A fish-based assessment tool for the ecological quality of the brackish Schelde estuary in Flanders (Belgium).

This paper presents a new approach which was developed to find an optimal combination of candidate metrics for creating afish-based estuarine biotic index (EBI) for defining the quality status of an estuarine area. The key idea was that a powerfulindex should minimise two prediction errors simultaneously: falsely declaring the status of a site as disturbed while it isnot (Type I error) and the reverse, falsely declaring a disturbed site as undisturbed (Type II error). The balance betweenboth errors is an inherent characteristic of an index and can be displayed as a curve. The area under this curve (AUC) isa measure of the misclassification rate (smaller=better). This criterion was the basis for a stepwise approach whereby ineach step a metric resulting in the highest reduction of AUC was added. Five metrics were selected and the distribution oftheir 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 sitesduring the period from 1995 to 2004. The sites ranged in quality from moderately impacted to very disturbed, classes 3 to5 respectively. Despite there being neither of the highest classes 1 (high) and 2 (good), the EBI presented can serve as anevaluation tool in the highly impacted situation in Flanders as it discriminates well between moderate and highly impactedsites. In addition, its definition complies with the biological status classes of the European Water Framework Directive (WFD).