Substrate choice, swimming activity and risk to predation by burbot (Lota lota) of the well established Gammarus roeselii and the invader Dikerogammarus villosus were studied in mixed and single-species aquarium experiments. We used stones, gravel and aquatic weeds (Elodea, Chara) as substrates. We hypothesized that both species have different substrate preferences and that substrate affects the predation risk. We also assumed that presence of D. villosus influences substrate preference and predation risk of G. roeselii since the invader is known to affect the behavior of other gammarids. Adults of D. villosus in single species experiments and juveniles in mixed and single species experiments were evenly distributed over the different substrates but adults in mixed species experiments were more likely to prefer stone substrate. In contrast, adults and juveniles of G. roeselii clearly preferred aquatic weeds independent of the presence/absence of the invader. Both species preferred substrates with fissured surface over substrates with smooth surface. Gammarus roeselii was observed swimming more often than D. villosus in the open water but its swimming activity was lower when its preferred substrate was present compared with its swimming activity if non-preferred substrates were present. Predation rate of burbot on D. villosus was comparatively low and independent of the substrate. Burbot consumed many more G. roeselii than D. villosus, both in mixed and single species experiments. But when the preferred substrate of G. roeselii (weeds) was used in the experiments, predation rate of burbot on G. roeselii was somewhat lower than that when non-preferred substrates were present. The results of the experiments support our hypothesis that the gammarids studied here have different substrate preferences and that presence of the preferred substrate can affect predation risk. However, there is no evidence that presence of D. villosus affected substrate choice or predation risk in G. roeselii. We consider that differences in use of spatial niches permit co-existence of G. roeselii and D. villosus in the wild when substrates are diverse. The fact that G. roeselii than D. villosus is more often observed swimming in the open water may explain its higher risk of being captured by fish.

We compared feeding rates of the well-established, non-invasive amphipod Gammarus roeselii with those of the invasive