| Abstract: | Non‐native fish species pose a major threat to local fish populations and aquatic ecosystems in general. Invasive gobies are a particular focus of research, but with partly inconsistent results. While some studies reported severe detrimental impacts on native species, others have concluded less serious or neutral effects. We provide results from a large‐scale, multi‐annual fish monitoring program on the occurrence and abundance of non‐native fishes in the main stem of a free‐flowing section of the Austrian Danube. Special emphasis was placed on identifying positive or negative interactions of invasive gobies with native species. Whereas most non‐native species occurred too sporadically or were too few in number to infer a direct threat on the local fish community, invasive gobies were among the most common fishes throughout all sampling years. Co‐occurrence analyses revealed species‐ and mesohabitat type‐specific associations of gobies with native species, which were primarily positive. Notably, native predators such as asp, burbot, or perch probably benefit from the ubiquitous gobies. Two characteristic fluvial fishes revealed negative associations with invasive gobies, namely barbel (Barbus barbus) and Danube whitefin gudgeon (Romanogobio vladykovi): they appear to avoid habitats occupied by gobies. Accordingly, high abundances of round and bighead goby most likely resulted in population losses of barbel and whitefin gudgeon, respectively. Overall, our results indicate a limited negative impact of non‐native species in the sampling area. This is because only two out of 51 occurring species were found to be adversely affected by gobies, the share of co‐occurrences with native species was high, and other non‐native species were generally rare. Nevertheless, invasions are highly dynamic, and new non‐native species are likely to occur in the Austrian Danube, calling for continued monitoring and awareness. |
