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The intrusion of non-native species into freshwater ecosystems can lead to rapid shifts in fish community dynamics, posing significant threats to biodiversity and ecological stability. In this study, we examined the changes in the fish community within the Kerkini Reservoir, a Ramsar Site in Greece, influenced by the establishment of invasive fish species. Through standardized seasonal surveys conducted in 2007–2008 and 2023, we assessed changes in fish species composition, abundance, and functional traits. Our findings revealed a shift in dominance, with invasive species such as Gymnocephalus cernua emerging as key players in the reservoir's fish fauna. Moreover, high numbers of fish specimens and biomass were recorded, highlighting the reservoir’s high productivity. However, concerns arise regarding the ecological impacts of invasive species, particularly G. cernua , which exhibits traits associated with ecological disruption and competition with native species. A shift towards the dominance of species with shorter lifespans, smaller sizes and lower trophic levels was observed. In addition, the observed decline in native species abundance, coupled with poor ecological quality assessments, underscores the need for proactive management measures to preserve ecosystem integrity. This study contributes valuable insights into the ecological dynamics of a system considered a biodiversity hotspot. It also highlights the proliferation of invasive fish species as a primary driver and underscores the importance of ongoing monitoring and conservation efforts. However, future research is needed, focusing on further understanding the long-term effects of invasive species establishment and their implications for native fish populations and ecosystem functioning.

Aim Formerly introduced for their presumed value in controlling mosquito‐borne diseases, the two mosquitofish Gambusia affinis and G. holbrooki (Poeciliidae) are now among the world's most widespread invasive alien species, negatively impacting aquatic ecosystems around the world. These inconspicuous freshwater fish are, once their presence is noticed, difficult to eradicate. It is, therefore, of utmost importance to assess their geographic potential and to identify their likely ability to persist under novel climatic conditions. Location Global. Methods We build species distribution models using occurrence data from the native and introduced distribution ranges to identify putative niche shifts and further ascertain the areas climatically suitable for the establishment and possible spread of mosquitofish. Results We found significant niche expansions into climatic regions outside their natural climatic conditions, emphasizing the importance of integrating climatic niches of both native and invasive ranges into projections. In particular, there was a marked shift toward tropical regions in Asia and a clear niche shift of European G. holbrooki. This ecological flexibility partly explains the massive success of the two species, and substantially increases the risk for further range expansion. We also showed that the potential for additional expansion resulting from climate change is enormous—especially in Europe. Main conclusions Despite the successful invasion history and ongoing range expansions, many countries still lack proper preventive measures. Thus, we urge policy makers to carefully evaluate the risk both mosquitofish pose to a particular area and to initiate appropriate management strategies. The study aimed to investigate the invasion success of one of the most widespread freshwater fish in the world: the closely related mosquitofish (Gambusia affinis and G. holbrooki). For this, we first explored potential niche shifts in the invaded regions and then provide risk assessments for future range expansion in the course of climate change.

To avoid the spread and environmental impacts of the European catfish (Silurus glanis), which arrived in Portugal around 2006, it is fundamental to understand anglers’ behaviors and perceptions. To address this need, 339 Portuguese anglers were surveyed online and face-to-face and divided into two groups (catfish anglers, n = 115; and non-catfish anglers, n = 224). The majority of surveyed anglers were males (>95%) who consider that catfish is not beneficial to the development of angling (>62%) and acknowledge its negative impact on other fish species (>84%), mainly by predation (>89%). Catfish anglers perceive the species as more positive for angling and for other fish species. Principal Component Analysis showed that anglers’ behaviors are mainly driven by their awareness of the impacts of European catfish and by their angling experience. Regression modeling inferred the probabilities of two anglers’ behavior (targeting catfish and practice of catch and release) and one perception (catfish is beneficial to angling), which are considered promoters of catfish invasion, and detected strong correlations among these three variables. Therefore, it is strongly advised to implement an educational and law enforcement program to stop illegal practices that promote the dissemination of invasive fish species like European catfish.

Wetlands are of great importance for biodiversity and nature conservation, especially geographically isolated wetlands (GIW). Yet literature about the ecological value of such GIW is missing, especially at the edge of the distribution of endangered species such as amphibians. In 2018 and 2022, we monitored amphibian communities in 15 isolated (GIW) and 12 non-isolated (nGIW) ponds by counting individuals using three methods: (1) capturing with hand nets, (2) visual counting, and (3) capturing with fyke traps. The three methods provided similar results, showing the great importance of GIW for amphibians, especially newts, whose abundance was 5–13 times greater in GIW compared to nGIW. The largest numbers of species and individuals (adults and larvae) were found in isolated wetlands (GIW). In non-isolated water bodies (nGIW) where more than 10 individuals of the Chinese sleeper Perccottus glenii, an alien invasive fish, were found, amphibians were not found at all. Importantly, between 2018 and 2022, the northern crested newt, T. cristatus, dramatically decreased in the nGIW. As a result of our work, it was revealed that the reconstruction of geographically isolated wetlands is very important for the conservation of amphibian biodiversity in a changing climate. One of the most effective measures aimed at protecting amphibians from negative factors—the spread of alien invasive fish species and diseases—is the reconstruction of wetlands in historically exploited landscapes with the creation of a wide range of water bodies yet broadly dominated by geographically isolated ponds.

Lake Dianchi is the largest freshwater lake on the Yunnan Plateau and the sixth largest one in China. In recent decades, a number of non-native fish species have been introduced into the lake intentionally or accidentally while human-accelerated eutrophication has been occurring. In this study, we provided a whole-lake assessment of species composition and abundance of the accidentally-introduced fishes, and described the spatial variability of invasive small fish community in relation to eight physicochemical environmental factors. Fish were sampled quantitatively at 30 sites throughout the lake with multiple mesh-sized gillnets in the autumn of 2008. A total of 2,050 individuals from seven non-native small fish species were captured. Thin sharpbelly Toxabramis swinhonis and piper halfbeak Hyporhamphus intermedius were recognized as dominant species, followed by barcheek goby Rhinogobius giurinus, topmouth gudgeon Pseudorasbora parva and redfin culter Culterichthys erythropterus, according to their abundance and occurrence. Principal component analysis (PCA) suggested that the fish community structure in the northern part was distinct from those in the middle and southern parts of the lake, corresponding to the increase of lake eutrophic level from south to north. Redundancy analysis (RDA) showed that redfin culter, piper halfbeak and thin sharpbelly were positively related to nutrient variables (total phosphorus, total nitrogen), while topmouth gudgeon and barcheek goby were associated with relatively higher secchi depth and lower nutrition level. Water depth seemed to have moderately negative influence on spatial distribution of all the fish species. Regression tree analysis was applied to visualize decision rules for the prediction of fish abundances (CPUE, ind/fishing pass/h) from key environment variables, and explained 51.5–69.0 % of the total variance of response values. The details of these findings are beneficial to understanding the establishment of invasive fish in degraded environment, and to developing suitable conservation strategies for the restoration and management of fish resources in Lake Dianchi as well as the other lakes of Yunnan Plateau.

Invasive alien species have become the second major threat to biodiversity affecting all three major ecosystems (terrestrial, marine, and freshwater). Increasing drivers such as habitat destruction, expanding horticulture and aquaculture industries, and global pet and food trade have created pathways for exotic species to be introduced leading to severe impacts on recipient ecosystems. Although relatively less studied than terrestrial ecosystems, freshwater ecosystems are highly susceptible to biological invasions. In India, there has been a noticeable increase in the introduction of alien fish species in freshwater environments. In the current study, we aimed to understand how climate change can affect the dynamics of the biological invasion of invasive alien fishes in India. We also evaluated the river-linking project’s impact on the homogenization of biota in Indian freshwater bodies. We used species occurrence records with selected environmental variables to assess vulnerable locations for current and future biological invasion using species distribution models. Our study has identified and mapped the vulnerable regions to invasion in India. Our research indicates that the interlinking of rivers connects susceptible regions housing endangered fish species with invasive hotspots. Invasive alien fishes from the source basin may invade vulnerable basins and compete with the native species. Based on the results, we discuss some of the key areas for the management of these invasive alien species in the freshwater ecosystems.

Invasive alien species (IAS) influence the trophic organisation and food web structure in an invaded ecosystem, and therefore, it is imperative to quantify the resultant ecological impacts. The globally recognised ecosystem modelling platform, Ecopath with Ecosim, was used to delineate the impacts of IAS on a tropical freshwater pond ecosystem in India. We analysed the trophic interactions, consumption patterns, prey overlap and mixed trophic impacts of three co-existing invasive alien fish species, African catfish ( Clarias gariepinus ), suckermouth catfish ( Pterygoplichthys pardalis ) and Mozambique tilapia ( Oreochromis mossambicus ), on other functional groups in the ecosystem. Together, the three IAS shared 11% of the total energy consumption and about 50% of the energy consumption by the fish species/groups. There was no predation mortality for African catfish and suckermouth catfish, and a very low estimate for the same was recorded for tilapia (0.64 year −1 ). The IAS shared high mean prey overlap with the native fish groups ( Garra sp., Etroplus suratensis , Systomus sarana , Chanda nama and various small species of the cypriniform genera Puntius , Rasbora and Devario ) indicating a substantial competition between alien and native species in the ecosystem. Consequently, the three IAS exhibited higher mean negative mixed trophic impacts on these functional groups. A very high Finn’s cycling index (39.59%), a low relative ascendency (28.5%) and a very low system robustness (0.07) were observed compared to similar ecosystems, and the baseline values. These indices exposed the vulnerability of the ecosystem towards perturbations, which could be due to the presence of multiple alien invasive species. Mitigating the impacts of IAS should involve a combination of approaches, including eradication through draining and harvesting, high-density stocking of similar trophic level fish in the pond, and local and national level policy interventions.

Topmouth gudgeon ( Pseudorasbora parva ) is one of the most invasive aquatic fish species in Europe and causes adverse effects to ecosystem structure and functioning. Knowledge and understanding of the species’ interactions with the environment and with native fish are important to stop and prevent the further spread of the species. Creating species distribution models is a useful technique to determine which factors influence the occurrence and abundance of a species. We applied three different modelling techniques: general additive models, random forests and fuzzy habitat suitability modelling (FHSM) to assess the habitat suitability of topmouth gudgeon. The former two techniques indicated that the abundance of native fish (i.e. biotic variables) was more important than environmental variables when determining the abundance of topmouth gudgeon in Flanders (Belgium). Bitterling ( Rhodeus amarus ), stone loach ( Barbatula barbatula ), three-spined stickleback ( Gasterosteus aculeatus ) and predator abundance were selected as the most important biotic variables and implemented in the FHSM to investigate species interactions. Depending on the preferred food source and spawning behaviour, either coexistence or interspecific competition can occur with bitterling, stone loach and three-spined stickleback. In contrast, the presence of predators clearly had a top down effect on topmouth gudgeon abundance. These findings could be applied as a biological control measure and implemented in conservation strategies in order to reduce the abundance of earlier established populations of topmouth gudgeon.

Globally, introductions of alien species are increasingly common, with invasive predators potentially having detrimental effects via predation on native species. However, native prey may avoid predation by adopting new behaviors. To determine whether invasive fish populations consume endemic shrimp in invaded Hawaiian anchialine habitats or if adopted patterns of diel migration prevents predation as previously hypothesized, a total of 183 invasive poeciliids (158 Gambusia affinis and 25 Poecilia reticulata ) were collected for gut content analyses from four anchialine sites during wet and dry seasons on the islands of Hawai‘i and Maui. Predation on shrimp was not detected in habitats where they retreat exclusively into the underlying aquifer diurnally and only emerge nocturnally. However, low levels of predation were detected (7/65 fishes, only by Gambusia affinis ) at Waianapanapa Cave, Maui, where shrimp retreat into both the aquifer and a cave during the day. Thus, adopted behavioral responses to invasive fishes generally, though not universally, prevent predation on endemic Hawaiian anchialine shrimps. However, non-consumptive effects resulting from behavioral modification of shrimps may have appreciable impacts on the Hawaiian anchialine ecosystem and warrant further study.

Although freshwater invasions have not been targeted for maintenance management or eradication as often as terrestrial invasions have, attempts to do so are frequent. Failures as well as successes abound, but several methods have been improved and new approaches are on the horizon. Many freshwater fish and plant invaders have been eliminated, especially by chemical and physical methods for fishes and herbicides for plants. Efforts to maintain invasive freshwater fishes at low levels have sometimes succeeded, although continuing the effort has proven challenging. By contrast, successful maintenance management of invasive freshwater plants is uncommon, although populations of several species have been managed by biological control. Invasive crayfish populations have rarely been controlled for long. Marine invasions have proven far less tractable than those in fresh water, with a few striking eradications of species detected before they had spread widely, and no marine invasions have been substantially managed for long at low levels. The rapid development of technologies based on genetics has engendered excitement about possibly eradicating or controlling terrestrial invaders, and such technologies may also prove useful for certain aquatic invaders. Methods of particular interest, alone or in various combinations, are gene-silencing, RNA-guided gene drives, and the use of transgenes.