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Indicator Specification
MAIN ADVANTAGES OF THE INDICATOR
The indicator shows in spatial and quantitative terms, fragmentation due to the presence of artificial structures that a) may affect the passage of migratory fish and so restrict their range and/or abundance and b) changes substantially the natural habitat distribution within rivers and modify their ecological capacity. It thus describes the difference between the potential range and actual range of migratory fish in river systems due to artificial obstacles on the one hand and the change in habitats on the other hand.
No units have been specified
To be healthy, fish communities require free access to river systems and healthy rivers that offer the different ranges of habitats required to fulfill their life cycles. River fragmentation is understood as more threatening to fish (aquatic) communities than pollution.
All fish species migrate in the water system. Most are short-distance migrants with requirements in the range of 10 to some 100 km Some are amphibiotic and their life cycle requires journeying between sea and specific rivers. For example, anadromous migratory fish (adults living in the sea and migrating up rivers to spawn in freshwater), such as Atlantic salmon (Salmo salar) and sea trout (Salmo trutta), rely for their movement and life cycle upon a favourable conservation status of their waterbodies, including unimpeded access to freshwater spawning sites and adequate river conditions during their fresh-water life. The European eel (Anguilla anguilla) is catadromous (migrating to the sea to spawn and growing in rivers), and relies similarly on movement between the sea where it breeds and within the rivers in which it grows. Obstacles of any kind (dams, cascades, diversions, quality, etc.) affect not only the movement of fish but other groups as well (invertebrates, mammals, plants, etc.).
Several types of habitat modification alter fish populations. A key change is related to the presence of numerous small dams, changing flowing rivers into stretches of river with still water conditions. Even though fish may actually pass the dam wall, they may find adverse living conditions upstream, which may make passing facilities ineffective at the population level.
All causes together alter fish communities by disrupting their structures (size components of the community, functional groups, species diversity and relative abundance) and in extreme cases result in the extinction of a population or even of the species.
Due to data constraints, the first produced indicators will be biased: non-large dams in the height range of 2.5 to 14 meters are impervious to fish journeying upstream; hence they have to be considered as a minimum pressure (underestimated) and not actual pressure. In parallel, comparisons with catchments where all dams are registered will be carried out.
Unfragmented rivers support a full range of ecosystem services and the majority of species and habitats within the river. Fragmentation decreases the size of undisturbed rivers and puts the integrity of the ecosystem at risk. This in turn might affect the potential of the river to deliver services.
No targets have been specified
No related policy documents have been specified
The CBD Subsidiary Body on Scientific, Technical and Technological Advice (SBSTTA) has reported on river fragmentation globally (UNEP/CBD/SBSTTA/10/INF/20, 17 December 2004), based on a fragmentation analysis by the World Resources Institute.
The French authorities, with special mention to the Loire-Bretagne Water Agency has developed a specific approach and carried out surveys on the Loire and Britany systems where LIFE supports Atlantic salmon and European eel habitat and population restorations.
The current development carried out at the EEA consists of implementing in a geographical database system the calculation of migratory routes and the impact of the three variables that affect migration of any kind and apply in both directions:
The variables apply individually or jointly on a river system for each species and allow assessing chains of small obstacles. The simplest application consists in assessing to what extent it is possible to access the spawning areas for anadromous fish along time, only based on permeability of large dams.
In a first step, the indicator focuses on the difference between the potential range and actual range of migratory fish in river systems due to artificial barriers and on the change of river structure resulting from works. Salmonids are the most emblematic fishes whose reproductive migration is jeopardised by physical obstacles. Their requirements and routes are the best documented.
By locating and counting the obstacles (large dams first, abstractions and small dams in a second step), the indicator defines the difference between the potential extension and actual range of migratory fish in river systems due to man-made obstacles. The indicator of passing considers both directions of migration because most obstacles do not have a symmetric impact on movement -- they may not pose a problem for upstream movement but a problem for downstream movement, or vice versa.
No methodology for gap filling has been specified. Probably this info has been added together with indicator calculation.
No methodology references available.
No uncertainty has been specified
No uncertainty has been specified
MAIN DISADVANTAGES OF THE INDICATOR
ANALYSIS OF OPTIONS
Fish is only one dimension of river fragmentation impacts, along with changes in sediment flow and alteration of hydrological cycles. The environmental impacts must be addressed as well because meeting the targets of renewable energy triggers the development of small hydropower plants. These other dimensions however are less directly relevant to biodiversity.
Migratory fish populations may be influenced by a range of pressures and management measures, at sea and within rivers. As a wider biodiversity indicator, the status of fish populations in river systems should be assessed (e.g. as under the Water Framework Directive/Natura 2000 riverine habitats).
Work specified here requires to be completed within 1 year from now.
Work specified here will require more than 1 year (from now) to be completed.
No resource needs have been specified
For references, please go to https://www.eea.europa.eu/data-and-maps/indicators/fragmentation-of-river-systems or scan the QR code.
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