The present study investigates the impact of northern pike (Esox Lucius) on the growth of juvenile burbot (Lota lota) in northern lake ecosystems. It is hypothesized that the presence of pike will positively affect the growth of juvenile burbot by reducing interspecific competition and increasing the abundance of benthic invertebrates. To test this hypothesis, electrofishing for juvenile burbot was conducted in the littoral zones of lakes with and without pike in the region Jämtland Härjedalen, Sweden. Data on length and age of the captured burbot were used to compare growth between the two lake types. Contrary to the initial hypothesis, the results show that juvenile burbot were significantly larger at a given age in lakes where pike was absent. This outcome suggests that juvenile burbot may reduce their foraging activity in lakes with pike to minimize predation risk. Since adult burbot has shown to be of greater size in lakes with pike, future research could investigate whether the shift towards piscivory in burbot leads to changes in how they are affected by pike.

Invasive species are distributed all over the world and defined as organisms that have been introduced to a specific geographic area where they were not originally native. Such species have the ability to change the invaded ecosystem by for example competing with native organisms for limited food resources or occupying habitats with resulting environmental impacts. Therefore, knowledge about the characteristics of these invasive species as well as of the affected habitat types are very relevant to understand arising changes for endemic biodiversity and ultimately to protect it. For several large rivers in Germany, the round goby Neogobius melanostomus (Pallas 1814), a benthic fish species of Ponto-Caspian origin, represents a highly invasive fish species. In this thesis, I traced the invasion stage by conducting a literature research on the distribution pathways and documenting occurrence spots of N. melanostomus within two large rivers in Germany, the Elbe and the Oder. Within the Elbe River I determined the invasion front by applying electrofishing and environmental DNA (eDNA) analysis. This kind of knowledge enabled the results on diet composition gained from gut content and trophic niche width gained from stable isotopes (δ13C and δ15N) to be linked with the ongoing invasion stage. I found a bimodal distribution for N. melanostomus, from upstream and downstream. On examination of the downstream-directed invasion gradient, differences in N. melanostomus food composition as well as macroinvertebrate occurrence could be identified, with habitat structure and abundance being key determinants. Furthermore, N. melanostomus from sites invaded earlier exhibited the widest trophic niche size and individuals from the most recently invaded sites showed smaller niche sizes, most probably due to stronger competition for resources. Within the Oder River I focused on the competitive feeding interaction of N. melanostomus with the native fish species burbot (Lota lota). The main findings revealed that both fish species consumed preferred the same prey taxa (Crustacea, mainly Gammaridae) indicating potentially high competitive interactions for invertebrates. Whereas isotope ratios revealed a potential shift in the L. lota food web at the invaded site, represented by an enrichment of ẟ15N in muscle tissue and ẟ13C in liver tissue. This is indicative for a niche differentiation among both fish species.This thesis provides new insights into intra- and interspecies-specific impacts of N. melanostomus on riverine ecosystems under natural conditions, considering traceable invasion stages.:Table of contents 1 Abstract 3 Zusammenfassung 5 General introduction 7 Biological invasions: their introduction, dispersal and establishment 7 Application of environmental DNA to detect aquatic invasive species 10 The effects of invasive species on river food webs 10 Study Species: Round goby (Neogobius melanostomus) 12 Place of origin and spread 12 Round goby characteristics 13 Round goby feeding behaviour and effects on food webs 14 Round goby impacts on native fish species 15 Thesis Objectives 16 CHAPTER I 18 Tracing the colonization process of non-native gobies into a large river: the relevance of different dispersal modes 18 CHAPTER II 33 Diet compositions and trophic niche differentiation of Neogobius melanostomus along an invasion gradient in a large lowland river 33 CHAPTER III 54 Round goby (Neogobius melanostomus) impacts on diet composition and isotopic niche of native burbot (Lota lota) within the large Lowland River Oder, Germany. 54 General discussion 71 Factors influencing the invasion success of round goby 71 The importance of early invasion patterns 75 Invasion-induced alteration of riverine food webs 77 Challenges in the study of riverine invasions 80 Suggestions for research actions 82 Conclusion 85 References 86 Appendix 106 Chapter I 106 Chapter II 109 Danksagung 113 Selbstständigkeitserklärug 115