SummaryIn Ireland, the Water Framework Directive (WFD) Freshwater Morphology Programme of Measures and Standards (POMS) identified barriers to fish migration as one of the principal issues placing channels Ôat riskÕ in terms of failing to achieve hydromorphology status. To quantify the degree of risk posed, and to develop methods to appraise and quantify risk associated with barriers, the POMS undertook a pilot study on the River Nore catchment. This provided an opportunity to compare the outcomes of an intensive fieldbased qualitative assessment of barriers to fish migration with a rules-based appraisal of risk, based on layers of georeferenced data, in a Geographical Information System (GIS) analysis. The underlying aim of this risk-based assessment was the restoration of the fisheries resource, in the context of WFD, grounded on knowledge-based assessment. The comparison focused on the Atlantic salmon (Salmo salar L.). A rapid-assessment field survey compiled a geo-referenced inventory of 508 structures (497 artificial and 11 natural) and classified each with regard to presumed risk of preventing the upstream migration of adult Atlantic salmon. This inventory was then subjected to a rigorous GIS-based analysis, using field-generated datasets on Atlantic salmon ecology in the Nore catchment. The rules-based analysis provided a very substantial re-classification of artificial barriers with a major reduction in the numbers classified as impassable or high-risk. It confirmed that no Ôhigh riskÕ barriers to adult salmon occurred on the R. Nore main stem but identified clusters of high risk barriers in the Dinin subcatchment and a series of moderate risk obstructions in the KingÕs River system. The GIS-based approach enabled an examination of the costs and benefits of stepwise removal of barriers, in terms of additional wetted habitat and salmon spawning area made accessible upstream of each barrier. A multi-species approach was then explored, examining the risk to upstream passage for eel (Anguilla Anguilla L.), sea lamprey (Petromyzon marinus L.) and Twaite shad (Alosa fallax Lace´pede). Substantial clustering of sea lamprey spawning effort was reported downstream of the lowermost weirs on the R. Nore. A combination of barriers identified from a salmon perspective and via the multispecies approach provided a listing of priority barriers for attention. The strategy of step-wise removal or modification of artificial barriers in an upstream direction would provide optimal conservation value for the relevant species. The study identified the primacy of the field survey in geo-referencing site locations. However, it also identified the shortcomings of the qualitative, Ôexpert-judgementÕ approach to risk category allocation of the barriers and, hence, the need for more rigorous criteria in implementing a field-based rapid assessment.

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