Comparison of diver operated stereo-video and visual census to assess targeted fish species in Mediterranean marine protected areas

Abstract

Underwater visual census (UVC) is currently the primary tool used to survey shallow water fish assemblages in the Mediterranean Sea. However, the rapid development of digital technologies, such as underwater video cameras and photogrammetric techniques, are providing new sampling opportunities in marine ecosystems. In this study we compare two non-destructive sampling methods, UVC and diver operated stereo-video systems (stereo-DOVs), to characterize assemblages of targeted fish species. Surveys were undertaken in three zones with different levels of protection from fishing (MR, marine reserve; PP, partially protected reserve; NP, non-protected area) in the Montgrí, Medes Islands and Baix Ter Natural Park (NW Mediterranean Sea). There were no statistically significant differences between stereo-DOVs and UVC surveys in terms of species richness, relative abundance and biomass of targeted fish species across the three protection levels. Both methodologies found statistically significant differences between areas closed to fishing (MR) and areas where fishing is regulated (PP) or allowed (NP), whereby higher fish biomass, density, species richness and larger sizes were found in MR surveys. Length estimates of fish were comparable between methods albeit with some species-specific differences in the size-distribution, and a tendency for UVC to overestimate the size of larger fishes. Our results suggest that stereo-DOVs provide an equivalent and complementary technique to UVC to survey assemblages of targeted fish species. In addition to structural data (e.g., biomass, abundance, and length) recorded by UVC, stereo-DOVs provide a permanent visual record of a fish community and enable additional factors such as fish behaviour, precise distance and length, and benthic composition to be quantified. Stereo-DOVs may, therefore, facilitate the continuity and expansion of long-term monitoring programs of fish assemblages in the Mediterranean Sea.