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

Abstract

BackgroundUnderwater 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.MethodsThe 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.ResultsLine 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.DiscussionOverall, 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.

Aliens in the Aegean – a sea under siege (ALAS)

“…The former will be created by combining existing datasets, e.g. by retrieving data from the European Alien Species Information Network -EASIN - , and from the Ellenic Network on Aquatic Invasive Species -ELNAIS - , and new data collected by extensive underwater surveys organized in the framework of ALAS, following the methodology and field protocols described in Issaris et al (2012), Thanopoulou et al (2018). GIS layers of environmental variables will be based on the Marine Environment Monitoring Services of Copernicus, and on the outputs of previous research projects such as MARISCA.…”

“…GIS layers of environmental variables will be based on the Marine Environment Monitoring Services of Copernicus, and on the outputs of previous research projects such as MARISCA. Species distribution modelling will be based on a Machine Learning Ensemble Modelling approach, as it offers better predictions and is more robust than single-model approaches (Zhou 2012, Shabani et al 2016.…”

“…Habitats will be used as the basic unit to identify impacts associated with individual invasive alien species, as they are easily defined spatially. Habitat maps will be created by improving/expanding previous efforts in the framework of the research project MARISCA (Sini et al 2017. This improvement will be based on an object-based image analysis of fine resolution satellite images (e.g.…”

“…Specific gaps will be identified, and field experiments or surveys will be designed and implemented to enrich existing knowledge. Special emphasis will be given to fragile habitats harboring unique diversity, such as marine caves which constitute an emblematic feature of the Aegean rocky coasts (Giakoumi et al 2013, Sini et al 2017 and are vulnerable to marine biological invasions ). An indicative non-exhaustive list of such field work includes: large-scale caging experiments to study the effects of alien herbivores on macroalgae, manipulative experiments to assess space competition between invasive species and important indigenous structural species, natural experiments to compare communities over invaded and uninvaded habitats, and underwater visual surveys of fish communities to estimate the alien-to-native ratio and invasibility of alien fish in relation to environmental gradients and fishing pressure.…”

“…Marine caves constitute an iconic feature of the Mediterranean Sea with more than 600 caves being distributed along the Aegean rocky coastline and specifically in the islanddominated South Aegean Sea (Giakoumi et al 2013, Sini et al 2017. They are characterized as "biodiversity reservoirs" of great conservation value, harboring a considerable number of rare, protected, endemic and still undescribed species (Gerovasileiou and Voultsiadou 2012).…”