Feeding and niche differentiation in three invasive gobies in the Lower Rhine, Germany

Since 2006, three invasive Gobiids from the Ponto-Caspian area established in the River Rhine and their abundances nowadays regularly exceed 80% of the fish community. Between 2009 and 2011, densities of gobies in the Rhine increased while their condition decreased, assuming that the populations are approaching or even reached the capacity of the ecosystem. Consequently, we hypothesized a high level of competition on food resources within this group of invasive gobies that all exhibit the same sedentary life style, which might strengthen the differentiation of the ecological niche on a spatial and temporal axis. Invasive gobies were caught with electro fishing and beach seining in different types of habitats over a period of two years in the Lower Rhine, analyzing the food of more than 1500 gobies of the three species Neogobius fluviatilis (NF), Neogobius melanostomus (NM) and Ponticola kessleri (PK). All species showed an opportunistic feeding strategy. In NF and PK, a clear shift in preferred food resources was observed between individuals smaller and larger 50 mm that occurred in parallel with a habitat shift from sandy areas to riprap structures in PK, but not in NF that were only found on gravel and sand. In contrast, there were no distinct changes in food and habitat preference in NM. Small NM were found from spring to autumn on the sandy nearshore areas where they competed on food resources with juvenile PK in spring, and with NF in late summer, respectively. Abundance of juvenile NF and NM increased during the night in sandy nearshore areas in October: This behavior is assumed as predator avoidance against large piscivorous NF as well as native pikeperch, because active feeding occurred mainly during the day. The results on the three invasive Gobiids in the Lower Rhine give important hints how fine-tuned spatial and temporal characteristics in intra- and inter-specific competition shape the ecological niche of these invaders in their new environment. (C) 2012 Elsevier GmbH. All rights reserved.