In 2022, the 22nd International Conference on Aquatic Invasive Species returned to Europe as a hybrid event. The conference welcomed representatives from 41 countries, including the largest group of students and early career professionals of any ICAIS conference. The theme of the conference was “Global Climate Change Amplifies Aquatic Invasive Species Impacts.” Keynote speakers discussed ongoing invasions and the damaging synergy between climate change and invasions, presented on the value of risk assessment, outreach, and education, highlighted new ways to estimate the economic costs of invasions, and told attendees about some of the work of NGOs in managing invasions. This special issue includes a selection of papers that were presented at the conference, along with a few related papers that were not presented which touch on the risk of specific vectors, improvements in survey techniques to detect new or spreading invaders, and advances in management approaches to control AIS.

The American bullfrog (Lithobates catesbeianus [Shaw, 1802]) is one of the hundred most destructive Alien Invasive Species (AIS) worldwide that has invaded more than 40 countries across 4 continents. In Belgium, bullfrogs have occupied a large area in a relatively short period of time despite a decade of intensive management interventions. Acquiring better insights into the distribution, abundance, and spatial spread of this invasive frog species is an important first step towards a successful management strategy. In this study, we sampled 382 permanent water bodies and combined environmental DNA (eDNA)-based analyses using quantitative droplet digital PCR (ddPCR) with assessments of habitat characteristics to generate an overview of the present distribution of bullfrogs in Flanders (Northern Region of Belgium) and the type of water bodies they have invaded. Our results revealed a fragmented distribution pattern covering an area of 364.76 km² that consisted of eight metapopulations located in five different river valleys, suggesting the occurrence of multiple anthropogenically-mediated introductions. Bullfrogs appeared to be firmly established in the valley of the Grote Nete, where invaded waterbodies have been found along 72% of the length of this river, divided into three distinct metapopulations. Unlike refuge sites, bullfrogs were found to be highly selective in their choice of breeding sites, which were characterised by abundant emergent vegetation and sparse tree cover along the shoreline. The division of the vast occupied area into well-defined, accurately delineated metapopulations facilitates the identification of functional management units. Furthermore, the obtained knowledge of the patterns of range expansions and the spatial configuration and associated environmental features of breeding sites can be used to prioritise management interventions in strategically located invasion hubs. Overall, we conclude that eDNA-based monitoring combined with environmental assessments provide important information that can be used to manage widespread aquatic AIS more effectively.

The present study aimed to investigate the role of multispectral images from Sentinel-2 satellites to map and monitor in the Mediterranean island of Sardinia (Italy), from 2018 to 2021, Pontederia crassipes and Hydrocotyle ranunculoides, two aquatic macrophytes included in the list of invasive alien species of European Union concern. The vegetation index NDVI was used to assess the level of vigor of these species and the area of invasion on waterways to understand their expansion over time. At the same time, the correlation between recorded monthly mean air temperatures and NDVI values was calculated. The coverage, during this 4-year-study, affected almost entirely the study area, with high coverage values in the summer and autumn periods. This study demonstrates the usefulness of Sentinel-2 satellite imagery in mapping and monitoring invasive alien macrophytes. In fact, large areas can be monitored and mapped quickly in a short time and have measurements every five days due to the images released by European Space Agency (ESA), unless there is excessive cloud covering. In combination with GIS and modelling tools, the tested multitemporal methodology could represent a suitable tool for prevention, drafting of dedicated action plans, priority setting for intervention both for P. crassipes and H. ranunculoides to meet the requirements of the Regulation (EU) No. 1143/2014 on Invasive Alien Species (the IAS Regulation).

Clogmia albipunctata (Williston, 1893) is a moth fly (Diptera: Psychodidae) which occurs in synanthropic habitats in tropical and subtropical climates all over the world, particularly drains and kitchen sinks which has earned it the moniker “drain fly”. Since the 1990s, however, its range has expanded rapidly northwards into temperate climates and it now occurs as far North as Karelia (Russia) and Finland. As it occurs together with humans and can be detected easily from photographs it is a species whose invasion can be tracked easily; inspection of Norwegian, Swedish and Danish nature photography web sites www.artsobservasjoner.no, www.artportalen.se and www.naturbasen.dk reveal its established presence in Denmark and Sweden, whereas no records hitherto have been presented from Norway. The records are all from large metropolitan areas, except one Danish record which is on the Southern border with Germany where the species has been established for a much longer time. The oldest Swedish images are from 2016 and the first Danish specimens appear in 2018. It remains unknown to which degree C. albipunctata poses a risk to native biodiversity, but records from Slovakia in 2012 revealed that it can breed in habitats used by native species also in temperate Europe.

Marinas are known to act as species reservoirs, facilitating the persistence and spread of non-native species. Monitoring of marinas for non-native species is therefore an essential element of any integrated monitoring programme. Baseline and updated survey data are needed to understand the abundance, distribution and rate of spread of non-native species within marinas over time in order to support management strategies, develop applicable biosecurity measures and inform rapid response plans. Rapid assessment surveys (RAS) were employed to survey floating pontoons, as well as opportunistically survey intertidal structures, for non-native biofouling species in 22 marinas across the Republic of Ireland. A total of 25 non-native species were recorded: 19 invertebrates and 6 macroalgae, with ascidians being the most represented group. Four non-native species not previously reported in published literature from the Republic of Ireland were recorded: devil’s tongue weed, Grateloupia turuturu Yamada, 1941, the compass sea squirt, Asterocarpa humilis (Heller, 1878), the San Diego sea squirt, Botrylloides diegensis Ritter & Forsyth, 1917 and the branching bryozoan, Bugulina simplex (Hincks, 1886). Other species identified as part of this study and previously reported from Ireland were shown to have increased their range.

Wildlife and natural resource institutions play key roles in invasive species monitoring and management. Paradoxically, the extensive fieldwork undertaken by these institutions and their partners may result in the inadvertent movement and spread of invasive species within and between sensitive ecosystems. In this work, we consider the potential effects of internal guidelines and policies designed to prevent the spread of invasive species by the field activities of management institutions and their partners. Such policies could be high-leverage tools for invasive species prevention, however, as large, complex organizations seek to implement policies to limit the spread of invasive species, it may be challenging to accommodate the wide diversity of potential invasion pathways and mitigation efforts facilitated via the programs, activities, and ecosystems they manage. Prevention policies may also be met with resistance due to the costs of implementation unless concrete benefits can be demonstrated. Assessing and communicating the effects of prevention policies could motivate improved implementation and adherence by institutional units and partners and could help inform adaptive policy changes. However, assessing the effectiveness of invasive species prevention presents a unique set of challenges, including incomplete data on invasive species distribution and pathways, that make it difficult to measure the effects of prevention efforts. In this work, we present a conceptual framework for evaluating institutional policies for invasive species prevention. We describe a flexible, multifaceted approach that considers policy implementation and adherence as well as ecological outcomes. We discuss potential application of this framework using a policy recently implemented by the Pacific Region of the U.S. Fish and Wildlife Service to prevent the introduction and spread of invasive species by service personnel and partners during field activities as a case study.

The Buddhist practice of life release is a known pathway of invasive species within the United States. However, there are no established outreach programs designed to address this pathway within the US. To facilitate the process of creating an outreach program, we surveyed members of the regional panels on aquatic nuisance species to determine their opinions on a series of potential life release practices that could both meet the cultural need of the practice and reduce invasion risk. Aquatic invasive species managers generally agreed that some practices were acceptable, including working with a wildlife rehabilitation center to release recovered wildlife and working with a wild bait harvester to release bait into the waterbody from which it was harvested. We hope that by identifying various life release practices and the associated opinions about these practices, aquatic invasive species managers can facilitate low risk methods of practicing life release.

Zebra mussels (Dreissena polymorpha) are an aquatic invasive species that cause extensive economic and ecological impacts and are a management priority in areas outside of their native range. Survivorship and distribution of zebra mussels within a waterbody are thought to be influenced by temperature and dissolved oxygen conditions, but detailed information to confirm the importance of these environmental controls is necessary to inform management efforts. We measured planktonic zebra mussel veliger density and adult survivorship in San Justo Reservoir in central California to determine distribution and timing of spawning in relation to temperature and dissolved oxygen throughout winter, spring, and summer. We found seasonal patterns in adult survivorship, with high mortality late in the summer and higher than expected survivorship during the spring when dissolved oxygen concentrations were approximately 1 mg/L. Veliger abundance peaked several meters above the thermocline from June to August. Dissolved oxygen concentrations limited veliger distribution, with few to no veligers collected in the anoxic hypolimnion. Veliger settlement out of the water column appears to be possible in San Justo Reservoir at any time of year. A better understanding of how veligers, juveniles, and adult mussels respond to fluctuating dissolved oxygen and temperature conditions will further knowledge of timing and duration of water drawdowns or other control methods for managing this harmful invasive species.

Introductions of invasive non-native species often have severe effects on environments and can lead to the collapse of local populations. Freshwater systems are highly biodiverse habitats that are particularly sensitive to the introduction of non-native species, which is recognized as one of the leading causes of animal extinctions worldwide. Effective management firstly depends on the early detection of incipient invasions and subsequent rapid response. Here, we focus on the cryptic and difficult-to-detect amphipod, Crangonyx pseudogracilis, which is already established in several countries in Europe, and C. floridanus, which was recently found in the UK and Ireland. Their exact distribution is relatively unknown, and their morphological similarities to each other makes them taxonomically difficult to distinguish. To obtain better insights on the actual distribution of both species, we developed and validated species-specific assays for the detection of environmental DNA traces from both C. floridanus and C. pseudogracilis. Next, we analysed a large number of eDNA samples collected from several freshwater systems in Ireland where the presence of both alien species is known, and from unknown sites in Belgium, Netherlands, France, and Norway (countries where only C. pseudogracilis has previously been recorded). Using qPCR analyses, we successfully detected eDNA from both C. pseudogracilis and C. floridanus in the known locations in Ireland. Sites investigated in Belgium, Netherlands, France, and Norway were negative for the presence of both species. The qPCR based detection of freshwater amphipods from aquatic eDNA is a cost-effective and sensitive method to monitor establishment and spread of invasive species as well as species of conservation priority. Based on our results, we recommend that eDNA surveys can be used as a powerful tool for stakeholders, including ecologists, especially in at-risk areas to detect potential early invasion of these species.

The threats posed by invasive freshwater crayfish species are one of the greatest concerns for freshwater ecologists, environmental managers, policy makers and local communities in the invaded regions. The invasive Australian redclaw crayfish Cherax quadricarinatus is rapidly spreading in southern Africa. Fishers in the Zambezi Basin have reported that crayfish affect their catch through partial consumption of fish caught on static gillnets. Owing to the immeasurable contributions of fishery to socioeconomic livelihoods in Africa, the catch losses that are due to crayfish are of concern. With this problem in Africa, it is imperative to continue researching on adaptive strategies to cope with the invasion. This study tested the effectiveness of “misdirection traps” to prevent fish spoilage and gear damage for gillnet catches by crayfish in Lake Kariba, Zimbabwe. The method involved placing baited traps along a gillnet to misdirect crayfish into the traps rather than fishing gear to prevent fish spoilage by scavenging. The misdirection traps significantly lowered the amount of catch spoilage. Gillnet damage by crayfish in general was minimal, although it was higher for gillnets without baited traps. The misdirection traps pose a pragmatic management initiative to reduce socioeconomic fishery costs from crayfish. We consider this method to be applicable in all the invaded regions where artisanal fishers are experiencing catch spoilage problems making a huge step towards preventing socioeconomic impacts associated with crayfish invasion.

A solid formulation of the pesticide TFM (4-nitro-3-(trifluoromethyl)-phenol) was developed in the 1980s for application in small tributaries during treatments to control invasive sea lamprey (Petromyzon marinus Linnaeus, 1758). Several initial inert ingredients were discontinued and substituted, culminating with an interim formulation that unacceptably softens and rapidly decays in warm conditions. A new TFM bar formulation was developed to resolve poor thermal stability and it was registered with the U.S. Environmental Protection Agency and Health Canada Pesticide Management Regulatory Agency in 2020. Laboratory studies compared the thermostability and dissolution (i.e., TFM release) of the interim and new formulation of TFM bars that were held at 20 °C or 45 °C for 24 hours prior to evaluation. Field tests compared the dissolution of the interim and new formulation of TFM bars when applied in three small tributaries in Michigan. Laboratory tests show that the new formulation bars remain usable when held at 45 °C for 24 hours; whereas, the interim formulation bars partially liquify and are not usable. Field tests indicate the new formulation bars have superior characteristics including a near consistent release of TFM for 10–13 hours when applied in waters with a velocity of < 0.06 m/sec. A near consistent release of TFM was observed for a maximum of about 6 hours in one field application of the interim formulation bars. Water temperature and water velocity influenced both formulations; however, the greatest effects were observed with interim formulation bars where higher initial TFM concentrations were followed by precipitous TFM concentration decreases in tributaries with the highest water temperature or velocity. Field treatment applications will provide data for refining application parameters such as the number of bars required per unit discharge at various water temperatures and the acceptable water velocity range for applications.

Bigheaded carp Hypophthalmichthys spp. are invasive species native to Asia expanding in the Mississippi River Basin in North America. An understanding of spatiotemporal distribution and aggregation of invasive carp is key to establishing when and where to focus surveillance designed to monitor expansion, and to managing harvest programs designed to curb population densities. We applied a two-stage hurdle model to assess three aspects of bigheaded carp ecology: distribution, relative abundance, and aggregation. Stage 1 was a binary 0/1 model that represented fish presence (p), and stage 2 was a truncated count distribution that had no zeros and included counts ≥ 1 only (C). Estimates of p and C varied temporally and spatially, but not in harmony and sometimes in opposing directions, indicating temporal and spatial swings in fish distributions and aggregations. Intense fish aggregations in channels in spring shown by low p’s and high C’s, eventually scattered by summer and fall as shown by high p’s and low C’s. An alternative but complementary interpretation of our observations is that p indexes incidence of aggregations and C indexes size of aggregations. Partitioning catch into its zero and nonzero components provided insight into population ecology that can inform development of monitoring and management of harvesting programs targeted at lessening potential effects of the invasion.