Dynamics of omnivorous crayfish in freshwater ecosystems

Abstract: In this thesis I examine the dynamics of crayfish in streams and lakes. I have studied the influence of abiotic and biotic factors on abundance, growth, trophic position, niche breadth, and recruitment of crayfish, by conducting field studies, an outdoor channel experiment and timeseries analysis. Overall, the results show that abundance of crayfish is mostly affected by the abundance of predatory fish, in the absence of predators or at low densities the substrate size influenced crayfish abundance, both for native Koura (Paranephrops planifrons) in New Zealand and the introduced signal crayfish (Pacifastacus leniusculus) in Swedish streams. However, as the native noble crayfish (Astacus astacus) in Swedish streams was not influenced by substrate, this might be species specific due to differences in burrowing capacities. Crayfish abundances fluctuate from year to year and usually crayfish abundance is measured as catch per unit effort (CPUE), since baited traps are used to estimate how many crayfishes there are. By using CPUE data from Lake Bunn, that was inhabited by noble crayfish until 1974 and the introduced signal crayfish from 1982 and forward, I was able to model the population fluctuations observed against temperature data transformed into ecologically relevant parameters. The best model for both species revealed that the temperature during winter months explained most of the observed fluctuations in large adult crayfish. Further, I also found that the fluctuations were density dependent and at high density the CPUE of crayfish decreased in the lake. Crayfish are regarded cannibalistic and it is generally thought that the larger ones consume smaller ones, which may influence the recruitment success for crayfish. However, in the outdoor channel experiment I show that cannibalism was larger between juvenile crayfish and that the presence of large adults increased the survival of juveniles. But, the most important factor for survival and growth in juveniles were attributed to habitat complexity. A habitat of high complexity (i.e. high amount of cobbles) increases the shelter availability for the juveniles and, thus increases the survival- and growth rates of juveniles. In my thesis I also show that growth rate, trophic position, carbon signature and niche breadth are influenced mostly by the biomass of invertebrates. However, it was not all invertebrate groups that were responsible for this pattern. A high biomass of large, sedentary and less mobile invertebrate groups increased growth rate, trophic position and niche breadth of crayfish. These types of invertebrates are probably easier to catch than smaller, faster and free swimming invertebrates and are also commonly found in crayfish guts. The main conclusion from my results is that crayfish dynamics, such as abundance, growth rate, niche breadth and recruitment are affected by specific abiotic and biotic factors in a complex way. The knowledge this thesis has revealed may have great implications for conservation and management purposes.