Occurrence of non‐native fishes in the Danube east of Vienna (Austria) and potential interactions of invasive gobiids with native fishes

Non‐native fish species pose a major threat to local fish populations and aquatic ecosystems in general. Invasive gobies are a particular focus of research, but with partly inconsistent results. While some studies reported severe detrimental impacts on native species, others have concluded less serious or neutral effects. We provide results from a large‐scale, multi‐annual fish monitoring program on the occurrence and abundance of non‐native fishes in the main stem of a free‐flowing section of the Austrian Danube. Special emphasis was placed on identifying positive or negative interactions of invasive gobies with native species. Whereas most non‐native species occurred too sporadically or were too few in number to infer a direct threat on the local fish community, invasive gobies were among the most common fishes throughout all sampling years. Co‐occurrence analyses revealed species‐ and mesohabitat type‐specific associations of gobies with native species, which were primarily positive. Notably, native predators such as asp, burbot, or perch probably benefit from the ubiquitous gobies. Two characteristic fluvial fishes revealed negative associations with invasive gobies, namely barbel (Barbus barbus) and Danube whitefin gudgeon (Romanogobio vladykovi): they appear to avoid habitats occupied by gobies. Accordingly, high abundances of round and bighead goby most likely resulted in population losses of barbel and whitefin gudgeon, respectively. Overall, our results indicate a limited negative impact of non‐native species in the sampling area. This is because only two out of 51 occurring species were found to be adversely affected by gobies, the share of co‐occurrences with native species was high, and other non‐native species were generally rare. Nevertheless, invasions are highly dynamic, and new non‐native species are likely to occur in the Austrian Danube, calling for continued monitoring and awareness.     

New records of non-indigenous gobies (Neogobius spp.) in the Austrian Danube

Beginning in the early 1990s, species of the genus Neogobius rapidly expanded their native range from the Black Sea and the lower reaches of the River Danube upstream to include the middle and upper Danube. In 2002, 83 rip‐rap sites at 25 locations along the Austrian section of the River Danube were sampled by catch per unit effort (CPUE) with electrofishing to assess goby distribution. Bighead goby N. kessleri was found in all locations and at 76% of these sites, it was the third‐most abundant species (17% of CPUE). Densities of Neogobius spp. were highest in industrial harbours (IH), whereas the native species European bullhead Cottus gobio was found primarily in the main channel and tubenose goby Proterorhinus marmoratus in non‐industrial harbours. The history of records and the present distribution of Neogobius spp. suggest that IHs are the main dispersal points.     

Ontogenetic variability in external morphology of bighead goby Neogobius kessleri from the Middle Danube, Slovakia

Over the last decade, four species of goby have invaded the Middle Danube area, and all of them have spread rapidly. In the early 1990s, bighead goby Neogobius kessleri appeared in the Middle Danube, where it now seems to thrive. Relatively little is known of the environmental biology and ontogeny of this species in its native and non‐native ranges. In this paper, preliminary results on the external morphology of bighead goby from the Slovak stretch of the Danube are presented within an ontogenetic context. Patterns of relative growth with no apparent changes at small size suggest direct development in bighead goby, although not as profoundly direct as observed in round goby N. melanostomus. Differences in life history between these two closely related species may have important implications for their success in novel environments, favouring the latter in short term (several years) and the former in long term (decades and longer) perspective.     

Metazoan parasites of Neogobius fishes in the Slovak section of the River Danube

Metazoan parasites were investigated in three non‐native fishes (monkey goby Neogobius fluviatilis, bighead goby Neogobius kessleri and round goby Neogobius melanostomus) collected from the former and current main channel of the River Danube and from the River Hron, Slovakia, in November 2003. Thirteen parasite species were identified: Triaenophorus crassus, Diplostomum sp., Tylodelphys clavata, Metorchis xanthosomus, Nicolla skrjabini, Gyrodactylus proterorhini, Pomphorhynchus laevis, Contracoecum sp., Raphidascaris acus, Anguillicola crassus, Unio tumidus, Anodonta anatina and Pseudoanodonta complanata. The maximum parasite diversity was found in N. fluviatilis. Total parasite abundance was significantly higher in N. kessleri, but no significant differences among sampling sites were observed. Pomphorhynchus laevis and glochidia of Anodonta anatina reached 100% prevalence in N. kessleri in the new channel of the Danube and, in general, these species were also the most prevalent parasites in all three goby species. For endoparasites, gobies served mostly as intermediate (digenean, cestodes and nematodes) or paratenic (acantocephalan and nematodes) hosts. All parasite species found are common parasites in the Middle Danube basin. No parasites specific to Neogobius, known from their native populations, were observed.     

The recent distribution and abundance of non-native Neogobius fishes in the Slovak section of the River Danube

The distributions of invasive Neogobius species were investigated in the Slovak section of the River Danube from Bratislava downstream to the village of Chl'aba. During October 2004, the main channel of the Danube was sampled, including by‐pass, head‐race and tail‐race canals of the Gab?íkovo dam, backwaters and the lower‐most sections of the tributaries Malý Dunaj, Hron, Váh and Ipel’. Three Neogobius species already documented in Slovakia were captured (monkey goby Neogobius fluviatilis, bighead goby N. kessleri, round goby N. melanostomus), with the latter two species being found in almost all stretches of the Slovak Danube. Monkey goby had a most limited distribution, and no racer goby N. gymnotrachelus were observed. The abundance of particular Neogobius species appeared to depend on the character of the shoreline habitat, and a possible association between larger towns and the abundance of bighead and round gobies requires further investigation.     

Ontogenetic variability in external morphology of round goby Neogobius melanostomus from Middle Danube, Slovakia

In the previous decade, four species of non‐native goby have invaded the middle section of the River Danube. To understand the problems associated with these invasions better, we examined the external morphology of 184 round goby Neogobius melanostomus specimens using both triple regression (distance‐based measurements) and geometrical (coordinates‐based measurements) analysis within an ontogenetical and epigenetical aspect. Using geometrical analysis of distance‐based characters, we found that the external body shape in four size groups of round goby remained essentially unchanged. Such direct development represents a strongly precocial (i.e. specialized) life‐history, whereas earlier maturation in this non‐native population suggests a shift back towards more altricial (i.e. less specialized, more generalized) life history. This combination of altricial–precocial trajectories seems to be typical for invasive round gobies and may be one of key factors for their successful colonization of new environments.     

Spread of an introduced parasite across the Hawaiian archipelago independent of its introduced host

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Invasive species are less parasitized than native competitors, but for how long? The case of the round goby in the Great Lakes-St. Lawrence Basin

There is increasing evidence that parasitism represents an unpredictable dimension of the ecological impacts of biological invasions. In addition to the risk of exotic pathogen transmission, other mechanisms such as parasite-release, could contribute to shaping the relationship between introduced species and native communities. In this study, we used the Eurasian round goby (Neogobius menalostomus) in the Great Lakes-St. Lawrence River ecosystem to further explore these ideas. As predicted by the parasite-release hypothesis, recently established populations of round goby were parasitized by a depauperate community of generalist helminths (8 taxa), all commonly found in the St. Lawrence River. In comparison, two native species, the logperch (Percina caprodes) and spottail shiner (Notropis hudsonius), were the hosts of 25 and 24 taxa respectively. Round gobies from each of 3 sampled localities were also less heavily infected than both indigenous species. This is in contrast to what is observed in round goby’s native range where the species is often the most parasitized among gobid competitors. This relative difference in parasite pressure could enhance its competitiveness in the introduced range. However, our study of an older population of round goby in Lake St. Clair suggests that this advantage over native species could be of short duration. Within 15 years, the parasite abundance and richness in the round goby has more than doubled whereas the number of parasite species per fish has increased to levels of those typical of fish indigenous to the St. Lawrence-Great Lakes watershed.     

Abundance and night-time habitat use patterns of Ponto-Caspian gobiid species (Pisces, Gobiidae) in the littoral zone of the River Danube, Hungary

We examined the abundance and meso‐habitat use of gobiid species during both day and night along 43 stretches (500‐m long) of the littoral zone at five locations of the Middle Danube, Hungary, in spring and in summer 2004. Electro‐fishing catch per unit effort sampling revealed significantly higher relative densities at night than during the day. Gobiids occupied all the available habitats encountered during sampling. Habitat‐abundance relationships from night‐time samples revealed that the two most abundant species, round goby Neogobius melanostomus and bighead goby Neogobius kessleri, were found in highest relative density along gravel beach and artificial rip‐rap habitats; these species were in relatively low density in sandy areas. Monkey goby Neogobius fluviatilis was more abundant in natural shorelines than in rip‐rap habitats and dispersed more consistently between sandy and gravel beaches. Tubenose goby Proterorhinus marmoratus demonstrated great plasticity in meso‐habitat use. In conclusion, Ponto‐Caspian gobies occurred rather consistently along the Hungarian section of the Danube, occupying all available habitats examined. Abundance‐habitat relations suggest plasticity in meso‐habitat use but partial segregation between species. Further studies are required at multiple spatial scales to assess how micro‐ and meso‐habitat use varies with fluctuating population densities.     

Checklist of parasites for Ponto-Caspian gobies (Actinopterygii: Gobiidae) in their native and non-native ranges

This study provides a comprehensive checklist of parasites reported for native and non-native populations of seven Ponto-Caspian goby species, i.e. Babka gymnotrachelus, Neogobius fluviatilis, N. melanostomus, Ponticola gorlap, P. kessleri, Proterorhinus semilunaris and P. semipellucidus, all of which have recently expanded their natural range, either by transoceanic translocation or along the three main European intracontinental invasion corridors. In some cases, invasion success is facilitated by loss of natural enemies, such as predators and parasites; thus, knowledge of a species natural parasite fauna is essential for assessments of enemy release in the host's non-native range. The study is based on published literature and personal unpublished data related to parasites of Ponto-Caspian gobiids. A total of 249 parasite taxa have been reported during 1931—2019, with higher numbers reported from native (214) than non-native (151) ranges. This checklist indicates apparent reduction in native parasites in non-native ranges in all goby species, although parasite colonisation in non-native ranges differs between fish species and invasion corridors. Interestingly, the cumulative number of species reported is still gradually increasing in both ranges. Acquisition of high number of local parasite species also indicates competence of Ponto-Caspian gobies to a wide range of generalist parasites.     

Condition status and parasite infection of Neogobius kessleri and N. melanostomus (Gobiidae) in their native and non-native area of distribution of the Danube River

The success of introduced species is often facilitated by escape from the effects of natural predators and parasites. Introduced species can profit from this favourable situation, attaining higher population densities and greater individual sizes in novel areas. In this study, somatic condition and parasite infection were compared between native and non-native populations of Neogobius kessleri Günther; introduced only within the interconnected Danube and Rhine River system, and N. melanostomus (Pallas); widely introduced throughout several river systems in Europe and North America. Higher values of Fulton’s condition factor were observed in non-native populations of both goby species. Neogobius melanostomus attained higher gonadosomatic index values in non-native populations, indicating potential increased investment in reproduction in its new area. A lower splenosomatic index was observed in non-native populations, especially in N. melanostomus. Parasite infracommunity richness and mean abundance were higher in N. kessleri in both native and non-native populations, suggesting higher susceptibility of N. kessleri to these parasites. Non-native populations of both hosts showed higher infra-community richness as a result of acquiring parasites native to the new area, but lower parasite abundance. Differences in success of the introduction and establishment in new areas between the two fish species may be associated with a relatively low parasite infection rate and a higher gonadosomatic index in non-native populations of N. melanostomus in comparison to N. kessleri.     

Ontogenetic variability in the external morphology of monkey goby, Neogobius fluviatilis (Pallas, 1814) and its relevance to invasion potential

In the previous decade, four species of non-native gobies have invaded the middle section of the river Danube and its tributaries. An effective tool for understanding biological invasions is the evaluation of various biological traits (morphological, life history, ontogenetic) within an epigenetic context. The present study examines the external morphology of monkey goby Neogobius fluviatilis (Pallas, 1814) from the mouth of the River Hron, the morphological differences among three goby species (monkey, bighead and round) and the relevance of these differences for invasive potential. Monkey goby reach their definite phenotype very early in their ontogeny and thus represent a strongly precocial (specialized) species with direct development. The morphological differences between monkey and two other goby species also reflect its strong specialization for sandy substrata and smaller prey types. Thus, monkey goby are not expected to spread to new areas as fast as the round and bighead gobies, and their distribution is likely to be limited to sandy and/or sandy-gravel substrata. If this assumption is correct, then the potential adverse impact of monkey goby on native fauna or even ecosystem is likely to be less than that of the bighead and round gobies. Handling editor: K. Martens     

Shifting dominance from native C4 to non‐native C3 grasses: relationships to community diversity

Many field studies have examined how site fertility, soil differences and site history influence the diversity of a plant community. However, only a few studies have examined how the identity of the dominant species influences the diversity in grasslands. Plant species differ widely in phenology, growth form and resource uses; thus, communities dominated by different species are also likely to strongly differ in the environment that they create and in which the subdominant species exist. We examined the correlation between the four most dominant species and community diversity in 2100 plots, located in 21 abandoned agricultural fields in central Minnesota over a 23‐year period. The four most common species were two non‐native C₃ cool season species, Poa pratensis and Agropyron repens, and two native C₄ warm season species, Schizachyrium scoparium and Andropogon gerardii. We found that the differences in the dominants explained up to 27% of the community diversity. Thus, the identity of the dominant species can have a strong influence on community diversity and studies examining factors that influence plant community diversity need to incorporate the effect of the dominants. Secondly, we found that the non‐native C₃ grass dominated communities had lower overall and lower native species richness relative to the native C₄ grass dominated communities. Therefore, a shift in dominants from C₄ to C₃ may lead to a large community diversity decline. We found that Poa pratensis, the most abundant non‐native C₃ grass increased in abundance over the 23 years; thus, the negative influence of non‐natives on the community diversity is not decreasing over time and active management is required to restore native grassland plant communities.     

Secondary invasion of the round goby into high diversity Great Lakes tributaries and species at risk hotspots: potential new concerns for endangered freshwater species

The round goby (Neogobius melanostomus) first invaded North America in 1990 when it was discovered in the St. Clair River. Despite more than 15 years of potential invasion, many Great Lakes’ lotic systems remained uninvaded. Recently, we captured the round goby from several Great Lakes tributaries known as species-at-risk hotspots. With a combination of field sampling of round gobies and literature review of the impact of round gobies on native taxa, we assess the potential impacts of the secondary invasion to native species using three mechanisms: competition; predation; and indirect impacts from the loss of obligate mussel hosts. We estimate that 89% (17/19) of benthic fishes and 17% (6/36) of mussels that occur in these systems are either known or suspected to be impacted by the secondary invasion of round goby. In particular, we note that the distribution of potential impacts of round goby invasion was largely associated with species with a conservation designation, including seven endangered species (1 fish, 6 mussels). As these recent captures of round goby represent novel occurrences in high diversity watersheds, understanding the potential impacts of secondary invasion to native biota is fundamental to prevent species declines and to allow early mitigation.     

Parasites of native and exotic freshwater fishes in south‐western Australia

In this study, 1429 fishes of 18 different species (12 native and six exotic) were sampled from 29 localities to compare the levels of parasitism between native and exotic fish species and to examine the relationship between environmental degradation and parasite diversity. Forty‐four putative species of parasites were found and most of these appear to be native parasites, which have not previously been described. Two parasite species, Lernaea cyprinacea and Ligula intestinalis, are probably introduced. Both were found on or in a range of native fish species, where they may cause severe disease. Levels of parasitism and parasite diversity were significantly greater in native fishes than in exotic species, and this may contribute to an enhanced demographic performance and competitive ability in invading exotics. Levels of parasitism and parasite diversity in native fishes were negatively related to habitat disturbance, in particular to a suite of factors that indicate increased human use of the river and surrounding environment. This was due principally to the absence in more disturbed habitats of a number of species of endoparasites with complex life cycles, involving transmission between different host species.     

Ontogenetic variability in external morphology of native (Canadian) and non-native (Slovak) populations of pumpkinseed Lepomis gibbosus (Linnaeus 1758)

External morphology of native Canadian (River Otonabee, Looncall Lake) and non‐native Slovak (River Danube) pumpkinseed was examined using both triple regression analysis (distance‐based measurements) and geometrical analysis (coordinate‐based measurements) within an ontogenetical aspect. In general, the results from the geometrical analysis comply with those from the triple regression analysis. The smallest pumpkinseed (predominantly juveniles) differed significantly from the largest pumpkinseed (predominantly adults) in all the three populations. The major difference was that adults had a deeper body and larger belly area than juveniles, which is probably associated with more space for gonads in mature fish. Developmental patterns and external morphology in pumpkinseed from the River Otonabee seemed to be closer to pumpkinseed from the Danube than to those from Lake Looncall. This suggests that, in the pumpkinseed examined, ontogenetic changes in external shape depend on environmental conditions (epigenetical information) rather than on geographical and/or genetical isolation. Further examination of early development, fecundity, number of spawning acts per season, parental care, egg size, age at maturation, etc., will follow to test this hypothesis.     

Sampling indigenous ground‐living beetles in a stand of non‐native tree privet (Ligustrum lucidum) in New Zealand raises new management questions

Small urban forest reserves in New Zealand have been shown to have value in conserving indigenous beetle diversity. However there is little information available on the ability of non‐native vegetation areas such as tree privet to support indigenous beetle assemblages. To investigate this for one site, ground‐living beetles were collected using pitfall traps over a year at a small urban forest of the invasive tree Ligustrum lucidum (tree privet) in Auckland, New Zealand. A total of 815 beetles were found, from 20 families and 42 relative taxonomic units. Using monthly data, there was no correlation between soil moisture and diversity index (P = 0.805) or species richness (P = 0.375). These results raise the question of whether urban patches of non‐native tree privet may have potential as reservoirs of beetle diversity, if only until they are replaced with native vegetation.     

Diet and feeding habits of monkey goby (Neogobius fluviatilis) in a newly invaded area

The monkey goby (Neogobius fluviatilis) is one of the Ponto-Caspian species that in recent decades made substantial East-to-West invasions in European inland waters. Now the species is present in the Danube as far as its Slovakian section. In the last decade the species also invaded the Western Bug and Vistula rivers, in the latter it is numerous nearly as far as its delta. The purpose of our study was to define the diet spectrum, feeding preferences, spatial and size related changes in diet and diurnal feeding activity of this goby. Another goal was to predict which groups of native prey would be most affected by the presence of this exotic predator in newly invaded areas. The study was carried out at three sampling locations in the Vistula River system. The food spectrum of the monkey goby was broad. The fish consumed insect larvae and pupae, crustaceans, annelids, gastropods and fish. Chironomid larvae were a prevalent food category in all sampling sites, followed by amphipod crustaceans at one site and by trichopteran larvae and chironomid pupae at another. At the third site, there was no distinct subdominant food category. According to the values of the Ivlev’s selectivity index, the preferred food category were chironomid larvae. No significant differences in diet were found over the 24-h cycle. There was no variation among different fish size groups. From our studies and from the available literature data it can be concluded that the species, due to its ability to use locally available food resources, displays a generalistic and highly flexible feeding strategy.     

Genetic and biogeographic relationships of the racer goby Neogobius gymnotrachelus (Gobiidae: Teleostei) from introduced and native Eurasian locations

The racer goby Neogobius gymnotrachelus , along with several other neogobiin fishes, has been spreading north and west from its native Ponto-Caspian range for the past two decades via shipping and canals. It has been predicted as a likely future invader of the North American Great Lakes, where it would join its neogobiin relatives – the round and freshwater tubenose gobies. The present study is the first to analyse the population genetic and biogeographic relationships of the racer goby, establishing a baseline to aid interpretation of its future spread patterns and likely donor-recipient population relationships. The mitochondrial DNA cytochrome b gene was sequenced from representative areas of the racer goby's range, including rivers of the northern Black Sea and areas of spread upstream in the Danube River and outside the Ponto-Caspian region to the Vistula River in Poland. Results discerned nine haplotypes, with few shared among drainages of native rivers and most sites housing unique alleles. Racer goby populations significantly diverged among watersheds, supporting historic low migration and little non-anthropogenic gene flow. The Dnieper River was identified as a likely donor source for the Vistula River colonization, where appreciable variability suggests a relatively large number of founding genotypes.     

Rapid response of invasive round goby (Neogobius melanostomus) (Pallas, 1814) to an environmental perturbation demonstrated in reproductive parameters of females

Plasticity in life‐history traits appears to be very important for adaptation of invasive species to a novel environment. Predictable changes in life‐history traits have been observed in the progression of invasions in several fish species. Nevertheless, the novel environment of the invaded areas can pose a challenge to the invaders caused by sudden perturbations in the ecological conditions. The aim of this study was to examine whether a sudden environmental disruption would reflect in significant changes in the reproductive parameters of the females of an invasive round goby (Neogobius melanostomus) population. Samples were collected from the middle section of the River Danube (Bratislava, Slovakia) from the 2008 post‐spawning period to the end of spawning in 2010. The results support the prediction that after a strong environmental perturbation, the females increase their absolute as well as relative numbers of oocytes and their real absolute fecundity; the differences in all of these parameters were statistically significant. Another strong environmental disruption occurred in June 2010, although the response differed from that of 2009. In 2009, the females responded by increasing their oocyte numbers; in 2010, they extended their reproductive season by producing an extra batch of eggs. This can be explained by the different timing of the sudden perturbations. In summation, a coincidence between the environmental perturbations and the significant changes in their reproductive traits was found in the round goby females in the two subsequent years. This indicates that the round goby can respond rapidly to environmental disruptions, and that such reproductive flexibility likely helps the species in its successful invasions.