PRIMARY RESEARCH PAPER Assessing fish metrics and biotic indices in a Mediterranean stream: effects of uncertain native status of fish L. Benejam Æ E. Aparicio Æ M. J. Vargas Æ A. Vila-Gispert Æ E. Garcı ´a-Berthou Received: 29 June 2007 / Revised: 11 December 2007 / Accepted: 27 December 2007 / Published online: 23 January 2008 Ó Springer Science+Business Media B.V. 2008 Abstract Implementation of the Water Framework Directive requires tools for measuring and monitoring the ecological status of aquatic ecosystems. Several indices are in use in the Iberian Peninsula, although there has been little comparison among them. We sampled the fish assemblage and limnological fea- tures of the Tordera stream (NE Spain) quarterly from September 2001 to May 2003 to evaluate the usefulness of several fish metrics and to compare habitat quality and biotic indices currently in use. Data for eight biotic and abiotic indices for this and three other Catalan river basins were also compiled in order to analyse the relationships among indices. In the Tordera stream, fish abundance and richness increased with stream order except in the last sampling site that had the lowest fish abundance owing to the effects of drought and water abstraction. Although most indices were positively correlated, some displayed low or null correlations particularly for the Tordera basin which is more affected by water abstraction and less by pollution; a commonly used physico-chemical index (ISQA) was the least correlated. In a regional fish index (IBICAT) under development, the brown trout (Salmo trutta) has been previously considered as introduced in the Tordera basin. Here, we report an old published record that demonstrates that trout was present before 1845 and we argue that its status should be considered as uncertain given the current information available. Whether brown trout is treated as native or intro- duced to this river basin has profound effects on the results of fish metrics because of its dominance in the upper reaches. We briefly discuss the role of intro- duced species, particularly in headwater streams, in the development of fish indices. Our study exempli- fies the need for careful, basin-specific assessment of native/introduced status in the development of fish metrics. Keywords Water Framework Directive Á Biotic integrity Á Introduced species Á Brown trout Á Salmo trutta Introduction In December 2000, the European Union enacted a new and ambitious water policy, the Water Frame- work Directive (WFD). According to the WFD, Member States are required to protect and restore all aquatic ecosystems with the aim of achieving good ecological status by 2015. The WFD also requires Member States to measure the ecological status of Handling editor: K. Martens L. Benejam Á E. Aparicio Á M. J. Vargas Á A. Vila-Gispert Á E. Garcı ´a-Berthou (&) Institute of Aquatic Ecology, University of Girona, 17071 Girona, Catalonia, Spain e-mail: emili.garcia@udg.edu URL: http://ciencies.udg.edu/w3/EGarcia/ 123 Hydrobiologia (2008) 603:197–210 DOI 10.1007/s10750-007-9272-1