Assessment of the ecological status of coastal areas and estuaries in France, using multiple fish-based indicators: a comparative analysis on the Vilaine estuary

When reviewing the various fish-based community indicators aiming at assessing the ecological status of marine communities and/or ecosystems, a typology consisting of three major components emerges. The first component highlights the choice of relevant metrics associated with a level of organization (e.g. population or community). The second relies on the method used to combine the metrics (an aggregated indicator or a synoptic table). The third refers to the type of analysis (direct or indirect) that is used to establish the link between the metrics and a given pressure. In this paper we use the Vilaine coastal-estuarine fish communities as a case study to discuss and relate two different approaches to the suggested typology. The first approach (time-trend approach) is based on historical series of data, whereas the second approach (multi-metric index, MMI) is based on a geographic series including various French estuaries. They were developed or adapted from French studies but are representative of the approaches used in Europe. When applied to the case study, they differed in their diagnosis: the time-trend approach indicated that the bay of Vilaine habitat was deteriorating, whereas the MMI, which compares many French estuaries, indicated a good environmental state. Differences and complementarities between the approaches are thoroughly discussed with respect to the three mains components of the typology. This work appears at a particular moment, when several fish-based indicators are being developed in France and the rest of Europe. It intends to serve as an element of the ongoing reflection concerning the limitations of the various approaches that can be used to develop such indicators.