How many fish? Comparison of two underwater visual sampling methods for monitoring fish communities

WOS: 000435832400008PubMed ID: 29942703Background: Underwater visual surveys (UVSs) for monitoring fish communities are preferred over fishing surveys in certain habitats, such as rocky or coral reefs and seagrass beds and are the standard monitoring tool in many cases, especially in protected areas. However, despite their wide application there are potential biases, mainly due to imperfect detectability and the behavioral responses of fish to the observers. Methods: The performance of two methods of UVSs were compared to test whether they give similar results in terms of fish population density, occupancy, species richness, and community composition. Distance sampling (line transects) and plot sampling (strip transects) were conducted at 31 rocky reef sites in the Aegean Sea (Greece) using SCUBA diving. Results: Line transects generated significantly higher values of occupancy, species richness, and total fish density compared to strip transects. For most species, density estimates differed significantly between the two sampling methods. For secretive species and species avoiding the observers, the line transect method yielded higher estimates, as it accounted for imperfect detectability and utilized a larger survey area compared to the strip transect method. On the other hand, large-scale spatial patterns of species composition were similar for both methods. Discussion: Overall, both methods presented a number of advantages and limitations, which should be considered in survey design. Line transects appear to be more suitable for surveying secretive species, while strip transects should be preferred at high fish densities and for species of high mobility.Research Project "PROTOMEDEA-Toward the establishment of Marine Protected Area Networks in the Eastern Mediterranean" - DG for Maritime Affairs and Fisheries of the European Commission [SI2.721917]This work was supported by the Research Project "PROTOMEDEA-Toward the establishment of Marine Protected Area Networks in the Eastern Mediterranean," funded by DG for Maritime Affairs and Fisheries of the European Commission, under Grant Agreement SI2.721917. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript