Towards environmental design in hydropower reservoirs - Developing a handbook for mitigation measures in regulated lakes
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Due to their ability to store large amounts of water, reservoirs are the supporting beam in the Norwegian hydropower system. Norway has approximately 1200 lakes that are regulated for hydropower usage, but despite the large number, there is relatively little public awareness of environmental impacts in lakes regulated for hydropower, at least compared to the focus on regulated salmon rivers. The Handbook for environmental design in regulated salmon rivers was published in 2013 and has become an important tool for local and national authorities, as well as hydropower companies in Norway. The aim of this report is to investigate the feasibility of developing a handbook for environmental design related to fish in reservoirs, similar to the one for salmon rivers. Given the large variation among Norwegian hydropower reservoirs, it is not realistic to create a first handbook for environmental design that will be useful for all reservoir types. Hence, it would be useful to narrow the scope and identify some reservoirs that are of particular interest and start with method development targeting these reservoirs. In this report, we have focused on environmental measures targeting brown trout populations, as this well-studied species is the most common in Norwegian hydropower reservoirs. The first phase of the environmental design methodology is the diagnosis phase, which aims to identify key bottlenecks for fish populations. Based on many years of fish monitoring in Norwegian reservoirs, several ecological bottlenecks are already known. An aspect that is not included in the salmon handbook, but of high importance for a future handbook for reservoirs, is collecting information on lake productivity. Unlike the anadromous salmon, experiencing abundant food recourses in their oceanic stage, resident species are highly influenced by the food availability in the lake. Hence, when looking for bottlenecks in reservoirs, an additional population-regulating factor to habitat-related and hydrological conditions may be food limitation. We suggest that the diagnosis phase for environmental design for reservoirs should consist of two parts: one based on data on fish populations and their main prey, and the other on hydrological and habitat data. In addition, the hydropower system must be described to understand potential environmental impacts. In contrast to data on fish populations and their main prey, habitat mapping and hydrological analyses have been lacking in most reservoir surveys. In many cases, appropriate habitat mapping is done in the adjacent spawning streams, but habitat mapping is rarely done within lakes. We believe it is realistic to identify bottlenecks and undertake a full diagnosis phase of environmental design based on today’s knowledge. However, there is a need to implement standardized surveys to ensure that sufficient data is collected in all reservoir monitoring. In this report, we have suggested available sampling methodology that we believe should be part of such standardized surveys. The second phase of environmental design is to identify design solutions. To our knowledge, no one has so far performed a full environmental design project in a reservoir. However, there are a number of different mitigation measures that have been used in Norwegian reservoirs and could be further developed for a future handbook. To obtain information about the type of already tested measures, we performed a survey among all County Governors in Norway. The results show that there is limited experience with mitigation measures in reservoirs. Of > 1200 Norwegian reservoirs, only 37 were reported to have known mitigation measures targeting brown trout, and a few targeting other species. Further, almost all measures targeting reservoir fishes have been implemented in the surrounding streams and rivers. These measures are more similar to the environmental design already developed for salmon rivers. For development of a handbook for reservoirs, it is important to develop environmental design methodology within the reservoir itself, i.e. in the lake habitats. The County Governors reported 14 cases of measures targeting trout within a reservoir, covering habitat-related measures, altered pattern of water level fluctuations and creation of “lake in reservoir”. We find these examples particularly interesting and relevant for a future handbook for environmental design in reservoirs. Although we have some examples of existing mitigation measures to learn from, it is important to identify optimal ways to balance the need of the fish populations with the need for power production. We believe that the most important knowledge gap to fill prior to development of an environmental design handbook for reservoirs is to develop tools that can assist balanced decisions between the environmental needs and the power production, similar to those already developed for the salmon handbook. The best way forward for environmental design in hydropower reservoirs is to establish a multidisciplinary research project, where scientists, power producers and managers can work together, focusing on one or two reservoirs in detail and use this as a pilot.