Video transects as a complement to underwater visual census to study reserve effect on fish assemblages

Abstract

The present study focuses on the use of diver-operated video (DOV) as a tool to eval- uate the impact of a marine reserve on a fish assemblage, in comparison to underwater visual cen- sus (UVC). Samplings were conducted in the vicinity of the marine protected area (MPA) Cerbere- 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 sig- nificant 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 tar- get 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 pat- terns. Sites outside the MPA were mainly characterized by the absence of fish or few fish aggre- gations, and sites inside were characterized by high fish density (i.e. dispersed or patched aggre- gations). 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 simultane- ous work on several sites to limit temporal bias.