This paper presents a new approach which was developed to find an optimal combination of candidate metrics for creating a fishbased estuarine biotic index (EBI) for defining the quality status of an estuarine area. The key idea was that a powerful index should minimise two prediction errors simultaneously: falsely declaring the status of a site as disturbed while it is not (Type I error) and the reverse, falsely declaring a disturbed site as undisturbed (Type II error). The balance between both errors is an inherent characteristic of an index and can be displayed as a curve. The area under this curve (AUC) is a measure of the misclassification rate (smaller = better). This criterion was the basis for a stepwise approach whereby in each step a metric resulting in the highest reduction of AUC was added. Five metrics were selected and the distribution of their average was the basis to derive the thresholds for the classes of the EBI. This paper presents the fish-based index (EBI) for the brackish Schelde estuary in Flanders (Belgium). The index was calibrated against fyke net data from five sites during the period from 1995 to 2004. The sites ranged in quality from moderately impacted to very disturbed, classes 3 to 5 respectively. Despite there being neither of the highest classes 1 (high) and 2 (good), the EBI presented can serve as an evaluation tool in the highly impacted situation in Flanders as it discriminates well between moderate and highly impacted sites. In addition, its definition complies with the biological status classes of the European Water Framework Directive (WFD).Keywords Brackish estuary AE Fish-based index of biotic integrity AE Schelde AE Flanders IntroductionWorldwide, estuaries suffer from ever increasing human pressure (Dennison et al., 1993;Simenstad

Fourteen non-indigenous fish species have been successfully introduced to the wild within the territory of Flanders; nine are considered naturalized. Most of the introductions occurred prior to 1950, with six species introduced since then. This paper reviews the available, hitherto scattered, information (including 'grey literature') on these 14 non-indigenous fish species introductions, and evaluates a decade of data from fisheries surveys to assess the recent development of these non-indigenous populations. Gibel carp Carassius gibelio and topmouth gudgeon Pseudorasbora parva are the most widespread of the non-indigenous species in Flemish waters, and both continue to expand their ranges. A reduction in range has been observed in brown bullhead Ameiurus nebulosus only. A case is presented for not including European catfish Silurus glanis, sunbleak Leucaspius delineatus and European bullhead Cottus gobio on the list of non-indigenous freshwater fishes in Flanders. Also discussed are non-indigenous fish species that are likely to colonize Flanders inland waters in the near future.

Nearly 263 000 individual length-weight (L/W) data, collected during 2839 fish stock assessments between 1992 and 2009, were used to calculate L/W relationships (W = aL b ) of 40 freshwater fish species from Flanders (Belgium). Roach Rutilus rutilus, perch Perca fluviatilis, three-spine stickleback Gasterosteus aculeatus, gudgeon Gobio gobio and European eel Anguilla anguilla were the most abundant species in our surveys. Mean a and b values were in accordance with those available in FishBase. Mean b across all examined species is 3.094 (95% CL = 3.041 to 3.148) indicating a tendency towards slightly positive allometric growth in most fishes (thirty-three species with b > 3). On the basis of a form factor equation, we characterized 2 species as Ôeel-likeÕ, 7 as ÔelongatedÕ, 22 as ÔfusiformÕ and 9 species as Ôshort and deepÕ. This paper adds a substantial dataset to the yet scarcely available a and b values of European populations of freshwater fish species.

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