Distribution history of non-native freshwater fish species in Germany: how invasive are they?

Distribution patterns of non-native fishes in Germany have been analysed starting from their first introductions reported, with the objectives to determine indicators of their invasiveness, to identify the most important distribution pathways, and to predict their further development. In Germany, 14 out of 93 non-native freshwater and migratory fish species ever registered are established today, a further six are possibly established, eight have failed to establish, and the rest were single records. Five of the established and three potentially established species were restricted to single locations. Of the remaining species, only the gobiid species are actually spreading throughout the waterway network, whereas other stocks rather tend to decline or did not change. The most wide-spread species is rainbow trout Oncorhynchus mykiss, with 5,872 records in open waters being the most important fish for German's commercial inland fisheries with an annual yield of > 21 000 tons. But rainbow trout is widely stocked and its establishment status is a subject of debate. Inland fisheries management has contributed most to the recent distribution of non-native fishes, but it became also the best regulated pathway. Therefore, it is expected that ornamental fish trade and aquaculture will become most important for introducing new non-native fishes in the near future, even if unintentionally. Every single fish escaping from a garden pond or holding facility is a potential founder of an invasive species stock and in particular raising water temperatures in winter might promote the establishment of ornamental fish. Once established, there are nearly no legal means to remove a non-native species successfully from a water body.