The FIThydro project not only compiled, assessed and analyzed published knowledge on the effects of hydropower to fish and the environment, but also generated new knowledge and recognized still existing knowledge and research gaps. These gaps concern diverse aspects and challenges that planning, construction and operation of hydropower plants pose in terms of mitigating and lowering environmental impacts. The aspects relate to the following fields of action:

  • Legislation

  • Operation of existing and new plants

  • Modifications to existing plants or in the affected watercourse sections

  • Design of new facilities or the replacement of parts of an existing facility.

As the name of the project acronym suggests, FIThydro (fish friendly innovative technologies for hydropower) was mainly about hydropower and its impact on fish individuals and populations. After completing the project, we know much more about fish and their behaviour and we have found and developed new solutions and technologies for individual elements of hydropower plants, but we have also realized that there is still room for improvement to ensure optimal protection of fish at hydropower plants. River fishes have evolved in disturbance-dominated ecosystems triggered by droughts and floods and developed life history traits providing resilience against such disturbances. Hydropower operation deviates from natural disturbances in several ways, e.g. by reducing amplitude and increasing frequency of water level changes, diverting water from the river and creating complex turbulent flows, which can cause injury or mortality. Therefore, hydropower operation as man-made challenge might exceed the resilience of fishes, i.e. their capacity to resist or recover from disturbances and their ability to adapt to them. It seems a major challenge to plan and build hydropower in a way that considers and respects the life history traits of the various fish species. An example of such an approach is the application of behavioural guiding systems like the curved bar rack (CBR) developed in FIThydro and described in this book. Although fish could easily swim through the large spacing between bars, they will swim along the CBR to a bypass for downstream passage. This guiding effect is supported by small eddies detaching at the CBR and creating a turbulent shear layer that fish do not like to cross. Such shear layers could also be artificially created elsewhere, for example, to guide fish. This underlines the importance of considering fish behaviour for successful mitigation of barrier impacts. Future research could use means of artificial intelligence to better understand and predict fish behaviour.

Hydropower is an intervention in the natural habitat of aquatic organisms, which is severe and usually not reversible as long as the hydropower plant exists and is operated. Therefore, the most obvious thing to do would be compensating for loss of habitat area and quality, as is usually mandatory according to environmental legislation. This thinking is found in this book, but perhaps not fully developed. Since hydropower plants are technical achievements, mitigation is usually sought in improving the technology. It is very unlikely though that this will lead to greater cost efficiency.

We can positively influence the environmental conditions at the hydropower site, the operation mode or the technology or a combination of these possibilities. Environmental conditions can be improved by creating and rehabilitating lost habitats in close proximity of the hydropower plant, but also by replacing them elsewhere, e.g. by large, nature-like constructed bypass sections. The operation of hydropower plants can also create conditions that are closer to the natural, undisturbed state with mostly small adjustments. In general, the natural dynamics of watercourses are lost through many hydropower plants, but this does not necessarily have to be the case, at least to a certain extent. Often it is due to the technical setup that a hydropower plant cannot at least partially follow the natural dynamics, but often it is simply due to the regulations used, which prescribe static and unnatural operation. Changing this would be very simple and quickly realizable. Changes in technology are usually the most difficult and cost-intensive option. Of course, there is potential for improving turbines to reduce the likelihood of lethal damage on fish. For cost efficiency reasons, this is only an option if a turbine gets too old, becomes uneconomical and needs repair or replacement. One may assume that creating improved habitat conditions costs less than replacing turbines, and that habitat measures will not only serve single fish individuals but the whole populations. Although the FIThydro project does not provide further-reaching, quantitative cost efficiency comparisons, it provides preconditions to do so.

Apart from the Cumulative Impact Assessment, FIThydro mainly dealt with individual hydropower plants and improvements at the Testcases considered. Unfortunately, the mitigation of cumulative impacts from multiple barriers could not be investigated within this project; for example, the benefits of removing obsolete transverse structures or even old, low output hydropower plants without changing anything at the other plants compared to on-site technical mitigation measures. The project scope did not allow for such assessments, which should be carried out in the course of a future project. The replacement of a cascade of many small hydropower plants in a river by a large diversion power plant aims in a similar direction. In the remaining section of the river, with sufficient residual water and structural morphologic improvements, a dynamic can be re-established that comes close to the original, natural conditions.

The following section explains opportunities and promising strategies for more sustainable hydropower use through mitigating environmental impacts, improving habitats, operating a hydropower plant, or developing hydropower technologies that result from the work presented in this book for the future.

To support mitigation planning a first sensitivity classification of European lampreys and fishes has been developed in the FIThydro project, which scored 168 species according to their sensitivity against adult mortality. This classification informs about both species’ response to mitigation measures and severity of individuals losses. The latter is also used to assess hazard risk for fish depending on their conservation value.

The sensitivity of species is used as weighting factor within the European Fish Hazard Index (EFHI) for fish at hydropower plants. The EFHI is the first standardized, transparent, and comparable assessment tool for screening the hazard risk for fish that considers constellation and operation related risks as well as the conservation value of the ambient fish assemblage and scores up- and downstream habitat changes and migration facilities, fish entrainment and mortality, and mitigation measures implemented. The scoring ranges from low to high risk and helps to prioritise mitigation planning or in-depth environmental impact assessment. The EFHI also allows prediction about risk lowering as a result of planned mitigation measures.

In European rivers there are about 30 times more barriers than hydropower plants. Therefore, assessing cumulative impacts goes beyond hydropower. Barrier effects, in particular migration obstacles and impoundments, i.e. habitat fragmentation and loss, are similar for transverse structures with and without hydropower. Correspondingly, the cumulative impact assessment in FIThydro considers both cumulative length of impoundment in river section and cumulative number of barriers, i.e. barrier density. To enhance the ecological status at the level of whole river sections it is probably much more efficient to remove obsolete barriers and to rehabilitate hydromorphologic processes in the river rather than to equip hydropower plants with expensive fish protection and guidance facilities. The latter does not hold true for rivers with diadromous species, i.e. species using both freshwater and marine habitats during their life cycle. These species have to pass all obstacles on their way including hydropower plants, where they become subjected to enhanced mortality. However, there are increasingly better fish protection tools available today, as e.g. the curved bar rack behavioural guidance system, that is applicable to even larger hydropower schemes. In addition, turbine management with shut-downs during fish migration peaks appears as very promising fish protection measure also for large and very large hydropower schemes.

Mitigating hydropower impacts can be further supported by a purposely designed decision support system, the FIThydro DSS open-access web tool (https://fithydro.eu/dss). The DSS summary assessment of the risks posed by a scheme focuses on prioritising hazards for mitigation in relation to ecological status and objectives and then selecting appropriate hazard-specific mitigation measures.

The DSS was designed to directly use outputs and innovations from the research in FIThydro or to act as a gateway to more specialised tools and results applicable to specific circumstances to enable informed and evidence-based choices. Further, the FIThydro wiki (https://www.fithydro.wiki/) links within the DSS to enable users to access supporting materials detailing the summary data held by DSS and to find detailed descriptions of solutions, methods, tools and devices that could be applicable for further site-specific impact assessment and mitigation. Further, the wiki is a gateway to further information compiled in FIThydro on hydropower policies in Europe, public acceptance of hydropower and costs of implementing ecological mitigation measures (solutions).

However, despite all these achievements in terms of assessment, screening and support tools as well as guidance information, it must be noted that there still remains a significant knowledge gap on how the empirically observed fish mortalities translate to population effects for non-diadromous species. In any larger river, tremendous efforts are needed to estimate the total number of fish therein. Without knowing this number, it is hardly possible to relate empirically observed migrating fish and mortality rates to the total population and thus to answer the question for population impacts. For sure, high mortality rates cannot be ignored; but further research is needed to estimate the share of a population that is affected by high mortality rates. This proportion not only determines the severity of population effect, but is also relevant for planning mitigation and compensation measures. Alternatively, spatially explicit population models could be used to calculate the area of spawning and nursing habitats that would be required to compensate for the number of adult fish that have been empirically determined to be likely to suffer a fatal injury during the passage of turbines, locks or weirs.

Many people immediately associate fish-friendly hydropower with turbine mortality and call for turbines with less fish damage. In the course of the FIThydro project, we have not investigated the influence of individual fish mortality on the respective population. Science it is not certain what proportion of fish in a population even moves across one or more hydropower plants within a year. However, optimising the survival probabilities of fish during turbine passage is certainly not the only technical way to support populations. This book extensively deals with classical options of fish passage and descent systems, where standards have been created and the FIThydro Testcase studies have shown quite high success rates in both passages. For the conservation of populations, it is important to create optimal conditions to ensure their basic needs, despite the construction of hydropower facilities. These basic needs include the presence of resting places and hiding places for protection from predators, suitable feeding places, and sufficiently large areas with optimal conditions for spawning and nursing. Hydropower plants change the environment for fish to such an extent that these basic needs require conscious support, including technical solutions, to maintain sustainable populations. It is therefore also a question of creating the conditions for maintaining or improving these basic needs by means of structural measures, supporting technology and suitable regulations when building new hydropower plants or refurbishing old ones. Perhaps one could say: Technology should enable and support diversity—diversity in the sense of variable flow patterns, natural fluctuations in discharge, differences in the composition of bed substrates and a diversity of morphological structures and landscape elements—to enable sustainable fish populations with targeted management. In most cases, the regulations in particular are also detrimental to such variability, often without any really obvious reason. Water levels in impoundments do not have to be fixed; it would be advantageous if they exhibited a certain dynamic, as in nature. By lowering reservoir water levels in advance, the dynamics and volumes of a natural sediment transport can be achieved sooner during flood events. For this, however, structural preconditions must be in place or created to use the flexibility on the regulatory side for variable environmental management at hydropower plants. Then it will be possible to activate the transport of sediments to support the formation of morphological structures similar to anthropogenically uninfluenced watercourses. Such variability creates a diversity of flow patterns, flow velocities and water depths and thus potential habitats for different life stages and aquatic populations. The technical prerequisite for this is that in impoundments the water level of the undammed watercourse can be established at least temporarily, which requires near-bottom regulation options that can be used for active sediment management. One such example is the Vortex Tube, which was investigated at the Schiffmühle Testcase. However, many existing controlled weirs on rivers could also contribute to making sediment transport more dynamic solely by changing their operation.

E-flow was a catchword of our proposal and a challenge which we intended to address in the Testcase studies. The term shows that, in a very simplified view, the only thing that matters is to provide enough water. However, fish do not only need water to live, as described in detail above. Today, we already have the possibilities to numerically simulate changes in sediment transport, bed structures and bed substrates at given discharges or to numerically run through scenarios to find optimal solutions for habitats of aquatic populations. Therefore, in the future, it will be increasingly required to guarantee not only the discharges in a restricted view, but also the habitats, and thus e-flows might then become e-habitats.

Storage hydropower plants generate flexible energy during times of high peak demand and stabilize the power grids. Hydropeaking is thus an increasingly important element for the integration of volatile, renewable energy generation and is subject to extensive research. The FIThydro project contributes with the further development of the COSH and CASiMiR tools to quantify the effects of hydropeaking on fish and to compare scenarios. The GKI Testcase on the upper Inn in Austria investigated a hydropeaking diversion hydropower plant combined with a buffer reservoir to mitigate the effects of hydropeaking on the environment and especially on fish. The analyses with the CASiMiR hydropeaking tool showed that the diversion and retention of hydropeaking, accompanied by morphological adaptations and improvements in the receiving water course, are innovative and future-oriented options to mitigate hydropeaking effects.

In the coming years, sensor technology and artificial intelligence will create possibilities that we cannot yet fully assess and foresee. In our Testcases we used sensors to estimate fish damage in turbines, the Barotrauma Detection System (BDS). The Lateral Line Probe (LLP) device allows to determine velocities not only at single measurement points, but to record flow signatures in a way that fish can do with their lateral line sensorium. This will perhaps enable better understanding of which swimming paths fish choose, which signatures tend to attract or deter them. The development and use of such sensors will contribute to a much more active environmental management, not only by serving the natural conservation of runoff and sediment dynamics, but also by being able to make visible what is hidden today and thus contribute to a better understanding of fish responses. In the course of FIThydro, we have investigated and improved the TRL level of two fish tracking technologies that allow us to detect, track and control fish in a water body even without previously implanted probes. One can imagine that weir gates are opened when a school of fish approaches a hydropower facility, or that discharges in fishways are increased to attract fish that want to ascend. One can even envision that one day there will be robotic fish that swims ahead of fish schools to safely guide them to fishways.

Related to turbine passage the findings from FIThydro could be used to optimise the operation of existing turbines so that it is less dangerous for fish, before perhaps installing new fish-friendly turbines. The research conducted has shown how turbines should be operated to achieve maximum fish protection during turbine passage, rather than maximum power output. We have developed and presented tools to determine what the overall statistical probability of turbine damage is, and have shown that the entry point of a fish into the turbine has decisive influence on the survival probability, and where the entry point of highest survival probability is located. All this knowledge can be used to guide fish accordingly, as was shown with the Induced Drift Application (IDA) Device in a first trial.

All the knowledge and technologies presented in this book and even more compiled in the FIThydro wiki allow for informed decision making in planning, constructing, refurbishing, and operating hydropower plants with regard to environmental concerns, riverine habitats, aquatic biodiversity and especially fish conservation. They contribute to making hydropower more ecologically sustainable and have opened up ways how such improvements can be achieved. We hope that this book gets the reader impetus and ideas that will be taken up and put into practice. When in coming years further steps are taken beyond this and lead to innovative ideas and developments, then this book has achieved its goal.