Invasion of round goby (Neogobius melanostomus) from the Ponto-Caspian to the Baltic

The spread in the distribution of the round goby in the Gulf of Gdansk over the last decade can be clearly shown

The invasion of round goby (Neogobius melanostomus) from the Ponto-Caspian region is a relatively new introduced species into the Baltic Sea, currently over a hundred species are known as man introduced into the Baltic.

In 1990, the first round gobies were caught in the Gulf of Gdansk and in the Great Lakes in the North America indicating the start of an invasion!

In June 1990, despite the origin and the route of transport being unknown (it is now surmised that the route of transport was in ships ballast waters), the first round gobies were caught in the vicinity of port in Hel, on the tip of the Hel Peninsula. In the same year, more round gobies were caught near Hel and Gdynia harbors. Over the next few years, the colonisation of new regions and an increase in round goby numbers occurred and from 1999, the round goby became one of the more dominant species in the shallow water zone in the west part of the Gulf of Gdansk.

Currently, three geographically separated groups of round gobies exist in the Gulf of Gdansk and its vicinity. The largest occupies the shallow waters of the west part of the Gulf of Gdansk; it is stable and well established. The second one on the west border of the Gulf of Gdansk is small and slowly growing. The third one in the Vistula Lagoon has started to grow rapidly in 2001. The new groups originate from the population inhabiting the west part of the Gulf of Gdansk and the discrete distribution suggests that transport in ballast waters was also the way the new groups were established.

Why invasion is a success

A number of biological features of the round goby support its invasion success, the most important being its high tolerance of a range of environmental factors and its effective spawning strategy.

Round gobies typically occur in shallow waters (from less than one to about 20m depth) and they always occur near bottom. Hard seabed habitats are preferred, however the invasion of the general sandy bottomed Gulf of Gdansk started near the harbour regions, where the concrete piers provided refuge for the fish.

In the Gulf of Gdansk, the round goby can grow up to 25cm in length. However the majority of individuals are between 8 and 18 cm in length making them one of the biggest fish in area. Their life span is rather short, three to four years, with the females living about three years and the males one year longer and reaching a larger size.

In the Gulf of Gdansk, they generally spawn from the beginning of May to the end of September. Although this is prolonged during warmer years. The male to female ratio is approximately three or two to one in contrary to native populations where the sex ratio is more or less equal. Round gobies are multi spawners, that is females spawn more than once during spawning season. In the Gulf of Gdansk spawning takes place between two or four times a year. Round gobies lay their eggs in nests guarded by males. The nest substratum must be solid and all hard object found on the seabed are used to form the basis for round goby nests including stones, rocks, wood, roots of vascular plants or even dumped waste. In the Gulf of Gdansk, where places suitable for nests are limited, the density of nests is much higher than in other regions.

Adaptability

Round gobies can live in marine, brackish and freshwater conditions. In that scale Baltic can be included into the brackish waters category. Hypothetically they can also live in full oceanic salinity, but no such population exists. The range of round goby migration is short, mostly hundreds of metres distance however longer migrations of up to several kilometers do take place in late autumn and early spring, when they migrate to and from deeper water.

Round gobies typically eat bivalves (mainly blue mussel) constituting about 60% of their diet in the Gulf of Gdansk. The arthropods (chiefly Idotea) are also an important food resource particularly for the young and smaller individuals and in places of smaller bivalves quantities. Round goby may also be food for predatory fish, but in the shallow water ecosystem of the Gulf of Gdansk there are almost no fish predators but the round goby is very important food source for cormorants constituting about 60% of fish eaten by this species.

Further invasions

The populations of round goby in the Gulf of Gdansk could provide the springboard for the establishment of new populations in Baltic region. Maybe an invasion is now in its starting phase and a new population established.

The round goby Neogobius melanostomus