Abstract
Monitoring fish assemblages is needed to assess whether Marine Protected Areas (MPAs) are meeting their conservation and fisheries management goals, as it allows one to track the progress of recovery of exploited species and associated communities. Underwater Visual Census techniques (UVC) are used to monitor fish assemblages in MPAs. UVCs should be adapted to fish abundance, body-size and behaviour, which can strongly affect fish detectability. In Mediterranean subtidal habitats, however, UVC strip transects of one surface area (25x5 m2) are commonly used to survey the whole fish assemblage, from large shy fish to small crypto-benthic fish. Most high trophic level predators (HTLPs) are large shy fish which rarely swim close to divers and, consequently, their abundance may be under-estimated with commonly used transects. Here, we propose an improvement to traditional transect surveys to better account for differences in behaviour among and within species. First, we compared the effectiveness of combining two transect surface areas (large: 35x20 m2; medium: 25x5 m2) in quantifying large, shy fish within and outside Mediterranean MPAs. We identified species-specific body-size thresholds defining a smaller and a larger size class better sampled by medium and large transects respectively. Combining large and medium transects provided more accurate biomass and species richness estimates for large, shy species than using medium transects alone. We thus combined the new approach with two other transect surface areas commonly used to survey crypto-benthic (10x1 m2) and necto-benthic (25x5 m2) species in order to assess how effectively MPAs protection the whole fish assemblage. We verified that MPAs offer significant protection for HTLPs, their response in terms of biomass and density increase in MPAs was always higher in magnitude than other functional groups. Inside MPAs, the contribution of HTLP reached >25% of total fish biomass, against < 2% outside MPAs. Surveys with multiple transect surface areas allow for a more realistic assessment of the structure of the whole fish assemblage and better assessment of potential recovery of HTLPs within reserves of HTLP.
Highlights
The use of marine protected areas (MPAs) as a tool for fish conservation and fisheries management throughout the world’s oceans has encouraged the development of non-destructive methods to monitor biodiversity and assess the performance of MPAs [1,2,3]
Trends were consistent for M. rubra, S. viridensis, D. cervinus, E. costae, S. umbra, E. marginatus and S. aurata
Previous studies have demonstrated how the abundance of high trophic level predators can increase within MPAs up to several years after the introduction of protection [53], and longterm monitoring programs are, essential to establish when carrying capacity is reached [53]
Summary
The use of marine protected areas (MPAs) as a tool for fish conservation and fisheries management throughout the world’s oceans has encouraged the development of non-destructive methods to monitor biodiversity and assess the performance of MPAs [1,2,3]. UVC with transects and fixed points are often more convenient than video methods Despite their known biases (i.e. observer effects, errors in fish body-size and sampling surface-area estimations) [12,13,14] and constraints (depth and time dive-limits), SCUBA observations are usually cost-effective. They enable the detection and identification of a high number of species, as well as quantitative descriptions of the fish assemblage such as density and biomass which are difficult if not impossible to obtain using video methods [3,15,16]. Distances from the observer adopted in the literature for these fish range from 5 to 15 meters [1,6,17,19]
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