Monkey goby Neogobius fluviatilis and racer goby N. gymnotrachelus, both native to the Ponto-Caspian region, were discovered in Polish rivers during the mid-1990s and currently they are invading the River Vistula. Preliminary studies were carried out on the diet of both species at two sites in the lower River Vistula, one lotic (main channel of the river downstream of the Włocławek river reservoir) and the other lentic (left bank, middle section of the Włocławek Reservoir). Significant differences were noted in diet between the two species and within species according to habitat, in particular the types of chironomid taxa taken. Over a single diel cycle, monkey goby were captured between 07:00 and 19:00 hours only, feeding more intensively on chironomid larvae than the racer goby, which was captured at most times of day. The wide diet plasticity of these non-native fish species favours their expansion.

We investigated habitat preferences of two invasive Ponto-Caspian gammarids (Dikerogammarus haemobaphes and Pontogammarus robustoides) and a native European species (Gammarus fossarum) in laboratory experiments. The habitats consisted of the following objects:(1) living zebra mussels; (2) empty mussel shells (clean or coated with nail varnish) with both valves glued together using aquarium silicone sealant to imitate a living mussel; (3) stones (clean or varnished); (4) empty plates. Ten objects of the same type were glued to a plastic plate (10 · 10 cm) with methyl acrylic glue. The plates were placed in experimental tanks in various combinations. A single gammarid was put into the tank and its position was determined after 24 h. The studied species responded differently to the presence of zebra mussels. D. haemobaphes preferred living mussels rather than their empty shells and these two habitats over stones and empty plates. It responded positively to shell shape, selecting varnished shells rather than varnished stones, and to shell surface properties, selecting clean shells rather than varnished shells. It did not respond to waterborne mussel exudates. P. robustoides did not exhibit any preferences for the above-mentioned substrata. G. fossarum was attracted by empty mussel shells (but not by living mussels). It responded only to their shape, not to surface properties. The strong affinity for zebra mussels, exhibited by D. haemobaphes, might help it survive and develop stable populations in newly invaded areas.

The Włocławek Dam Reservoir located on the lower Vistula River (central Poland) is part of the central corridor used by Ponto-Caspian species to migrate in Europe. It provides a number of habitats suitable for sustaining populations of several non-indigenous taxa. Four Ponto-Caspian amphipod species were recorded in the reservoir: Chaetogammarus ischnus, Chelicorophium curvispinum, Dikerogammarus haemobaphes and Pontogammarus robustoides. We found significant differences in amphipod densities and species composition among various microhabitats in off-channel areas. P. robustoides was the only amphipod species that occurred on very shallow (< 1 m) sandy bottom near the shore. It inhabited also other sites but its share in the total number of amphipods and abundance decreased with the distance from the shore. Furthermore, at sites more distant from shore its affinity for plant substratum was higher. D. haemobaphes, C. ischnus and C. curvispinum clearly preferred sites distant from shore, overgrown by macrophytes or covered by mussel shells. Furthermore, the abundance of C. ischnus was negatively correlated with the presence of P. robustoides.

Relationships between the occurrence ofPopulations of