Invasive Freshwater Fish Research Articles

Phylogenetic and environmental DNA insights into emerging aquatic parasites: implications for risk management

Species translocation leads to disease emergence in native species of considerable economic importance. Generalist parasites are more likely to be transported, become established and infect new hosts, thus their risk needs to be evaluated. Freshwater systems are particularly at risk from parasite introductions due to the frequency of fish movements, lack of international legislative controls for non-listed pathogens and inherent difficulties with monitoring disease introductions in wild fish populations. Here we used one of the world’s most invasive freshwater fish, the topmouth gudgeon, Pseudorasbora parva, to demonstrate the risk posed by an emergent generalist parasite, Sphaerothecum destruens. Pseudorasbora parva has spread to 32 countries from its native range in China through the aquaculture trade and has introduced S. destruens to at least five of these. We systematically investigated the spread of S. destruens through Great Britain and its establishment in native fish communities through a combination of phylogenetic studies of the host and parasite and a novel environmental DNA detection assay. Molecular approaches confirmed that S. destruens is present in 50% of the P. parva communities tested and was also detected in resident native fish communities but in the absence of notable histopathological changes. We identified specific P. parva haplotypes associated with S. destruens and evaluated the risk of disease emergence from this cryptic fish parasite. We provide a framework that can be applied to any aquatic pathogen to enhance detection and help mitigate future disease risks in wild fish populations.

Parasites of invasive freshwater fishes and the factors affecting their richness

AbstractFreshwater environments are threatened by nonnative species introductions, often involving fishes. Parasites and pathogens introduced with fishes have the potential to infect native taxa an...

A Preliminary Study on the Distribution Pattern, Feeding Behavior and Invasiveness of the Exotic Fish, Chitala ornata (Clown Knife Fish) in the Sub Water Ways of Attanagalu Oya

Increasing threat from the introduced invasive species has been identified as one of the major risk factors for the extinction of freshwater fishes. There are number of identified freshwater alien species which have become invasive within Sri Lankan natural waters. However, there is a dearth of information regarding many of these alien invasive species. Therefore, this research project focused on elucidation of distribution pattern, food and feeding habitats and invasiveness of Chitala ornata, an alien invasive freshwater fish species in potentially new site of infestation in the Attanagalu oya and its tributaries over a period of three months as a preliminary study. Key objectives of the study were, to find out the present distribution of C. ornata in Attanagalu Oya and its tributaries, to evaluate the food and feeding habits of C. ornata through gut analysis and to find out the presence of other invasive/introduced species in the area. Over the study peroid, 50 individuals of C. ornata ranging from 21cm to >80cm were collected using hook and line fishing gear and captured fish were deep-frozen immediately after capturing to avoid the deterioration of the food items within the fish gut. Fish gut analyses have been carried out under laboratory conditions. Morphological measures, frequency of occurrence and Index of fullness were calculated. The results indicated that C. ornata has established in the Attanagalu oya basin in areas such as Ekala and Kotugoda. Gut content analysis indicated that C. ornata feeds on significant number of native fish species. Five major prey fish species, consisting 58 Hirikanaya (Labeo dussumeri), 35 Rathu waral katu kuriya (Puntius kelumi), 23 Iri ankutta (Mystuss vittatus), 13 Walapoththa (Ompok bimaculatus) and 16 Illeya (Megalops cyprinoides) were found to make up the diet of 28 adults and 05 juveniles of C. ornata. The average Index of fullness was 378.19. Frequencies of occurrence are 0.606, 0.485, 0.424, 0.121 and 0.242 respectively for L.dussumeri, P. kelumi, M. vittatus, O. bimaculatus and M. cyprinoides. Out of these five species P. kelumi is listed as an endangered endemic species. Other than fishes, one shrimp species, molluscs and fish eggs were also recorded as their food items. Further, other exotic species such as Tank cleaner (Pterygoplichthys spp.) and giant gourami (Osphronemus goramy) were also recorded in the Attanagalu oya. It was concluded that immediate steps should be taken to manage this potentially harmful Alien invasive species in the Attanagalu Oya basin. Keywords: Invasive, Exotic, Fresh water, Chitala, Attanagalu oya

Comparative trophic impacts of two globally invasive cyprinid fishes reveal species‐specific invasion consequences for a threatened native fish

Abstract Predicting the ecological consequences of invasions by non‐native species is a fundamental aspect of their risk‐based management. As impacts can include the negative consequences of resource sharing with native species, the application of in situ cohabitation field experiments can test hypotheses relating to invasion ecology via competitive interactions and processes. As fishes are adaptable and tractable experimental organisms, they are strong model species for use in studies on competitive interactions. The trophic consequences of invasion by two globally invasive freshwater fishes, common carp Cyprinus carpio and goldfish Carassius auratus, were tested on the threatened native fish crucian carp Carassius carassius. Cohabitation experiments, completed in pond enclosures, used all species in allopatric and sympatric treatments using a substitutive design where the number of fish per treatment was kept constant. Stable isotope analysis (δ13C and δ15N) assessed alterations in the trophic ecology of each species across treatments, with growth rates used to assess any consequent impacts on the fish. When in sympatry with C. auratus and C. carassius, the Cy. carpio isotopic niche was at a significantly lower trophic position compared to allopatry. This resulted in niche overlap with C. auratus, while for C. carassius, their isotopic niche shifted to a higher trophic position compared with allopatry. The growth rate of Cy. carpio was always significantly higher in sympatry than in allopatry, whereas growth rates for C. carassius and C. auratus were significantly depressed in Cy. carpio presence. In contrast, the isotopic niche sizes and positions and growth rates of the Carassius fishes were not significantly different between allopatry and when they cohabited. Plasticity in the isotopic niche of Cy. carpio resulted in significant alterations in their trophic position between allopatry and sympatry and, when coupled with their depressed growth in allopatry, suggests the competitive processes driving this were intra‐specific rather than inter‐specific. This then resulted in detrimental impacts on cohabiting Carassius fishes. These results emphasise that ecological consequences of Cy. carpio in invaded freshwaters include impacts on the trophic ecology of native fishes.

Identification of potentially invasive freshwater fishes, including translocated species, in Turkey using the Aquatic Species Invasiveness Screening Kit (AS-ISK)

Screening tools are being increasingly used to identify more effectively non-native species that pose an elevated risk of being invasive. Of the available decision-support tools, the Fish Invasiveness Screening Kit (FISK) has been widely used, but has recently been replaced by a generic screening tool, the Aquatic Species Invasiveness Screening Kit (AS-ISK), which is applicable to any aquatic species and complies with the minimum requirements for risk tools under the new EC alien invasive species Regulation. With its unique zoogeography and rich native fauna, Turkey is highly susceptible to non-native species’ introductions and translocations. In order to inform non-native species policy and management regarding fishes in Turkey, AS-ISK was used to re-assess species previously screened using FISK and to assess additional non-native and translocated fish species. In this first calibration study of AS-ISK for Turkey, a basic score threshold of 28 was achieved, which reliably distinguished between potentially invasive (high risk) and potentially non-invasive (medium to low risk) fishes. Of the 64 species assessed, only one was ranked as ‘low risk’, 40 were categorised as ‘medium risk’, and the remaining 23 as ‘high risk’ of which five were translocated. Non-native species currently not present in Turkey, but that pose a high risk of being invasive, were Ameiurus melas, Ameiurus nebulosus, Hemiculter leucisculus, Hypophthalmichthys molitrix, Micropterus salmoides, Perccottus glenii, Pimephales promelas; whereas, the highest scoring translocated species were Cyprinus carpio, Esox lucius and Silurus glanis. When the potential effects of climate change on the assessments were considered, risk scores increased for some (sub)tropical fishes of which two are translocated species.

Optimising invasive fish management in the context of invasive species legislation in South Africa

Background: South Africa hosts a large number of non-native freshwater fishes that were introduced for various industries. Many of these species are now listed under the National Environmental Management: Biodiversity Act (NEM:BA) Alien and Invasive Species (AIS) lists and regulations, though the practical options available to conservation agencies to effectively manage these fishes vary greatly among species and regions. Objectives methods: We assessed the history and status of national legislation pertaining to invasive freshwater fishes, and the practical implications of the legislation for managing different species with contrasting distributions, impacts and utilisation value. Results: The smallmouth bass, despite being a potential conflict-generating species, is fairly straightforward to manage based on current legislation. Two species of trout, which remain absent from the NEM:BA AIS lists because of ongoing consultation with stakeholders, continue to be managed in regions like the Western Cape province using existing provincial legislation. To maximise the limited capacity for management within conservation agencies, we proposed a decision-support tool that prioritises invasive fish populations that represent high environmental risk and low potential for conflict with stakeholders. Using three case studies, we demonstrated how the tool can be used to set management goals of ‘eradicate’, ‘manage against impacts and further spread’ and ‘continue to monitor population’ as the most pragmatic solutions given the state of an invasion, its socio-economic impact and the capacity of the responsible agency to act. Conclusion: By choosing a pragmatic management strategy, conservation agencies can maximise the effective deployment of limited resources, while minimising avoidable conflicts with stakeholders.

Widespread and enduring demographic collapse of invasive common carp (Cyprinus carpio) in the Upper Mississippi River System

Populations of invasive species that undergo rapid expansions after establishment in a new system can also be subject to collapse. Although the dynamics of the establishment and expansion phases and their ecological effects are well documented, substantially fewer studies document collapses despite their importance for understanding invasion dynamics. Two long-term fish monitoring programs sample the fish assemblage of the Upper Mississippi River System. These data provide an opportunity to document the collapse of common carp (Cyprinus carpio), a globally invasive freshwater fish species. Here we describe their population trajectory over several decades and examine several hypotheses to explain the decline, including: boom-bust population dynamics; suppression by native predators; resource exhaustion; improvements in environmental conditions; and disease. The observed trends appear to be most consistent with the hypothesis that disease was the most important factor contributing to the collapse. In particular, cyprinid herpesviruses have been shown to affect common carp in a manner consistent with the observed decreases in catch rates and increases in size distributions. The apparent role of a viral agent in causing the decline of common carp across one of the largest river basins in North America suggests similar collapses may occur elsewhere.

Increasing agricultural land use is associated with the spread of an invasive fish (Gambusia affinis)

Land-use change and invasive species pose major threats to ecosystems globally. These stressors can act together, with disturbance due to changes in land-use facilitating invasion. We examined the potential for agricultural land use to facilitate the establishment and population growth (abundance) of a globally invasive fish (Gambusia affinis). To achieve this we examined Gambusia presence, abundance, and life history traits in 31 streams spanning an agricultural land use gradient in the North Island of New Zealand. We used regression models to quantify the relationship between agricultural land use and in-stream physiochemical and habitat variables, and zero-inflated models to explore the relationship among physiochemical, habitat and catchment-scale variables and Gambusia's distribution and abundance. The percentage of the catchment in agricultural land use was associated with changes to physiochemical and habitat conditions. Increasing agricultural land use was associated with increasing macrophyte cover and water temperature and decreasing velocity in streams. Catchment-scale variables (land use and site position in the network) and water temperature were the most important determinants of whether Gambusia occurred at a site. Local in-stream habitat (macrophyte cover and water velocity) and nutrient conditions were the most influential predictors of Gambusia abundance given Gambusia were present. Gambusia life-history traits, sex ratio and body length varied among sites but were not predicted by physiochemical gradients. The distribution of Gambusia in streams in New Zealand is partially controlled by catchment-scale conditions via a combination of dispersal limitation and environmental filtering, both of which are affected by agricultural land use. Agricultural land use alters local in-stream conditions, resulting in systems that are similar to those in Gambusia's natural range; these altered systems have the potential to support an increased abundance of Gambusia. This study provides preliminary quantitative evidence that agricultural land use is related to the spread of a globally invasive freshwater fish.

An approach to incorporate individual personality in modeling fish dispersal across in‐stream barriers

Animal personalities are an important factor that affects the dispersal of animals. In the context of aquatic species, dispersal modeling needs to consider that most freshwater ecosystems are highly fragmented by barriers reducing longitudinal connectivity. Previous research has incorporated such barriers into dispersal models under the neutral assumption that all migrating animals attempt to ascend at all times. Modeling dispersal of animals that do not perform trophic or reproductive migrations will be more realistic if it includes assumptions of which individuals attempt to overcome a barrier. We aimed to introduce personality into predictive modeling of whether a nonmigratory invasive freshwater fish (the round goby, Neogobius melanostomus) will disperse across an in‐stream barrier. To that end, we experimentally assayed the personalities of 259 individuals from invasion fronts and established round goby populations. Based on the population differences in boldness, asociability, and activity, we defined a priori thresholds with bolder, more asocial, and more active individuals having a higher likelihood of ascent. We then combined the personality thresholds with swimming speed data from the literature and in situ measurements of flow velocities in the barrier. The resulting binary logistic regression model revealed probabilities of crossing a barrier which depended not only on water flow and fish swimming speed but also on animal personalities. We conclude that risk assessment through predictive dispersal modeling across fragmented landscapes can be advanced by including personality traits as parameters. The inclusion of behavior into modeling the spread of invasive species can help to improve the accuracy of risk assessments.

A risk assessment for the introduction of invasive fish for Kejimkujik National Park and National Historic Site, Canada

Recent research has highlighted the need for invasive species risk assessments that consider multiple factors, such as habitat suitability and life history. With invasive species encroaching on the boundary of Kejimkujik National Park and National Historic Site (Kejimkujik), Canada, there is concern surrounding the potential ecological effects on the Park’s freshwater ecosystems. Herein we present a multifactor risk analysis for the risks of introduction of two invasive species, smallmouth bass and chain pickerel. Methods included computer-based analysis of cost–distance and habitat suitability, and a literature-based review of management techniques. Smallmouth bass were found to have a high risk of establishment and consequent ecological effects in Kejimkujik. Although chain pickerel can also negatively affect aquatic communities, their separation from the Park’s primary watershed and low dispersal ability resulted in lower levels of risk. Four recommendations were developed following a review of management techniques: (1) containment of the source population for smallmouth bass; (2) public education, outreach and communication; (3) monitoring and rapid response upon detection; and (4) collaboration with other parties. Although the present study focused on Kejimkujik, similar challenges for invasive freshwater fish assessment and mitigation exist elsewhere, and our methods may prove illustrative for researchers and managers working under similar conditions.

Environmental DNA detection of redfin perch, Perca fluviatilis

Invasive alien species are one of the leading causes of extinctions worldwide. Preventing their establishment, eradicating or containing their spread relies on low-density detection. Environmental DNA (eDNA) has been shown to have superior detection sensitivity compared to traditional methods, permitting detection at lower densities. Here, we develop a species-specific assay to detect Perca fluviatilis (redfin perch), an invasive freshwater fish present in Australia. We show that the assay is highly sensitive and highly specific to detect the species in Australian freshwater environments and demonstrate the utility of the assay for detecting the species from environmental water samples. This assay will prove useful for low-density detection of P. fluviatilis to assist in the management of this invader, with conservation benefits for the native species it threatens.

Female Body Size and Reproduction in Western Mosquitofish (Gambusia affinis) from Two Ponds in Central Ohio

Abstract Gambusia affinis (Western Mosquitofish) is one of the most widespread and invasive freshwater fish. Herein we describe and compare the body size and reproduction of female Western Mosquitofish from 2 small ponds in central Ohio: Wood Duck (WD) and Olde Minnow (OM). This population is one of the most northern populations of Western Mosquitofish to be studied to date. The 2 ponds differed in water temperature (OM > WD), dissolved oxygen (OM » WD), and depth (OM > WD). Body size and body condition of Western Mosquitofish collected was similar in the 2 ponds. Litter size averaged 15.2, increased with female size, and did not differ between ponds. This mean litter size is substantially smaller than that reported for other populations at a similar latitude. Females contained embryos or follicles throughout the study (May–July), but the proportion declined over time, especially in WD. Simultaneous presence of embryos and follicles suggests multiple broods per season, and was more common in OM than in WD...

Global distribution, entry routes, mechanisms and consequences of invasive freshwater fish

生物入侵已经成为全球面临的三大环境问题之一。鱼类入侵现象也随全球经济一体化的进程日益严重。本文综述了全球淡水鱼类入侵的现状和研究进展,包括鱼类入侵的定义及分布、入侵途径和机制、产生的生态和社会经济影响以及预防措施等。据统计,目前全球外来鱼类达624种,该数量超过30年前的两倍。外来鱼类主要通过水产养殖（51%）、观赏渔业（21%）、休闲垂钓（12%）、渔业捕捞运输（7%）等多种途径被引进。入侵鱼类对本地种产生了捕食、种内种间竞争、杂交和疾病传播等负面影响,破坏本地生态系统,但是其正面的生态及社会经济影响也不可忽略。近20年来全球鱼类入侵日益受到重视,相关论文发表数量翻了8倍。值得提出的是,近10年来全球鱼类入侵风险评价系统的研究显著增加,一些鱼类入侵模型已应用于五大洲的多个国家。我国淡水外来鱼类共计439种。然而,我国关于鱼类入侵的研究起步较晚,发表文献数仅占全球的3.7%,且主要研究方向仍集中在入侵物种的分布及生物学特性等基础研究上,缺乏对于鱼类入侵机制及风险评价预测的研究。因此,我们建议：（1）开展全国范围的本底调查并建立数据库,实现数据共享,明确鱼类入侵的历史与分布现状;（2）联合多个政府部门和机构,对鱼类入侵进行长期观测,从整个水生生态系统的角度出发,深入了解其入侵机制及其产生的正面和负面生态和社会经济影响;（3）加强增殖放流的科学研究和管理;（4）构建区域性外来鱼类入侵风险评价系统,有效预测鱼类入侵活动,评价入侵种的危害,并为相关政府部门的决策提供科学依据。

Potential Use of Direct Current Electric Fields to Eradicate Rainbow Trout Embryos from Freshwater Ecosystems

AbstractThe application of electric fields (i.e., electroshocking) may be an option in the eradication of invasive freshwater fishes. Electric fields can be used to facilitate capture or to kill vulnerable life history stages of fish. The objective of this study was to evaluate the relations between water conductivity and the electrical variables of voltage gradient and power density on the mortality of Rainbow Trout Oncorhynchus mykiss embryos at different developmental stages. Embryos in age‐groups ranging from 27 to 405 daily temperature units (TU) were exposed for 5 s to constant (nonpulsed) DC electric fields (2–20 V/cm) at a water conductivity of 220 S/cm to determine (1) the stage of embryonic development most susceptible to electroshock‐induced mortality, and (2) the lethal voltage gradient (V/cm) predicted to kill 50% (LV50) of embryos in each age‐group. Mortality was assessed 24 h postexposure. Using LV50 voltages determined for each age‐group, the relation between water conductivity (20–640 S/cm) and electroshock‐induced mortality from exposure to LV50 was determined for six different stages of embryonic development (81–364 TU). Results show that the mortality of embryos exposed to electroshock decreased with the stage of embryonic development. The mortality of embryos exposed to the LV50 for their respective age‐groups increased with water conductivity and was consistent throughout early development (81–292 TU), but the effects of increased conductivity were not observed in eyed embryos after 364 TU, as mortality remained constant with the determined LV50 values. Voltage was determined to be the most influential factor of embryo mortality. As conductivity increased, maintaining a constant voltage (LV50) elicited greater mortality than maintaining a constant power. These data suggest that a combination of DC electric field and increased localized conductivity could be effective at eradicating nuisance salmonids prior to eyed embryonic stages.Received March 18, 2015; accepted June 3, 2015

Diel Cycle and Effects of Water Flow on Activity and Use of Depth by Common Carp

AbstractCommon Carp Cyprinus carpio is among the most popular and commercially important fishes globally. For this reason, it has been introduced worldwide and is invasive in many regions, with well‐known ecosystems impacts. Like many other freshwater invaders it is thought not to tolerate strong flows well, but knowledge of the effects of flow on their activity, habitat use, and diel cycles are limited, despite this being crucial information for management and control. By means of ultrasonic telemetry we investigated depth use and activity of Common Carp in a small reservoir with a very low water residence time on the main stem of the Ebro River, the largest river by discharge in the Iberian Peninsula, over a 19‐month period. The activity of carp and their use of depth displayed low seasonality compared with abiotic factors. However, carp exhibited diel vertical migration patterns, mostly in the warm season, shifting from deep positions near the reservoir bottom during the night (with decreased activity) to shallow waters during the day. This pattern included extensive use of hypoxic waters (<1.1 mg/L dissolved oxygen) and occurred largely at night. Activity and habitat use also varied among individuals and were significantly related to water flow; carp were less active and used shallower water during increased flows. The individual variability, the behavioral adaptation to refuge from high floods flows, and the extraordinary resistance to hypoxic waters might help explain why Common Carp is one of the most successful invasive freshwater fish species.

Hierarchy formation and hormonal profiles in Australoherus facetus, an invasive freshwater fish in Portugal.

Hierarchy formation and hormonal profiles in Australoherus facetus, an invasive freshwater fish in Portugal.

Alien and invasive freshwater fish species in Iran: Diversity, environmental impacts and management

More than 32 alien (exotic) fish species belonging to 10 orders and 12 families (Cyprinidae, Gobiidae, Salmonidae, Anguilidae, Mugilidae, Centrarchidae, Heteropneustidae, Gasterosteidae, Cichlidae, Poecilidae, Adrianichthyidae and Pleuronectidae) were reported from Iran of which 25 species are confirmed by specimens. Some of the alien fishes (e.g., Cyprinus carpio, Carassius auratus, Pesudorasbora parva, Xiphophorus hellerii and Gambusia holbrooki) have already established breeding populations, acting as invasive species. Some others are regularly stocked by the Iranian Fisheries Company (e.g., Hypophthalmichthys molitrix, Hypophthalmichthys nobilis, Ctenopharyngodon idella) and few others are occasionally recorded from natural waters. Amatitlania nigrofasciata and Tilapia zillii are two alien cichlids recently reported from Iran. Aquaculture, sport fishing, control of malaria, ornamental purposes, research activities, demonstration in national fairs and accidental introduction are the main reasons for these introductions. Pseudarasbora parva was introduced accidentally along with other exotic carps and now is widely distributed in inland waters. The intentional introduction of alien species is an extraordinarily complex issue and requires taking into account a broad number of variables. Understanding the risks that alien species, especially aquatic invasive species (AIS) pose, comparative studies on the ecology and life history strategies of the introduced and invasive species and closely related indigenous species with similar ecological life traits, monitoring the rate of invasion, and the causal relation between population dynamics and species invasion and public awareness, could be effective management strategies to minimize the bio-invasion impacts.

Detection of invasive freshwater fish species using environmental DNA survey

Invasive species are one of the most significant problem in freshwater ecosystems. Most common non-native freshwater species in Turkish freshwater fish fauna are Prussian Carp (Carassius gibelio), North African Catfish (Clarias gariepinus), Nile Tilapia (Oreochromis niloticus) and Topmouth Gudgeon (Pseudorasbora parva).Recent studies showed that environmental DNA could be used to detect target species inhabiting the ecosystem with higher precision and less effort compared to traditional field surveys. In this study, eDNA approach was used to investigate non-native freshwater fish species from fifteen different locations of Upper Sakarya Basin. eDNA was successfully extracted from the water samples of locations where the species were visually observed. Mean amplification rate of eDNA was calculated as 77.03%.This study is the first environmental DNA study used in detection of four of the most common invasive freshwater fish species. Results clearly indicating that eDNA surveys could be used as an important molecular tool to monitor invasive fish species in freshwater ecosystems.

Do invasive freshwater fish species grow better when they are invasive?

A frequent assumption in invasion ecology is that invasive species have enhanced growth rates in their invasive ranges. However, invasions frequently occur in sub‐tropical and tropical environments where growth could be higher simply due to climatic conditions rather than novel habitat. In this study, a meta‐analysis of growth rates (length‐at‐age data) was completed for six invasive freshwater fish species: common carp Cyprinus carpio, largemouth bass Micropterus salmoides, brown trout Salmo trutta, brown bullhead Ictalurus nebulosus, flathead catfish Pylodictis olivaris and northern snakehead Channa argus. Significant effects of climate on growth were observed for all species except common carp, and following normalization of growth for climate effects, a range of growth responses between native and invasive populations were revealed. Two species (brown trout, flathead catfish) showed significantly increased growth rates in invasive compared to native ranges, but two species (common carp, largemouth bass) showed significantly faster growth in native ranges, and two other species (northern snakehead, brown bullhead) showed no difference in growth rates. No species showed both significantly enhanced growth rates and initial sizes in invasive compared to native ranges. Using the comparative method, countergradient growth variations were apparent for all species within their native ranges and for all but one species in invasive ranges. Invasive populations of freshwater fish do not always grow faster when invasive and future studies need to consider growth covariates (e.g. climate and countergradient growth) prior to comparing life‐history differences between invasive and native populations.

High prevalence of the parasite Sphaerothecum destruens in the invasive topmouth gudgeon Pseudorasbora parva in the Netherlands, a potential threat to native freshwater fish

The prevalence of Sphaerothecum destruens, a pathogenic parasite, was studied in two wild populations of topmouth gudgeon (Pseudorasbora parva), an invasive freshwater fish non-native to the Netherlands. Using genetic markers and sequencing of the 18S rRNA gene, we showed the prevalence of this parasite to be 67 to 74%. Phylogenetic analysis demonstrated a high similarity with known sequences of S. destruens. The topmouth gudgeon, which functions as a healthy carrier of the pathogen, is rapidly colonizing the Netherlands, its expansion showing no signs of saturation yet. Both the presence of S. destruens and the rapid dispersal of the topmouth gudgeon are considered to constitute a high risk for native freshwater fish.