Modification in the nest guarding strategy — one of the reasons of the round goby (Neogobius melanostomus) invasion success in the Gulf of Gdańsk?

The round goby was noted for the first time in the Gulf of Gdańsk in 1990. After a few years, the round goby had invaded all suitable areas in the west part of the Gulf of Gdańsk. Generally, the sandy bottom was the factor that do not prevent the fish invasion. The effectiveness of reproduction is one of the most important factors in deciding the success of an introduction into a new area. The round goby needs a hard bottom for egg laying and subsequent embryo development in nests guarded by males. We observed the round goby nests in the Gulf of Gdańsk during three subsequent years. Distances between nests, nest size, number of eggs per nest, and depth of nest locations were investigated by underwater survey, from May till September, throughout the round goby spawning season. Our investigation led us to confirm that any solid elements on the bottom may be utilized as nest substrate when nesting areas are limited. Distances between nests decrease noticeably in comparison with native round goby areas. Taking into account these findings, we conclude that shallow water areas, of almost any type of bottom, might be invaded by the round goby.