Dispersal and food web impacts of the round goby Neogobius melanostomus along an invasion gradient / Claudia Susana Nogueira Tavares

Abstract

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.