species. We analysed the number of human disease cases of tick borne encephalitis (TBE), tularaemia and hantavirus in norhern Sweden in relation to population size of their most common reservoirs, the mountain hare, European hare and the bank vole. We also analysed the effect by a common predator the red fox and we used the NAO index as a proxy for climate variations. The results show that hantavirus infection in man is more dependent on the population density of reservoirs than on the climate variable. In case of tularaemia, we found a more complex ecological situation that may be climate driven.We conclude that the outcome of warmer climate may vary among species but that species adapted to northern winter conditions may be particularly sensitive to climate change.

Zoonotic diseases are likely to be affected by climate variations but to reveal such dependence long time series extending over several decades are required. Human cases of Hantavirus infection in Northern Sweden shows regular peaks that occurs every fourth year and the virus is associated with the bank vole (Clethrionomys glareolus). From a perspective of climate change it is important to analyse variations in bank vole population size and human disease incidence and how these interactions is affected by climate forcing. Here I show that a time lag of two years exist between NAO index and response in the vole population and human disease incidence. In some way triggers the NAO index two years previously a build up and peak in vole density and with corresponding peak in human Hantavirus disease. Likewise, high NAO index two years previously is associated with low population density of voles. The change in number of human disease cases is positively correlated to density of bank voles and both bank voles and disease peaks at a regularity of about every fourth year. Further, the time series revealed a match between NAO index, vole population and human disease cases at a longer periodicity of 8.5 years that has not been anticipated.These contrasting climate effects makes it hard to predict how a future warmer climate will affect disease dynamics in this particular system because of complex relationship between long and short term effects.