Neogobius melanostomus - Round goby

The round goby is native to the shallow areas of the Black Sea, the Sea of Azov, the Sea of Marmara and the Caspian Sea. It also lives in the estuaries of rivers that flow into these seas, including the Dniestr, the Don and the Danube. The round goby is less abundant in the south of the Black Sea, where the salinity is higher due to the lower freshwater inflow from rivers ^{[2, 3]}.

First observation in Belgium

The round goby was first observed in Belgium on the 8^th of April 2010 at the mouth of the Scheldt near the Liefkenshoektunnel (a toll tunnel between Antwerp and Beveren). The first individual was found in a trap set by the Research Institute Nature and Forest (INBO) as a part of a fish monitoring program in the Scheldt ^[4].

Spreading in Belgium

Two months after its discovery in the Scheldt, 12 more round gobies were retrieved by an angler from the Albert Canal, near Hasselt ^[4]. That same month, a different individual was found in a fish trap in the Scheldt near Zandvliet ^[4]. Since then, the round goby has become widespread. Nowadays, the alien species is present in large numbers in the Albert Canal (from Antwerp to Kanne) and numerous other canals and rivers, such as the Ghent-Terneuzen Canal, the Sea Scheldt, the Meuse, the Brussels-Scheldt Maritime Canal, the Dessel-Kwaadmechelen Canal etc. ^{[5, 6]}. The round goby is also widespread in the Meuse in Wallonia ^[7].

Spreading in neighbouring countries

In 2004, before the first observation in Belgium, the round goby was already present in the Dutch Delta region of the Meuse and Rhine ^{[4, 8]}. This observation in the Lek River near Schoonhoven was the first time the species was sighted in the estuaries and river basins of the North Sea region. Afterwards, the number of observations increased considerably. The round goby colonised the big rivers from the lower river area and the North Sea Canal, where it also occurs in brackish water ^[9]. In the Elbe, the first individuals of the round goby were found in 2008 near Hamburg ^[10]. Between 2011 and 2013, they were frequently caught by anglers. Since 2015, the species occurs in the upper reaches of the Elbe near the Czech Republic ^[11].

In Western Europe, the species is found in Germany and France, where populations were observed in the Rhine, the Weser and upper reaches of the Danube ^{[4, 12]}, the French Rhine and the Moselle ^[13]. In the rest of Europe, the species is now present in the countries bordering the Danube basin, in the Baltic Sea and Russia. The Baltic Sea has a lower salinity than other seas, allowing the round goby to survive here. In 2011, the species was observed in Finland ^{[14, 15]}.

The first observations of the fish outside its original habitat date back to the 1980s and were in Russia ^[16]. In the early 1990s, the species occurred in the bay of Gdansk in Poland ^[16], from where it spread over large parts of the Baltic Sea, from Germany to Sweden and Estonia ^[17]

Ballast water from ships is the most important way of transport for the round goby. This transport method ensured that the species could spread rapidly across large parts of Europe and North America ^[18]. Recently, the introduction of the round goby got confirmed based on genetic and parasitic information ^[19]. Furthermore, it is likely that the goby spread naturally from colonized areas, although this species is not considered a good swimmer. However, they can sporadically cover relatively long distances ^[20].

The construction of a few important canals has facilitated its spread. In this way, the species ended up in Belgian waters along the Rhine-Main-Danube Canal, which connects the Rhine and Danube basins, and along the Scheldt-Rhine Canal ^[4]. Intentional or accidental releases from aquariums may have caused occasional introductions of the round goby outside its original distribution area ^[21]. In the 1950s, the species was released into the Aral Sea to enrich the local fauna ^[22].

The round goby has a broad salinity tolerance and occurs naturally in both fresh (European rivers) and brackish water (the Caspian Sea and the Black Sea, with a salinity of respectively 13 PSU and 19 PSU) ^{[16, 20, 21, 23, 24]}. This species would have been spotted in water with a higher salt concentration (up to 40.5 PSU) ^[25]. However, this seems exceptional and is most likely due to the different chemical composition of salt in that water ^[20]. A laboratory experiment showed that the round goby cannot survive in seawater with a salinity of 35 PSU ^[26]. Round gobies can survive a salinity of 30 PSU, but it appears to have a negative effect on their fitness ^[27]. This indicates that it is unlikely that populations of the round goby have settled in marine conditions ^[24] or will colonize the North Sea.

The round goby can withstand low oxygen levels (to 4 mg/L) for a couple of days and tolerates temperatures between 0 and 30°C ^[20]. The species needs a rich food base consisting of molluscs, crustaceans and juvenile fishes to thrive. Furthermore, the predation pressure by, among others, cod, pike, eel, and small marine mammals, should not be too high ^[21]. Hard surfaces or dense vegetation are preferred, although the round goby can also occur on sandy soils ^[21]. The variety of habitats and typical, non-natural rocky reinforcements in Belgian rivers, canals and estuaries ensure that the species thrives. The presence of rocks is beneficial for round gobies because it provides shelter and a place to deposit their eggs ^[28]. In the Netherlands, it is suggested to use more natural types of banks and substrate, as they contain a lower amount of alien gobies than non-natural ones ^[29]

The round goby is a ground-dwelling fish that prefers waters between 0 and 30 m deep ^[21], which is one of the reasons why the species cannot cross the Atlantic Ocean naturally (combined with unfavourable salinity levels and the fact that it is a poor swimmer). The populations in the Great Lakes of North America are therefore solely due to transport via the ballast water of ships ^[18]. Besides this, the use of ballast water (both transoceanic and inland waterway transport), together with the construction of canals, has made it relatively easy for this species to spread across the European continent. In this way, dispersion from the Black Sea to our regions, and possibly also to the Baltic Sea, was possible ^[4].

N. melanostomus larvae are pelagic at night when they feed on zooplankton near the water surface at a depth between 0 and 9m ^{[30, 31]}. The exchange of ballast water during the night can, therefore, increase the dispersal of thousands of juveniles ^[20].

Since the round goby lives on the bottom, lays its eggs there and does not have a true larval stage ^{[4, 21]}, it is almost certain that juveniles are the ones transported via ballast water ^{[19, 21]}. Changes to natural ecosystems, such as the construction of dikes or breakwaters, create new habitats for the round goby that promote its further spread ^{[29, 32]}

The round goby can negatively affect existing food webs and native communities of molluscs and crustaceans. Therefore, it is considered an invasive species ^[21]. Experiments in North America have shown that the abundance of zebra mussels (Dreissena polymorpha) decreased substantially in the presence of the round goby ^[33]. In that region, the zebra mussel is an invasive species and is the preferred food source of the round goby, compared to native American molluscs ^[34]. It is even speculated that the round goby has adapted to a diet of zebra mussels. To digest the mussel, its shell must be broken so that digestive enzymes can reach the mussel’s body. When studying the round gobies' faeces, it was found that the zebra mussels got digested, but other prey, such as seed shrimp (Ostracoda), were not digested and were sometimes still alive ^[35]. The non-native zebra mussel is also present in the North Sea ^[36] and has been shown to form an important part of the diet of the round goby ^{[5, 35]}. However, the latter species prefers crustaceans (such as amphipods, shrimps etc.) as a food source ^{[7, 37]}. Preliminary research in Flanders indicates that the round goby has an opportunistic feeding strategy, in which the number of available prey species partly decides the goby’s food choice ^[5].

Native fish species are negatively affected by the presence of the round goby ^[38]. This species feeds on the larvae and juveniles of other fish and in this way disrupt their natural population growth ^[18]. Due to the high reproductive capacity of the round goby (female individuals can lay eggs multiple times a year, and produce up to 10,000 eggs per year ^[21]), competition with native fish is often unbalanced. In North America, it has been shown that the round goby exerts a negative influence on, among others, the mottled sculpin Cottus bairdii ^{[39, 40]}. In the Netherlands, the decline in populations of Cottus perifretum since 2000 has been associated with the rapid expansion of the round goby, which competes with this species ^[38]. There are cases known of direct predation on other species, such as the European flounder ^[14]. The round goby itself can also be a food source for other species. In its native range, it is predated on by other fish, including northern pike (Esox Lucius), beluga sturgeon (Huso huso), asp (Aspius aspius) or zander (Sander lucioperca) ^[41]. In the introduced habitat of the North American Great Lakes, predatory fish are also the most important predators of the round goby ^[18]. Predation pressure is higher in open habitats with fewer places to hide ^[28]. Research in the Baltic Sea has shown that the great cormorant (Phalacrocorax carbo) is also an important predator ^[42].

As a benthic fish, the round goby can easily be exposed to polluted sediments, but, it survives these conditions relatively well ^[43]. However, this might pose a problem for humans if these fish are consumed since their flesh can accumulate certain toxins ^{[18, 43, 44]}.

Preventive measures have not yet been taken against the further spread of the round goby. However, the enforcement of the International Convention for the Control and Management of Ships’ Ballast Water and Sediments (September 2017) should lead to reduced transport of round gobies via the ballast water of transoceanic ships in the future. In addition, the construction of nature-friendly bank and substrate types is used as a mitigation measure in nature management ^[29]. On the other hand, the removal of migration barriers on rivers (as part of the Benelux Decision of 2009) may encourage the further spread of many invasive, non-native species (such as the round goby) to ecologically interesting upper reaches ^[7].

Effective action requires close monitoring of the distribution of the round goby. However, it is difficult, if not impossible, to eradicate established populations of non-native fishes and other organisms. Nevertheless, targeted fishing for round gobies could be a means of limiting the further spread of the species. The size of some populations would certainly make targeted fisheries commercially viable. In the Gulf of Gdansk in Poland, the round goby is already fished for commercial purposes ^[32]. However, the possible bycatch of other fish species, which are currently under threat, remains a problem. It is also possible to restrict fishing for natural (indigenous) predators of gobies, such as perch or pike, or to introduce additional predatory fish. Burbot (Lota lota) appears to be a suitable species for this purpose; they are also bottom dwellers and like to hide between stones. Research in the United States has already demonstrated that burbot has adapted its feeding behaviour to the presence of the round goby, which is one of the most abundant prey items in the stomach of burbots ^{[45, 46]}.

On top of that, it is also possible to selectively lure the round goby into traps. The male individuals emit a 175 Hz courtship sound that attracts female individuals. When this sound is artificially imitated, female individuals can be lured into traps ^{[37, 47].}

The round goby is a relatively small benthic fish with a maximum length of about 30 cm ^[48]. Individuals of invasive (freshwater) populations generally remain smaller, reaching lengths of 6-10 cm ^[37]. The individuals sampled in Flanders had an average length of 7.1 cm and varied between 1.3 and 19.2 cm ^[49]. The species resembles other goby species and can be confused with them. However, the round goby has a clear black spot on the first dorsal fin which is absent in all other non-native gobies in Flanders. Males grow slightly larger than females and turn completely black during the mating season ^[16]. Males take three to four years to mature, females only one or two years. However, outside their natural habitat, this might be quicker ^[18]. The mating season lasts from April to June (or until September) ^[50] and females lay eggs several times during the spawning season. The male makes a nest and guard it even after the eggs have been laid. After the eggs have hatched, the male often dies of exhaustion. To attract the female to his nest, he emits a courtship call of approximately 175 Hz, which also repels other males ^[37].

The second dorsal fin is spineless and, as in all species of goby, the ventral fins are fused to form a suction cup that enables the fish to attach itself to the bottom in fast-flowing water ^{[4, 21]}. The round goby mainly feeds on molluscs, fish eggs, larvae and amphipods ^{[37, 51]}.

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