The Use of Barriers to Limit the Spread of Aquatic Invasive Animal Species: A Global Review

Aquatic invasive species (AIS) are one of the principal threats to freshwater biodiversity. Exclusion barriers are increasingly being used as a management strategy to control the spread of AIS. However, exclusion barriers can also impact native organisms and their effectiveness is likely to be context dependent. We conducted a quantitative literature review to evaluate the use of barriers to control animal AIS in freshwater ecosystems worldwide. The quantitative aspect of the review was supplemented by case studies that describe some of the challenges, successes, and opportunities for the use of the use of AIS exclusion barriers globally. Barriers have been used since the 1950s to control the spread of AIS, but effort has been increasing since 2005 (80% of studies) and an increasingly diverse range of AIS taxa are now targeted in a wide range of habitat types. The global use of AIS barriers has been concentrated in North America (74% of studies), Australasia (11%), and Europe (10%). Physical barriers (e.g., weirs, exclusion screens, and velocity barriers) have been most widely used (47%), followed by electric (27%) and chemical barriers (12%). Fish were the most targeted taxa (86%), followed by crustaceans (10%), molluscs (3%) and amphibians (1%). Most studies have been moderately successful in limiting the passage of AIS, with 86% of the barriers tested deterring >70% of individuals. However, only 25% of studies evaluated barrier impacts on native species, and development of selective passage is still in its infancy. Most studies have been too short (47% < 1 year, 87% < 5 years) to detect ecological impacts or have failed to use robust before-after-control-impact (BACI) study designs (only 5%). Hence, more effective monitoring is required to assess the long-term effectiveness of exclusion barriers as an AIS management tool. Our global case studies highlight the pressing need for AIS control in many ecoregions, and exclusion barriers have the potential to become an effective tool in some situations. However, the design and operation of exclusion barriers must be refined to deliver selective passage of native fauna, and exclusion barriers should only be used sparingly as part of a wider integrated management strategy

Phenotypic Convergence in Sea Bass (Dicentrarchus labrax) Escaping From Fish Farms: The Onset of Feralization?

The impact of fish escaping from fish farms may depend on the extent to which escapees adapt to the natural environment, resemble wild conspecifics, and become feral. Yet, little is known about the process of feralization in marine fish. We examined phenotypic changes in body shape, body condition, and scale growth profiles of sea bass escaping from fish farms in the Canary Islands and quantified the extent to which escapees had diverged from farmed conspecifics. Most feral sea bass had sizes that overlapped with those of farmed fish, indicating that they had escaped throughout the production cycle. However, 29% of escapees were larger than the maximum size at harvesting, indicating growth in the wild. Analysis of scale growth profiles showed that some escapees had grown in the wild as fast as cultured fish, albeit at more variable growth rates. Feral sea bass tended to converge towards a similar body shape, having more streamlined bodies, lower body condition, and lower hepatosomatic indices (HSI) than fish in cages. Although our study cannot discriminate between phenotypic plasticity and differential mortality of escapees, we interpret phenotypic convergence as the likely result of a period of initial starvation, phenotypic plasticity, and selection against maladapted phenotypes. Our results warn against the risks of rearing sea bass in open-net cages and suggest that sea bass escapees could pose a threat to shallow coastal assemblages, particularly in areas where the species is not naturally found

Impacts of existing and planned hydropower dams on river fragmentation in the Balkan Region

The Balkan region has some of the best conserved rivers in Europe, but is also the location of ~3000 planned hydropower dams that are expected to help decarbonise energy production. A conflict between policies that promote renewable hydropower and those that prioritise river conservation has ensued, which can only be resolved with the help of reliable information. Using ground-truthed barrier data, we analysed the extent of current longitudinal river fragmentation in the Balkan region and simulated nine dam construction scenarios that varied depending on the number, location and size of the planned dams. Balkan rivers are currently fragmented by 83,017 barriers and have an average barrier density of 0.33 barriers/km after correcting for barrier underreporting; this is 2.2 times lower than the mean barrier density found across Europe and serves to highlight the relatively unfragmented nature of these rivers. However, our analysis shows that all simulated dam construction scenarios would result in a significant loss of connectivity compared to existing conditions. The largest loss of connectivity (−47 %), measured as reduction in barrier-free length, would occur if all planned dams were built, 20 % of which would impact on protected areas. The smallest loss of connectivity (−8 %) would result if only large dams (>10 MW) were built. In contrast, building only small dams (<10 MW) would cause a 45 % loss of connectivity while only contributing 32 % to future hydropower capacity. Hence, the construction of many small hydropower plants will cause a disproportionately large increase in fragmentation that will not be accompanied by a corresponding increase in hydropower. At present, hydropower development in the Balkan rivers does not require Strategic Environmental Assessment, and does not consider cumulative impacts. We encourage planners and policy makers to explicitly consider trade-offs between gains in hydropower and losses in river connectivity at the river basin scale

Water security determines social attitudes about dams and reservoirs in South Europe

River barriers affect river dynamics and aquatic biota, altering the entire ecosystem. Nevertheless, dams and reservoirs provide goods like water supply and low-carbon energy that are becoming increasingly critical under current climate change. To know to what extent dams and reservoirs are important to the population, we explored social attitudes towards dams and reservoirs using a face-to-face questionnaire in two regions of contrasting climate and water security in Spain, a country with one of highest densities of dams in Europe. Results (N = 613) revealed a higher support for dams, mediated by the recognition of the services they provide, in the drier Mediterranean Malaga province (Andalusia), than in the wetter Atlantic Asturias province (Bay of Biscay), where water shortages are rare. Awareness of the impacts of the dams was more pronounced in Malaga, coupled with a higher willingness to pay for reconnecting rivers. Social awareness of both impacts and services provided by dams and reservoirs may depend on local climate and water security; different dam acceptance emphasizes the need to involve local citizens in the decision-making processes about water management

Best practices for selecting barriers within European catchments

With over 1.2 million dams and other instream barriers, Europe has possibly the most fragmented rivers in the world, but also the opportunity to benefit enormously from barrier removal. Resources available for barrier removal, however, are limited and some form of prioritization strategy is thus required to select barriers for removal that will provide the greatest gains from restoring river connectivity in the most efficient possible way. To properly restore river function, barrier removal programs need to consider all types of artificial instream barriers that cause river fragmentation, not just those that impede fish movements. Opportunities for barrier removal depend to a large extent on barrier typology, as this dictates not only where barriers are typically located, but also their size, age, condition, and impacts. Crucially, the extent of river fragmentation depends chiefly on the number and location of barriers, not on barrier size. However, because barrier removal costs typically increase with barrier height, acting on many small barriers may be more cost-efficient than acting on fewer larger structures. Here we review the main strategies available to prioritize barriers for removal and mitigation, with special emphasis on removing non-ponding, low-head (<3 m) barriers, as these are the most abundant across Europe and other regions. To increase the success of barrier removal programs, we recommend that barriers considered for removal fulfill four essential conditions: (1) they would bring about a meaningful gain in connectivity; (2) are cost-effective to remove; (3) will not cause significant or lasting environmental damage, and (4) are obsolete structures. There are dozens of prioritization methods in use. These can be broadly grouped into six main types depending on whether they are reactive or proactive, whether they are typically applied at local or larger spatial scales, and whether they employ an informal or a formal approach. These include, in increasing order of complexity: (1) opportunistic response; (2) use of local knowledge and expert opinion; (3) scoring and ranking; (4) geographic information system (GIS) scenario analysis; (5) graph theory; and (6) mathematical optimization. We review their strengths and weaknesses and provide examples of their use. Overall, mathematical optimization sets the gold standard for effective and robust barrier mitigation planning, but to be practical, it needs to factor in the constraints imposed by uncertainties and opportunities. Accordingly, a hybrid approach that considers uncertainty, the presence of natural barriers, the importance of future-proofing, and opportunities provided by local knowledge is likely to be the best overall approach to adopt. Various studies have shown that a small proportion of barriers is typically responsible for the majority of river fragmentation. These ‘fragmentizers’ can be identified and located using the prioritization methods discussed herein and a targeted approach can produce substantial gains in connectivity by acting on a relatively small number of structures. Unfortunately, many of these ‘fragmentizers’ cannot be easily removed. Removal, therefore, is constrained by opportunities and what is practically feasible. Mapping of barrier removal projects according to the three axes of opportunities, costs, and gains can help locate the ‘low hanging fruits.’ Opportunities normally develop over time as infrastructure ages, so acting on some barriers now will likely open opportunities for acting on others later on to create a snowballing effect. The ability to simulate benefits and costs of barrier removal and select barriers for removal is critically dependent the quality of the data at hand, particularly with respect to the number of barriers, which can be grossly underrepresented. Uncertainty caused by incomplete barrier records diminishes the effectiveness of barrier mitigation actions but these can be overcome to some extent by (1) ground truthing via river walkovers or (2) predictive modelling. Other critical sources of uncertainly include those caused by inaccurate stream networks and spatial errors regarding the exact locations of barriers. Although uncertainties can be reduced by collecting more information, it needs to be weighed against the cost of waiting. Waiting to collect more data to reduce uncertainties tied to barrier removal may lead to ‘paralysis by analysis,’ while species and ecosystems continue to decline due to stream fragmentation. To better understand how barrier prioritization is implemented in the real world, we sent out an online questionnaire to river restoration practitioners located in Europe and North America. Results show that most organizations (~60%) have a plan to achieve free-flowing rivers. Most respondents (34%) use expert judgment, followed by consultation with stakeholders (17%) and a combination of methods (28%) to prioritize barriers for mitigation. Only 12% used specialized software or algorithms. Attributes most frequently considered by practitioners in barrier prioritization were barrier ownership and rights, results of field surveys, and the obsolescence and conservation status of barriers. The most important rational flagged by practitioners to prioritize barriers for removal was to improve fish passage. Our study suggests that no matter what prioritization approach is ultimately adopted, decision makers need to be mindful that no priorities should be set in stone. Planning needs to be agile and flexible enough to adapt to changes and react to opportunities

SNP analyses and acoustic tagging reveal multiple origins and widespread dispersal of invasive brown trout in the Falkland Islands

Biological invasions are important causes of biodiversity loss, particularly in remote islands. Brown trout (Salmo trutta) have been widely introduced throughout the Southern Hemisphere, impacting endangered native fauna, particularly galaxiid fishes, through predation and competition. However, due to their importance for sport fishing and aquaculture farming, attempts to curtail the impacts of invasive salmonids have generally been met with limited support and the best prospects for protecting native galaxiids is to predict where and how salmonids might disperse. We analysed 266 invasive brown trout from 14 rivers and ponds across the Falkland Islands as well as 32 trout from three potential source populations, using a panel of 592 single nucleotide polymorphisms (SNPs) and acoustic tagging, to ascertain their origins and current patterns of dispersal. We identified four genetically distinct clusters with high levels of genetic diversity and low admixture, likely reflecting the different origins of the invasive brown trout populations. Our analysis suggests that many trout populations in the Falklands may have originated from one of the donor populations analysed (River Wey). The highest genetic diversity was observed in the rivers with the greatest number of introductions and diverse origins, while the lowest diversity corresponded to a location without documented introductions, likely colonized by natural dispersal. High levels of gene flow indicated widespread migration of brown trout across the Falkland Islands, likely aided by anadromous dispersal. This is supported by data from acoustically tagged fish, three of which were detected frequently moving between two rivers ~26 km apart. Our results suggest that, without containment measures, brown trout may invade the last remaining refuges for the native endangered Aplochiton spp. We provide new insights into the origin and dispersal of invasive brown trout in the Falkland Islands that can pave the way for a targeted approach to limit their impact on native fish fauna