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Biotic indices, which reflect the quality of the environment, are widely used in the marine realm. Sometimes, key species or ecosystem engineers are selected for this purpose. This is the case of the Mediterranean seagrass Posidonia oceanica, widely used as a biological quality element in the context of the European Union Water Framework Directive (WFD). The good quality of a water body and the apparent health of a species, whether or not an ecosystem engineer such as P. oceanica, is not always indicative of the good structure and functioning of the whole ecosystem. A key point of the recent Marine Strategy Framework Directive (MSFD) is the ecosystem-based approach. Here, on the basis of a simplified conceptual model of the P. oceanica ecosystem, we have proposed an ecosystem-based index of the quality of its functioning, compliant with the MSFD requirements. This index (EBQI) is based upon a set of representative functional compartments, the weighting of these compartments and the assessment of the quality of each compartment by comparison of a supposed baseline. The index well discriminated 17 sites in the north-western Mediterranean (French Riviera, Provence, Corsica, Catalonia and Balearic Islands) covering a wide range of human pressure levels. The strong points of the EBQI are that it is easy to implement, non-destructive, relatively robust, according to the selection of the compartments and to their weighting, and associated with confidence indices that indicate possible weakness and biases and therefore the need for further field data acquisition.

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Arborescent macro-algae forests covering temperate rocky reefs are a known habitat for juvenile fishes. However, in the Mediterranean, these forests are undergoing severe transformations due to pressures from global change. In our study, juvenile fish assemblages differed between pristine arborescent forests (Cystoseira brachycarpa var. balearica) versus an alternate state: bushland (Dictyotales – Sphacelariales). Forests hosted richer and three-fold more abundant juvenile assemblages. This was consistent through space, whatever the local environmental conditions, along 40 km of NW Mediterranean subtidal rocky shores (Corsica, France). Among Cystoseira forests, juvenile assemblages varied through space (i.e. between localities, zones or sites) in terms of total abundance, composition, richness and taxa-specific patterns. More than half of this variability was explained by forest descriptors, namely small variations in canopy structure and/or depth. Our results provide essential cues for understanding and managing coastal habitats and fish populations. Further studies are needed to explain the residual part of the spatial variability of juvenile fish assemblages and to help focus conservation efforts.

The present study focuses on the use of diver-operated video (DOV) as a tool to evaluate the impact of a marine reserve on a fish assemblage, in comparison to underwater visual census (UVC). Samplings were conducted in the vicinity of the marine protected area (MPA) Cerbère-Banyuls (northwestern Mediterranean Sea). Four sites, with different levels of protection, were sampled 4 times, at depths of 5 and 10 m, to study abundance, species richness, structure of fish assemblages and fish aggregation. Results obtained by DOV and UVC provided evidence of significant effects of protection on abundance, species richness and fish assemblages. However, detection of weaker reserve effects using DOV may be limited. Furthermore, certain species are difficult to identify by video (e.g. Symphodus spp.), but DOV may be useful when studying a target and/or easily identifiable species. Contrary to UVC, DOV data can be archived and used for further study. Results concerning fish aggregation within fish assemblages showed specific patterns. Sites outside the MPA were mainly characterized by the absence of fish or few fish aggregations, and sites inside were characterized by high fish density (i.e. dispersed or patched aggregations). Thus, the fish aggregation parameter measured using DOV is an interesting metric to quantify reserve effect. Consequently, although this technique is time-consuming to produce results, DOV presents great potential for estimating MPA effectiveness while allowing simultaneous work on several sites to limit temporal bias.

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