The round goby Neogobius melanostomus ( P allas , 1814) (Perciformes: Gobiidae), an invasive species in the Albert Canal (Belgium)

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The round goby is native to the Ponto-Caspian area, where it is a euryhaline benthic species of some economic importance (3).Neogobius melanostomus was transported with ballast water of cargo ships to the region of the Great Lakes in North America and to different countries in Central, East, North and West Europe (4,5,6).In the Netherlands the round goby was found for the first time in 2004 in the River Lek (7).It was reported for the first time in Belgium on April 8 th , 2010 from the River Scheldt near the Liefkenshoektunnel (1).The round goby probably reached the River Scheldt in the ballast water of cargo ships.The species is also known from different places in the Albert Canal, which is connected to the river Scheldt at Antwerp.Since the canal is far too deep for electric fishery N. melanostomus has a fairly round head, and rather large eyes, which are slightly protruding from the top of the head.The pelvic fins are fused and form a single pelvic suction disc.A good diagnostic characteristic is the posterior dark spot of the first dorsal fin (Fig. 1), which is absent in the other species of Ponto-Caspian gobies in West Europe.There are 49-55 scales on the lateral line (1).Adult round gobies are mottled with olive green, brown and black irregular spots, whereas juvenile specimens are grey in colour.
Males and females of the round goby can be distinguished by their urogenital papilla: it is white to grey, long and pointed in males and brown, short and blunt tipped in females (4)  dimorphism; the males turn black and their cheeks become swollen.They are territorial and display male parental care (guarding the nests with eggs and hatchlings).Spawning can take place in fresh water as well as in salt water (8).
The females can spawn up to six times during the spawning season.This results in a fairly high reproduction rate and enhances rapid distribution of this invasive species.
The distribution of the round goby in the Albert Canal seems to be discontinuous.Apart from the sampling areas, where they are abundant, there are large parts in the canal where they are absent: e.g. at Grobbendonk not one single round goby was caught outside the section mentioned above in the first year.The fact that both new locations lay near regularly used mooring places of barges, pleads for the distribution of the round goby by disposal of ballast water by those barges in the Albert Canal.The construction of underwater bank protection with stone debris in that canal (2) probably also facilitates the sustainability of the round goby, because that kind of bank protection forms a suitable habitat for Dreissena sp., a regular prey for round gobies.Follow up in future years will determine whether the round goby is able to fill the gaps in its distribution by natural dispersal.In the second year, wider dispersal has already been confirmed by catches at five new locations at Grobbendonk of which only one location was a mooring place.
The round goby has recently been reported from two locations in the River Meuse in Belgium: one near the Dutch border and one near Liège (Verreycken 2013, personal information).This means, that the species is now present in both the rivers Scheldt and Meuse in Belgium, which are connected by the Albert Canal.It had already been reported from the Dutch part of the river Meuse earlier (RAVON, http://www.ravon.nl/Soorten/Vissen).
Round gobies can feed all day long (4), but at Grobbendonk and at Merksem round goby catches with fishing-rods decreased almost to nil after dark.In freshwater the diet of the round goby consists of worms, small crustaceans, insect larvae, molluscs, fish eggs, fish larvae and small fish (8,9,10,11).That diet puts them in competition for food with the native bullhead species -Cottus perifretum freyhof, kottelat & nolte, 2005 and Cottus rhenanus freyhof, kottelat & nolte, 2005 -resulting in a possible decline of the latter species (11).In the Albert Canal juvenile quagga mussels and juvenile zebra mussels -Dreissena polymorpha (Pallas, 1771) -form important prey of the round goby: the smallest shells are swallowed whole, whereas the larger ones are crushed with the laryngeal teeth.In fact, each time that we put some freshly caught round gobies in a clean aquarium, we found a few hours later some small bivalve shells and shell debris of larger specimens of dreissenids on the bottom.So far three juvenile doublets of the Asiatic clam have also been found.Since some pieces of that shell debris are far too large -larger than 1 cm -and too sharp to pass through the intestines or the anus, we presume that at least the large pieces were vomited out by the round gobies after the digestion of the soft parts.In nature, juveniles of those two exotic invasive species of Dreissena thus constitute an important part of the food of round gobies, which is probably the only positive aspect of the presence of the latter species in West European waters.The round goby, Neogobius melanostomus, in the Albert Canal (Belgium) Potential predators of the round goby in the Albert Canal are pike -Esox lucius linnaeus, 1758 -, perch -Perca fluviatilis linnaeus, 1758 -, zander -Sander lucioperca (linnaeus, 1758) -, European catfish -Silurus glanis linnaeus, 1758 -, big eel -Anguilla anguilla (Linnaeus, 1758) -, big flounder -Platichthys flesus (linnaeus, 1758) -, big plaice -Pleuronectes platessa linnaeus, 1758, the great cormorant -Phalacrocorax carbo (linnaeus, 1758) -, commonly observed at the Albert Canal, and the great crested grebe -Podiceps cristatus (linnaeus, 1758) (12).But pike, flounder and plaice are far too rare in the canal to be of great influence.Among the birds, the depth of the canal eliminates the grey heron -Ardea cinerea linnaeus, 1758 -as a predator.As the round goby has only very recently been recognized as an invasive species in the Albert Canal, no research on its predation has been confirmed yet.We observed, however, that round gobies were successfully used as bait by anglers to catch zander.List of distinguishing features in the otoliths between Neogobius kessleri and Neogobius melanostomus.The potentially rapid distribution of the two aforementioned invasive fish species (N.melanostomus and N. kessleri) can be followed up by direct sampling of river or canal stretches susceptible to yield catches, in particular the mooring places of river barges, where ballast tanks are drained.Another method is the inspection of bird pellets or droppings for fish otoliths along waterways.Predator birds feed on fish, but do not digest their hard parts (such as bones and otoliths), which are subsequently vomited out or ejected in droppings (e.g., 13,14).

Feature
Otoliths (sagittae) provide a useful tool in identifying fishes to species level and they are widely used to that effect in ichthyology, ornithology, cetology and palaeontology (15).For this reason we present here a list of distinctive features (Table 2), and illustrations of the sagittae of N. melanostomus and N. kessleri (specimens from the RBINS collection, unnumbered, Fig. 3) to enhance identification of otolith finds in pellets of birds.Moreover, using the OL and OH information from Table 2, the length and weight of the consumed fish can be reasonably well estimated.As no specimens of N. kessleri from Belgium were available for dissection, we figure otoliths from two specimens caught on the Danube, near Vienna in Austria.Otoliths of N. kessleri and N. melanostomus are quite thin in ventral view and have an overall rectangular to parallelogram outline.Those of N. melanostomus are characterized by a convex, sometimes notched dorsal rim (feature 1), with the postdorsal process (feature 2) well below the highest point of the dorsal rim.Only the sagittae extracted from a fish of 16 cm TL had a very concave dorsal rim.The otoliths show little marginal ornamentation, except for specimens from larger fish.The posterior part of the ventral rim (feature 3) is bent, whereas the ventral rim in otoliths of N. kessleri is more or less straight.The anterior rim (feature 4) is higher in N. melanostomus than in N. kessleri, due to the convexity of the dorsal rim of the former.The ostium (feature 5, anterior part of the sulcus) is wider in N. melanostomus than in N. kessleri.
In our small sample, there appears to be a clear relationship, for the larger sizes, between body weight and OL, but less so between TL and OL (Table 1).Two male specimens of 138 mm and two of 140 mm and 143 mm TL resp.have a different OL depending rather on the body weight than on the TL.This contrasts, however, to an earlier study concerning a large sample of the same species from the Baltic Sea, which showed a clear correlation between fish length and body weight (16).Moreover, our sample contains specimens of 105, 110 and 120 mm, represented each by one male and one female.In two cases (TL 110 and 120 mm) the otoliths of the males are larger than those of the females, but in one case (TL 105 mm) the opposite is true.The two largest specimens (one female of 160 mm TL and one male of 157 mm TL) are comparable in size, but the otoliths of the male are much larger than those of the female.Possibly, such differences can be attributed to sexual dimorphism.It must be noted, however, that sexual dimorphism in otoliths is a rarely observed phenomenon and when observed otoliths of male fish are not necessarily larger than those of female fish.It will, however, require much more additional material than presently available to verify our assumptions.

Fig. 1 .
Fig. 1. -Male specimen of Neogobius melanostomus from the Albert Canal at Grobbendonk (TL = 137 mm), exhibiting the characteristic black spot on the first dorsal fin.Specimen not preserved.
. During the spawning season they exhibit sexual

TABLE 1
Morphometric data of Neogobius melanostomus of the Albert Canal at Merksem and Grobbendonk.