Two of the most invasive freshwater bivalve species in temperate regions worldwide are the Ponto-Caspian dreissenids Dreissena polymorpha and Dreissena rostriformis bugensis. Throughout their range, observations have been made of a dominance shift favouring D. r. bugensis where the two species co-occur. Although both dreissenids have been widely studied, the mechanisms driving this dominance shift are not completely understood. Our long-term and short-term field experiments assessed a selection of species traits related to growth and settlement, which may be linked to a competitive benefit for either of both species. We assessed relative population densities in time and space, mortality, and inter- and intraspecific interactions in relation to environmental factors like temperature, salinity, and light intensity, using 14 × 14 cm PVC settlement plates. Dreissenids were identified, counted and measured over 11 years in the waterbody where D. r. bugensis was first discovered in Western Europe. Dreissena polymorpha appeared to have a competitive benefit over D. r. bugensis by being able to settle earlier, i.e. in spring, while other studies indicate the opposite. As salinities in the study region were relatively high in spring, this discrepancy may be explained by a higher salinity tolerance of D. polymorpha, as is known from the literature. In addition, Common Coots predated D. r. bugensis clusters. Dreissena polymorpha were not usually found in such clusters. Regardless of these competitive benefits for D. polymorpha, D. r. bugensis is the dominant species in the region. One possible explanation could be the fact that D. r. bugensis was found in similar densities on both light exposed and shaded fouling plates. Dreissena polymorpha on the other hand, was practically absent on light-exposed plates after 12 months. Therefore, a wider range of habitats seems to be suitable for the settlement of D. r. bugensis. Another driver of the dominance shift can be linked to it having faster growth than D. polymorpha, especially during spring. More generally, D. r. bugensis occurred in higher population densities on plates with D. polymorpha than on plates with only D. r. bugensis. Dreissena polymorpha individuals may thus induce the settlement of additional individuals of D. r. bugensis. A final reason for the dominance of D. r. bugensis concerns its low winter mortality. While the number of D. polymorpha individuals more than halved over the winter 2016–2017, no significant decrease in numbers of D. r. bugensis was recorded.