Invasive Ponto-Caspian gobies rapidly reduce the abundance of protected native bullhead

Invasions by alien species may cause a decline in populations of vulnerable protected species through interference and resource competition. During the last decade, four invasive goby species of Ponto-Caspian origin have displayed rapid dispersal in The Netherlands. High densities of these species have been recorded in large rivers and hydrologically connected water bodies such as canals and floodplain lakes. In the River Meuse, alien tubenose goby (Proterorhinus semilunaris), round goby (Neogobius melanostomus) and bighead goby (Ponticola kessleri) occupy similar habitat to native, protected river bullhead (Cottus perifretum), i.e., coarse substrates with large pebbles, and groyne stones and riprap that protect river banks against erosion and preserve river channels. In the years following the arrival in 2011 of N. melanostomus in the River Meuse, a rapid decline in native C. perifretum average density from twenty to one individual per 100 m was observed, most likely due to predation and competition for shelter and/or food. C. perifretum density also declined at sites colonized by Ponticola kessleri and/or Proterorhinus semilunaris only. However, when compared to sites where N. melanostomus was present, C. perifretum density remained relatively high. Similar effects on other native benthic fish species may occur in the near future due to the presence of alien gobies. Compliance with ecological status objectives relating to the European Habitats Directive and Water Framework Directive may not be achievable due to the loss of protected and endangered native fish species in areas invaded by alien gobies.