First report on the importance of alien gobiids in the diet of native piscivorous fishes in the lower Vistula River (Poland)

We conducted a snapshot study to check the importance of non-native Ponto-Caspian gobies (monkey and racer goby) in the diet of native obligate (northern pike, pikeperch) and facultative (Eurasian perch) predators in the Vistula River 3–4 years after the appearance of aliens. In total 71 fish with non-empty stomachs, taken randomly from net catches from various parts of the river were analyzed, including 32 pike, 20 pikeperch and 19 perch specimens. We found that gobiids prevailed in the diet of smaller (<30 cm standard length, SL) individuals of pike and pikeperch, as well as larger (>15 cm SL) specimens of perch, although the effect of predator size on the presence of gobiids in the diet was not significant in the case of perch. Our results indicate that gobiids as a prey fish can stimulate considerable changes in local food webs, which require further studies.