Spread of invasive Ponto-Caspian gobies and their effect on native fish species in the Neckar River (south Germany).
Fish assemblages in the Neckar River were investigated by electrofishing under the Water Framework Directive 2000/60/EC (WFD) to assess the ecological status of rivers. This monitoring program also provides information on the distribution of invasive Ponto-Caspian gobies along the Neckar River in Germany, a tributary of the Rhine River. The western tubenose goby Proterorhinus semilunaris was first recorded on the Neckar River in 2007, followed by bighead goby Ponticola kessleri and round goby Neogobius melanostomus in 2009 and 2012, respectively. The current distribution boundary for Ponto-Caspian gobies coincides with the navigation limit for cargo ships at the river port of Plochingen (about 200 km upstream of the city of Mannheim). Marked differences in fish assemblages took place following invasion of the round goby, with average CPUE of native stone loach Barbatula barbatula and gudgeon Gobio gobio declining from 19.6 and 21.5 individuals per 100 m, respectively, to fewer than one. Bighead gobies failed to build up a stable stock after round goby invasion but have still been caught in small numbers at most sampling sections. The earlier-arriving western tubenose goby could not be detected any longer. The results indicate a strong impact of the round goby on native fish assemblages in the Neckar River which will hamper efforts to improve the ecological quality of the river. Further investigations are needed to assess long term effects of round goby invasion and find solutions for achieving the mandatory European Water Framework Directive (WFD) target of "good ecological status" of the river.