Dams on Nalon-Narcea basin as case study from Asturias-Spain by University of Oviedo

Asturias (43°20′0″N; 6°0′0″W), in the northwest of the Iberian Peninsula (south Bay of Biscay), is a succession of mountains and valleys watered by short steep rivers. There are 8 small river basins throughout the region. The climate, landscape and ecosystem are similar to those of other temperate Atlantic European regions but with slightly warmer temperatures corresponding to relatively lower latitude. One of the most fragmented by dams and reservoirs is the Nalon-Narcea basin (Fig.1) where one of the AMBER case studies in the Iberian Peninsula is located, conducted by the University of Oviedo (Asturias-Spain).

The Nalon-Narcea hydrographic basin runs through the Asturias central zone. Nalon River, 145 kilometers long and average flow of 55.2 m3/s, and Narcea River, 97 km long and average flow of 43.4 m3/s contain in total seven dams and reservoirs. Main reservoirs’ uses are water supply and the generation of electric power, plus recreational fishing. Several dams are impassable and have no fish ladders so the rivers are fragmented and the ecosystem continuity is interrupted.

In AMBER’s Nalon River case study we will analyze the upstream area of this hydrographic basin and two reservoirs occurring therein: Tanes and Rioseco (http://www.embalses.net/pantano-773-tanes-rioseco.html, accessed February 2017). The two reservoirs are interconnected and water is pumped from Rioseco to Tanes during the night while it passes down (Tanes to Rioseco) during the day to produce energy. The turbulences produced by these connections impede the use of the two reservoirs for swimming and navigation activities. The uppermost zone is located within the UNESCO (United Nations Educational, Scientific and Cultural Organization) Biosphere Reserve and Natural Park of Redes. With a rich faunal diversity, the ecosystem is dominated by well-conserved forest. More than 9000 hectares of beech (Fagus sylvatica) with oak (Quercus robur), holly (Ilex aquifolium) and yew-tree (Taxus baccata). Among the fauna inhabiting the reservoirs of the Upper Nalon, we find many amphibians –most of them protected- like the common toad (Bufo bufo), the midwife toad (Alytes obstetricans), the green frog (Rana perezi), the Alpine newt (Triturus alpestris), the Iberian newt (Triturus boscai), the fire salamander (Salamandra salamandra) and the Gold-striped salamander (Chioglossa lusitanica). There is a large and diverse group of aquatic invertebrates that are the basis of the trophic chain of this ecosystem. In the top of the chain there are otters (Lutra lutra), and just below the top, the waters of the Nalon-Narcea rivers are populated by two native salmonid fish: brown trout (Salmo trutta) and Atlantic salmon (Salmo salar); however, Atlantic salmon cannot reach upstream areas due to the impassable dams, thus only natural populations of brown trout occur in the case study zone. As native fish there are also a few European eels (Anguilla anguilla), coming from occasional stocking because as Atlantic salmon this migratory species cannot pass the river dams.

Dams are sometimes a reservoir of exotic species because given their status of artificial ecosystems they are rarely subject of environmental protection and exotics may be deliberately released there. Tanes and Rioseco reservoirs are not different. In addition to the native fish, we find there rainbow trout (Oncorhynchus mykiss) and European minnow (Phoxinus phoxinus) that were introduced for different purposes: the first one in the reservoirs for recreational fishing, and in a few aquaculture farms; the second as fishing bait. Although some old dams may be considered as a part of the industrial heritage, this does not apply to Rioseco and Tanes reservoirs, operative as a complex since 1978. The Upper Nalon river is especially interesting because a new reservoir is projected in the small tributary River Caleao, a mountain stream located entirely within Redes UNESCO Biosphere Reserve. The purpose is water supply and flow regulation. In this area of exceptionally good conservation status, a new reservoir may endanger the pristine hydrographic network within the protected space; however, the recent climate change with more and more frequent droughts in the region seems to justify the need of more water for human consumption. The ecological and socioeconomic impacts of Tanes and Rioseco reservoirs are still unknown, so it is time to evaluate them and to assess the current attitudes of locals and regional stakeholders towards the new projected reservoir. Within AMBER we are working in parallel in two fields within the Upper Nalon: natural and social sciences.

From the side of biology we are developing and applying molecular tools for a thorough accurate inventory without disturbing local biota with electrofishing. For this purpose we are extracting DNA from water samples as a source of information about present biodiversity. The macroinvertebrates, fish, birds, and hopefully mammals visiting those mountain streams will be detected from simple water samples taken with bottles. From the side of social sciences we have already contacted the local stakeholders and are collaborating with city councils inside the area. We have developed and validated a questionnaire and will proceed with a survey to know the opinion of the people inhabiting Upper Nalon about the reservoirs. The same could be done in other AMBER case studies.