The spread and settlement of a new fish species can occur naturally, but today it is often supported and accelerated by human activities. Fish eggs and larvae travel around the world in the ballast water of ships, the construction of canals connects once isolated bodies of water and opens up new migration routes, aquarists and anglers sometimes release foreign species into native waters. An example of this is the spread of lionfish (Pterois spp.) in the Caribbean, where they were presumably released by aquarists out of a misguided love for animals and now threaten the biodiversity of the reefs. Economic motives also often play an important role in the introduction of new fish. Rainbow trout, originally at home in North Pacific coastal waters, are now farmed in more than 100 countries worldwide. In some places, escapes from trout farms have led to independent, self-sustaining fish populations, which then often endanger the native wildlife.
However, no one can say with absolute certainty why PontoCaspian gobies in particular, which barely grow larger than 20 centimetres, have managed to penetrate numerous water systems in Central Europe from their natural ranges in just three decades. In addition to the bighead goby (Neogobius kessleri) and tubenose goby (Proterorhinus marmoratus), monkey goby (Neogobius fluviatilis) and the racer goby (Neogobius gymnotrachelus), is the round goby (Neogobius melanostomus), which is considered particularly invasive. Colloquially, these five goby species are sometimes grouped together as “Black Sea gobies”, but this only describes one of the areas of origin. This quintet is complemented by the Chinese sleeper (Perccottus glenii) from Asia, another species that is already included in the first Union list of EU Regulation No. 1143/2014 on invasive alien species. Once established, goby populations often proliferate and can even become the numerically dominant fish species in the newly “conquered” waters. Thus, it is hardly surprising that in many places they are perceived as a nuisance and a serious risk to ecosystems because they threaten invertebrates, amphibians, and fish.
Frugal, undemanding and environmentally tolerant
If one looks for reasons for the gobies success in spreading, one will first encounter the enormous species richness of this family (Gobiidae), which comprises at least 1,100 species. Most of them live in the salt water of the oceans, but plenty are also found in brackish and fresh water. What all goby species have in common is their frugality and adaptability, their ecological plasticity, so to speak. These characteristics are also shared by the round goby, which seems to be even better at conquering new habitats than its relatives and has therefore become the most common goby neozoan in Central European waters. Round gobies are relatively tolerant of oxygen deficiency. This warmth-loving species feels most comfortable at temperatures around 26°C (which suggests that its spread is favoured by climate change), but also tolerates cold periods in winter. In addition, this goby has a pronounced salt tolerance and copes well in both fresh and salt water, as its spread in the brackish water of the Baltic Sea proves. In the saltier North Sea, however, its occurrence is limited to the sweeter estuaries. The spawning season extends from April to September, so that the animals can reproduce about four to six times a year. With an average of 200 to 1,000 eggs, the fertility per spawning cycle is relatively low, but because the male lures several females one after the other into his breeding cave to spawn with them and selflessly protects and cares for the batches of eggs, the reproductive success is nevertheless very high. This allows the fish to spread quickly and effectively in new habitats. Sometimes their population density increases so much in a short time that they compete with each other for food and habitats. This then increases the pressure to migrate to neighbouring and more distant areas. Usually, round gobies appreciate shallow water areas up to 3 metres deep with hard bottom or rocky bottom near the shore. In North American Lake Erie, however, individual animals have also been recorded at depths of around 130 metres on sandy and soft bottoms.
Expansion at breathtaking speed
Seemingly unstoppable, the round goby is conquering large European rivers, the coasts or estuaries in the Baltic Sea and North Sea. This increases the risk of native fish species being displaced and disappearing from these areas. The “invaders” spread across Europe along three routes:
– The northern route runs from the mouth of the Volga in the Caspian Sea upstream and continues via the Volga-Baltic Sea Canal to the eastern Baltic Sea coast.
– The central route starts in the Black Sea at the mouth of the Dnieper and runs upstream from there. It is connected to the Bug and the Vistula via the Dnieper-Bug Canal.
– The southern route runs via the Danube and the Rhine-MainDanube Canal to the Rhine. Current knowledge suggests that the rapid spread of gobies in European water systems is likely to have mainly taken place via this route.
Since the goby has no swim bladder, it is a relatively poor swimmer, making only short excursions into open water and holding its own with difficulty against currents. Nevertheless, the small fish spreads amazingly fast upstream in flowing waters, as a chronology of its advance in the Danube since the beginning of the 1990s proves. In 1997, the round goby was recorded for the first time in the Serbian section of the Danube, in 2000 it had reached Austria and in 2004 it had advanced as far as Straubing in the German section. It reached the Rhine via the Rhine-Main-Danube Canal, where the species was first recorded in 2008. The colonisation of this watercourse seems to have occurred from two directions. Apparently, some gobies entered the North Sea as stowaways in the ballast water of ships, because in 2004 this fish species was recorded for the first time in the Dutch Lek, an estuary of the Rhine. From there, the animals moved steadily upstream and reached the German section of the Rhine near Dormagen in 2008. At the same time, a second population coming from the Rhine-Main-Danube Canal spread downstream. Today, the round goby probably colonises the entire water system of the Rhine, including the large tributaries such as the Moselle and Neckar, and is already one of the dominant fish species in large parts of the river. This by no means stops its spread, however, as it was detected in the Belgian Scheldt in 2010 and quite far upstream in the Swiss Rhine in 2011.
In the Baltic Sea area, the first round gobies were caught in June 1990 at the tip of the Hel Peninsula (Gdansk Bay), where they most likely arrived in the ballast water of ships. Since the records were of adults that were already two to three years old, the initial colonisation could have taken place as early as 1987. From there, the species spread westwards along the Baltic Sea coast reaching the German island of Rügen in 1998, and further invading the Peene (2006) and the Oder (2013). The second direction of spread of the Polish “founder population” was towards the east and north. In 2002, the round goby reached the Gulf of Riga and in 2005 the Gulf of Finland. And now there are also records from the south coast of Sweden.
Human activities are forcing the spread
Particularly astonishing is the spread of the round goby in North America, where the species was first found in 1990 in the St. Clair River on the border between the USA and Canada. The leap “across the pond” can really only have succeeded in connection with transcontinental shipping. Since then, the goby has successfully spread throughout the Great Lakes ecosystem. It is not possible to say exactly where they came from, because analyses have shown an enormous phylogenetic diversity. The species is also spreading in North America. It entered the Illinois River and the Mississippi River via the Illinois and the Michigan Canal. Studies in the Calumet Sag Canal near Chicago have shown that the fish migrate downstream by up to 45 kilometres per year.
According to scientists, the fish spreads mainly in four ways:
– In the ballast water of ships. It is mainly fry and juveniles that are brought in when they rise to the surface at night to feed. In the process, they get caught in the suction cones of ships that pump ballast water on board. This hypothesis is supported by parasitological findings, because introduced round gobies are not as heavily infested with parasites as the original populations in Ponto-Caspian waters. The younger the introduced animals, the lower their parasite infestation.
– Since gobies sometimes attach their eggs to the outer layer of ships‘ hulls, the carry-over can also occur in this way.
– Canal construction to connect waterways. Log fills in the banks of artificial waterways provide ideal habitats and hiding places for round gobies. Baer et al. (2017) have observed that the gobies sometimes form large migratory swarms that occupy new territories almost „by ambush“.
– Dispersal by anglers using gobies as live bait. Individual animals can detach from hooks and survive; surplus bait fish are released in the fishing water even though they were caught elsewhere. This is how the fish is said to have entered Canadian lakes Lake Simcoe and Rice Lake as well as the Shiawassee River and Flint River, for example.
Serious threat to native wildlife
Due to their enormous resilience and massive reproduction in newly colonised waters, which can quickly make round gobies the numerically dominant fish species in the water body, they are one of the most invasive fish neozoa worldwide, whose spread should be intensively monitored and often needs to be regulated. In many countries, the fish has already been blacklisted as an invasive species because of fears of impacts on native biodiversity. The pan-European publicly accessible database DAISIE (Delivering Alien Invasive Species In Europe) lists it among the „100 of the worst“ (www.europealiens.org). Round gobies compete with autochthonous bottom-dwelling fish for food and territory and exert strong feeding pressure on aquatic molluscs. The goby eats almost everything that is available in the water: soft invertebrates such as amphipods and insect larvae, but especially mussels and snails, whose hard shells it cracks with its powerful pharyngeal teeth. However, the assumption that round gobies are vicious predators on the spawn of other species could not be confirmed in 2011 when the stomach contents of over 100 gobies from the Czech Danube tributary Thaya were analysed. Although the sample material for the study was taken in May and June, when most fish species reproduce in the water body, less than one percent of the food found in the stomachs consisted of fish spawn or larvae. Although this single finding does not completely dispel the suspicion of spawn predation, it may be taken as an indication that this risk may be lower than feared.
In many waters, the immigrant gobies in turn expand the food base of native predatory fish. In the North American Great Lakes, black sea bass, glassyeyed perch, American river perch and Arctic char are among the predators of the round goby. Bottom-dwelling burbot have benefited particularly strongly from the increased food supply due to the influx of gobies. In Lake Erie, almost two thirds of the adult round gobies are said to be eaten by burbot, which also grow much faster than before because of the abundant food supply. In European rivers, the gobies mainly fall prey to perch,zander, catfish and broadhead eels. In the brackish waters of the Baltic Sea, round gobies are preyed upon not only by perch but also by cod. A considerable number of the small gobies are also preyed upon by grey herons and cormorants, which are even said to be the main predators of round gobies in the Bay of Gdansk.
Permanent eradication seems unrealistic
It is almost impossible to suppress or completely eradicate the gobies, if only because of their hidden way of life. Control is usually limited to strict stocking bans and occasional reductions in population densities through targeted fishing. Anglers are not allowed to release caught gobies back into the water. In many places, the fish is perceived as a nuisance by sport fishermen because the gobies are more likely to take the bait and do so more frequently than the actual target species. It is difficult to reduce the goby population with fishing methods. In the shallow waters of rivers and lakes, electric fishing gear is occasionally used, and closemeshed fish traps are also quite suitable. However, the selective removals by anglers, who have little use for these fish beyond using individual gobies as bait, are particularly effective. For some years now, anglers‘ associations and fisheries authorities have therefore been promoting the „culinary“ exploitation of the gobies, which are relatively small but have a pleasant perchlike taste. Apart from the central bone, which is said to be perceptible only in gobies 12 cm or longer, the fish is „practically free of bones“. In their natural range, round gobies are popularly eaten and are valued food fish.
This was once also common in Central Europe, as the goby recipes in Friedrike Louise Löffler‘s „New Cookbook“ from 1806 prove. However, this tradition has been almost completely lost with increasing prosperity and the change in eating habits in favour of fillets and loins.
A wide range of culinary uses
Round gobies are suitable for many different types of preparation. The starting point is always the gutted fish without the head. To do this, open the abdominal cavity with a sharp knife in the direction of the head and cut off the head from above by making a slightly oblique cut to the back so that the pectoral and abdominal fins are removed along with the head. After the guts have been removed and the fish has been thoroughly washed, it is ready for preparation. In the simplest case, the gobies are only seasoned with pepper and salt, rolled briefly in flour and then fried in hot fat until crispy. But they can also be deep-fried, pickled, baked in the oven, salted and dried as a snack with beer or hot smoked. Depending on the preparation method and the size of the gobies, one should plan on about 5 to 10 per person.
To stimulate the consumption of gobies and provide culinary inspiration, the Rhineland-Palatinate Ministry of Food has even put together a small brochure with recipes. The Grundelkochbuch (goby cookbook) includes suggestions from gravad goby to the Portuguese fish stew Cataplana, as well as an Asian variant, lemon goby. If more people can be convinced of the good taste of the goby, the chances of relieving natural water systems at least somewhat of these pests will increase.