Editorial: Green or red: Challenges for fish and freshwater biodiversity conservation related to hydropower
Juergen Geist
Abstract
The global sustainable development goals of the United Nations (United Nations, 2015) and the associated necessary transformations, such as energy decarbonization (Sachs et al., 2019), have resulted in increased hydropower development around the world (Zarfl et al., 2015). Advocates of hydropower point to the important contributions of this form of energy to the reduction of CO2 emissions, its low cost, and its use as a stable source of energy, justifying it as a ‘green’ energy that makes an important contribution to electricity supply compared with other forms of regenerative energy. For the year 2018, the International Energy Agency (IEA) lists a global energy supply from hydropower of 4,325,111 GWh, compared with 1,273,409 GWh for wind energy and 554,382 GWh for solar/photovoltaic energy (IEA, 2020). In contrast to wind or solar power, hydropower production is available at any time of day and is less dependent on current weather conditions. According to the Collins Dictionary (www.collinsdictionary.com, accessed 14 February 2021), ‘green energy’ is defined as ‘power that comes from sources that do not harm the environment …’. The debate about how ‘green’ the green energies really are is controversial (Gibson, Wilman & Laurance, 2017). Opponents of hydropower production often refer to it as a ‘red’ energy because of the mortalities and injuries that fish face when passing turbines (Mueller, Pander & Geist, 2017; Mueller et al., 2020b), as well as other ecological harm to free-flowing rivers associated with habitat fragmentation and alteration. Hydropower is often referred to as one of multiple impacts on river ecosystems that has potentially synergistic effects with other stressors (Ormerod et al., 2010; Mueller et al., 2020a; van der Lee & Verdonschot, 2020). In many cases hydropower plants were built after other structural modifications of river systems, such as damming and straightening (some of them irreversible) that had already taken place (Auerswald et al., 2019). Still, the general global consensus remains that hydropower can significantly alter river systems. Several international targets in the conservation of natural habitats and wild fauna and flora (Habitats Directive, Council of the European Communities, 1992) conflict with further hydropower development. This includes achieving ‘good ecological status’ of water bodies (or ‘good ecological potential’ in the case of ‘heavily modified water bodies’) as required by the European Water Framework Directive (Council of the European Communities, 2000). Such policies result in enormous efforts (financial and physical) dedicated towards improving freshwater conservation and restoration (Geist, 2015; Geist & Hawkins, 2016). If applied as intended these would be expected to hinder any further expansion of hydropower development, and would also support an argument for dam removal if conservation targets are not met. Despite this paradox, global hydropower capacity is projected to approximately double from the 2010 installed capacity, requiring a dramatic increase in the number of hydropower dams in river basins around the world (Opperman, Grill & Hartmann, 2015). It would be highly ineffective – also from a financial point of view – if both the expansion of hydropower and the restoration of streams and rivers received support from public funds in spite of conflicting targets. It is thus important to bridge the gap between hydropower use and the conservation of aquatic biodiversity, ecosystems, and services that they provide to human society. This is clearly a difficult task and many experts question the feasibility of this ambitious target. From the perspective of conservation biology, the expansion of hydropower is of particular concern in rivers that are considered to be biodiversity hotspots, such as the Amazon (Latrubesse et al., 2017; Latrubesse et al., in press), the Congo and the Mekong (Winemiller et al., 2016), the São Francisco (Gomes et al., 2020), or regions in south-east Europe (Hudek, Zganec & Pusch, 2020). From a general perspective, aquatic conservation in the age of sustainable development goals and ecological assessments in connection with the ‘shifting baseline syndrome’ have already been broadly explored (Irvine, 2018). The intention of this editorial is to provide more specific guidelines that help to assess comprehensively the effects of hydropower in relation to aquatic biodiversity conservation. This requires the better integration of ecological effects, technological properties, and socio-economic factors, as well as conservation prioritization, and a transition from assessing the effects of single facilities on single species to an assessment of cumulative effects, followed by an open, fact- and evidence-based discussion that involves all stakeholder groups before proceeding with political decision making (Figure 1). At present, baseline information is rarely complete even within specific fields. For example, ecological information rarely includes effects on food webs, complete life cycles, or populations, and instead mostly focuses on the organismic damage of a few fish species. Whereas the conservation of fishes is typically prioritized in the context of ecological assessments of hydropower, other aspects relevant to biodiversity conservation deserve inclusion, such as managing the hydromorphological changes to flow regimes, thermal regimes, and altered substrates, which can all affect a range of processes and organisms, not just fishes. The focus of assessing the ecological impacts of hydropower has traditionally been on the direct consequences of the entrainment of fishes. This comprises mortality or injuries from contact with structures, such as screens and bars, collision with turbine blades, shear stress, and barotrauma arising from abrupt pressure changes during turbine passage. Direct and delayed mortality (Ferguson et al., 2006) as well as types of external and internal injury (Mueller, Pander & Geist, 2017; Mueller et al., 2020b) are affected by the physical and hydraulic forces encountered by the fishes and can be predicted to some extent (Deng et al., 2007; Boys et al., 2018), although strong species-specific differences related to anatomical properties prevail. Even less is known about how fish behaviour affects mortality and differences in injury, related to different species, personality types, and diurnal activity patterns, as well as route choice at hydropower facilities (Baumgartner et al., 2014b). Egg et al. (2017) demonstrated that an eel bypass system that was effective in laboratory experiments was not used by eels in a field setting during the silver eel migration. Knott et al. (2019) and Knott et al. (2020) showed pronounced diurnal differences in the downstream movement of different fish species as well as their route choice when approaching hydropower plants, and the resulting injury patterns. A major limitation is that many assessments of the direct impacts of hydropower facilities are based only on one or two fish species (often eel or salmon smolts), without considering the typically much more diverse fish community at its full size range in the specific habitat. In addition, there is little to no integration of information on all life stages of species (for instance, eggs or larvae), nor on other taxonomic groups such as primary producers or macroinvertebrates, which are key elements in the functioning of aquatic food webs. Another difficulty in relation to conservation is that it is often difficult or unethical to experiment with species that are already highly endangered and only present in low numbers. The subjective perception of the mortality or injury of individual fish, which is relevant from the perspective of animal welfare, is different from an assessment of population-level effects, which is more relevant from the perspective of conservation. As the quality of modelling effects at the population level depends strongly on the quality of input information (i.e. on the sensitivity of different life stages as well as the interactions among species), large data gaps concerning the autecology of many species of conservation concern still have to be filled as a first step (Smialek et al., 2019). For example, there is still little knowledge on the safe passage of drifting fish eggs and larvae when passing hydropower turbines. The published results point in different directions as they depend heavily on local conditions, hydraulic mechanisms, and the species studied (Boys et al., 2016; Navarro et al., 2019). Another difficulty is the general lack of studies investigating the effects of turbine passage on taxonomic groups other than fishes. Globally, hydropower development is contributing to one of the largest expansions of dams seen in history (Opperman, Grill & Hartmann, 2015). This is problematic in many ways: rivers are naturally four-dimensional systems that depend on a high degree of connectivity (Ward, 1989; Auerswald et al., 2019), and there are hardly any free-flowing rivers in the world any more (Grill et al., 2019). Practically all hydropower development depends on the introduction of concrete structures into rivers, resulting in habitat fragmentation; indeed, Cooke et al. (2020) have proposed that the concrete conquest of aquatic ecosystems should cease. The creation of impoundments, urbanization, and catchment land use have been identified as the most important factors affecting fish community composition in streams (Bierschenk et al., 2019a; Mueller et al., 2020a). It should be noted that the impacts of hydropower on habitat are not primarily caused by the generation of hydropower itself, but mostly result from the dams and weirs that reduce or fully block connectivity and migration, and alter flow regimes. The importance of free movement and migration is well understood for fishes (Vasconcelos et al., 2020), particularly for diadromous species that depend on migrations between oceans and fresh water to complete their life cycles (Smialek et al., 2019). The provision of fish passage for charismatic flagship species is often proposed as a solution (Silva et al., 2018a). In this respect, a clear distinction must be made between upstream and downstream movements of fishes, as these movements follow different principles (Calles & Greenberg, 2009). Although there is a wealth of scientific studies related to technical and nature-like solutions for facilitating the upstream movement of fishes, their safe downstream passage remains poorly understood (Knott et al., 2020). Even the upstream solutions are typically directed towards one or a few target species. If the target species are strong swimmers, such as the Atlantic salmon (Salmo salar), then a functional fish pass for the target species may still hamper the movements of other species that co-occur in the same habitat. In Europe, this is particularly true for small and weak swimmers that are hardly considered in the planning of fish passes, such as Cottus gobio, Gobio gobio, and Barbatula barbatula. They are often referred to as ‘stationary species’ yet have been found to demonstrate significant movement in fish passes (Pander, Mueller & Geist, 2013). A similar problem arises for downstream migration, as illustrated for the European eel (Anguilla anguilla). This target species for conservation receives great attention and has a unique life cycle in which adults metamorphose into silver eel that migrate down the rivers on their long spawning migration into the Sargasso Sea. Silver eel have a long, elongated body shape and thus are at high risk of getting injured when passing hydropower facilities in a downstream direction (Egg et al., 2017; Mueller, Pander & Geist, 2017; Mueller et al., 2020b). For this reason, there have been various proposals for adjusting turbine operations in relation to eel activity patterns, or for trapping eels, moving them around the barriers, and then releasing them near the ocean (‘trap and truck’). Although the usefulness of such measures that require continuous human management action remains controversial, there is no doubt that other, less prominent species, such as lampreys, are left behind. Compared with the many challenges in facilitating connectivity for fishes, much less is known on how other taxonomic groups are affected by although it that with the life cycle strongly impacts (Pander, Mueller & Geist, 2018). For instance, such as that can in their stages are less affected by habitat fragmentation than species that are only on the larvae of freshwater which are strongly in their by In to habitat by also to be upstream of dams and weirs are typically by increased water flow increased and altered water systems with a large number of dams and weirs from a system into a on their functional species can be affected by these habitat with of low conservation and of high conservation The resulting in the community at all from primary producers to fishes (Mueller, Pander & Geist, with species et al., 2018), the life cycles of species often be & Geist, because of the of or et al., 2019). an argument that hydropower use in natural river would increase biodiversity because it to diverse such as flow the dams and at the turbine From a conservation perspective these are as they the many impacts on the system as well as the fragmentation of key habitats for the life stages of species. Even if target species of conservation such as to in the downstream of a their larvae typically do not water habitats in highly modified & Geist, or to use the flow upstream of the dam as a habitat. solutions to the challenges requires to a of such should be to fish or habitat how can the effects of hydropower be and considering the cumulative effects river which principles can at the level of species, ecological and should the effects of mortality and fish injury related to fish passage hydropower facilities be or should the of fishes into the facilities and moving them around be to extent can hydropower provide and how can decision an between the ‘green’ and the ‘red’ aspects of the impacts of hydropower on aquatic biodiversity conservation is by Even for individual hydropower data are rarely available on all taxonomic groups and all life stages that are important for the modelling of population-level From a hydropower perspective, the population-level effects of fishes are a primary often requiring assessments based on modelling and fish fish field can help to the factors that fish mortality and and reduce the of hydropower on studies with can help to the specific conditions. In addition, such studies can help to that harm to aquatic fauna is This includes technological and such as adjusting the of turbine (Deng et al., as well as down turbines at of migration. the other such studies also with if fishes to be upstream of hydropower facilities in to assess their damage passage. of fishes may resulting in the introduction of that with local fishes, in and in the local From the of the use of species in such experiments as the European eel in the can also be if the river was not with these species in the The use of species that are already endangered is of their use in such experiments may result in population or – in the case of fishes – result in with related to conservation This for example, for the European a species that has in Europe (Mueller, Pander & Geist, 2018). of that and pressure changes may provide for this in the (Deng et al., yet they fully and differences among species that result in diverse injury among different species passing the same (Mueller, Pander & Geist, 2017; Mueller et al., 2020b). At the community interactions among species are difficult to and of results from laboratory experiments into field can be problematic (Egg et al., 2017). habitat changes requires and that are rarely within hydropower The should primarily target an assessment of the at hydropower facilities related to the and for example, water current and as these are used when assessing the effects of dams and weirs on habitat quality (Mueller, Pander & Geist, In the context of hydropower, properties such as changes that may result in of fishes as well as in the mortality of conditions, also to be It is important to that are often more relevant than because these can have impacts on aquatic It is that processes such as flow habitat and connectivity are rarely used as yet should be important conservation targets. to focus in particular on of or and hydropower passage to increase public of hydropower as a ‘red’ energy, a of habitats is to be more relevant for biodiversity conservation. Even when the effects of individual hydropower on taxonomic groups and habitats are well the with other stressors such as and land use are difficult to rivers often have a of hydropower their and not just single assessment of such cumulative effects is yet this is for are only when considering multiple For instance, Latrubesse et al. that the cumulative effects of dams a major to assessments and typically only individual A solution to this be to the and of individual hydropower plants for river systems on the of related to population of key species of conservation as well as to the habitat quality and on which they In a is available for ecological at hydropower such a system is to the focus from a of individual to river systems. It is well known that the impacts of hydropower facilities and on fishes can be by adjusting (Bierschenk et al., by the provision of (Knott et al., Knott et al., 2020), and by the use of barriers, such as (Egg et al., 2019), with species-specific differences in The same true for different types of hydropower plants that in their species-specific mortalities on the local conditions. from a conservation to be for management among rivers and among the of hydropower The question of which species should be prioritized is often by a few flagship species for conservation that are or but not the most endangered or important species (Geist, At the species to species eel and as well as to particularly and species with the in the most If such a can be with or services such as food then this can Hydropower are typically with This that any decision made to have a in the and that and management and should but without the effects of conservation such as turbines during the migration of key species. the high number of hydropower facilities in of the it is that be to to river systems in the near the targets for example, by the European Water Framework Directive (Council of the European Communities, 2000). A can be by the processes of for hydropower as well as dam and impacts in the case of prioritization, had proposed a in freshwater conservation that must clear and (Geist, 2015). this to the field of hydropower that the few river and systems should the conservation and that any form of hydropower development on these systems should be in with the on by (Opperman, Grill & Hartmann, 2015). In to the few large rivers in which the hydropower has not yet been fully explored the and Mekong this also to many of the in European streams further expansion of hydropower is often only if or in of the other in ecological of already highly or river systems should have first can be these systems are in and connectivity to important In river systems, different conservation are of particular importance in complete life cycles of and managing bypass as free-flowing as and downstream of hydropower plants and fish bypass primarily to fish migration are often the only with hydraulic to provide a habitat for species of conservation concern (Pander, Mueller & Geist, Pander & Geist, 2018). in to their as migration the habitat of bypass must be considered even their fully for in the & Geist, 2013). in rivers, free-flowing are important for the of fishes (Silva et al., et al., 2020). is also focus on flow in that the natural (Auerswald et al., Pander et al., or (Baumgartner et al., as well as other conservation that into the natural of systems (Geist, Geist, 2015). As a of hydropower are to flow and thus – at to a extent – can be used to help for conservation Such can also the impacts of hydropower on the of and hydropower with the of impacts can provide a for Several have been such as the hydropower and low as well as of these require as this is the only to for the that are less to aquatic systems than the key question – of it is more to the effects of turbine passage by in more hydropower than to fish or fish passage – is not an one to Although large fishes by screens and from various in European streams and rivers clearly that the of all fishes are less than in there are to their entrainment to technical of and field conditions. For fishes and other aquatic organisms, it is that as yet there are no effective of or even significantly their A better of population-level effects is yet it that the of effective to the damage with downstream passage makes it to focus on the of as well as on technical and such as turbine management to safe fish passage (Figure 1). The discussion on hydropower and conservation is highly controversial and often results in and solutions for the challenges can only be if an discussion based on scientific is used as a for and management (Figure 1). that aspects such as functioning and technical which may be to into clear based on and services relevant to to be most (Geist, From an ecological point of a must be made from considering individual and species to a of population-level effects, and more on ecological into conservation decision making et al., 2017). For instance, a by et al. demonstrated that effects of and can the effects of and increase to the which are that typically in the of As of both species can be affected by hydropower the consequences for other species, such as that depend on to be better and considered in decision Such to be applied more broadly to other a more global decision making in both hydropower development and conservation management is to and political It thus in the of that a natural or perspective as well as an perspective not be to the challenges natural and should provide important information from their as baseline information that then requires the integration of socio-economic and aspects as well as the of different technological and (Figure 1). such information should not only be at the level of individual hydropower but instead cumulative effects for river systems. Such a requires the of stakeholder and for example, between hydropower and et al., 2020), as well as among biodiversity, food and hydropower et al., for large river systems, such as the Mekong and the food for and provision for development are key that to be with conservation targets. The more that the of different of decision the it be to a that the about hydropower as green or energy. would to for to this which was by on the ecological effects of hydropower by the European and for hydropower and a on the effects of and hydropower by the of and at the of would also to Lee and Pander for of the and hydropower at for their great to