The fish community of the Gulf of Puck in the Baltic Sea has changed in recent years following the successful introduction of the Round Goby (Neogobius melanostomus). The first specimen of Round Goby was caught in 1990 and now is estimated in millions of individuals. The Round Goby was a main component of the Grey Heron diet in the colony at Mosty and made up 95% of regurgitated prey items. During chick rearing period, herons mostly flew to the coast to forage in shallow waters and feed on the Round Goby. The numbers of Grey Herons at Mosty have increased recently, especially since 1999 (from 238 in 1999 to 493 nests in 2003; an increase of 107%). The indirect evidence suggests that the rapid expansion of the Round Goby has influenced the Grey Heron and affected both the diet and the numbers of birds feeding and breeding in the vicinity of the Puck Gulf.