Activity and competitive behaviour of invasive Neogobius melanostomus and Ponticola kessleri (Gobiidae) from the River Rhine, Germany

Behavioural experiments were conducted to gather information on how competitive behaviour between the invasive gobies Ponticola kessleri and Neogobius melanostomus may affect the development of populations in the River Rhine. Short-term experiments that were designed to measure activity could not discern any consistencies, neither between species nor based on gender. Although both species needed time to adapt to laboratory shelters, N.melanostomus hid conspicuously more often than P.kessleri. Few directly competitive interactions between species were observed. However, when such interactions were observed, N.melanostomus was stronger and won most inter-specific conflicts, even when it was the smaller opponent. It was also revealed in long-term feeding experiments that N.melanostomus preferred gammarids over fish during direct competition, while the opposite was true for P.kessleri. Behavioural results of the present study show these two species to exhibit different strategies, giving some evidence that P.kessleri and N.melanostomus may occupy different niches that enable them to coexist in the River Rhine.