Using diver-operated stereo-video to monitor juvenile fish assemblages in Mediterranean coastal habitats formed by macrophytes

Trabajo presentado al 9th International Symposium Monitoring of Mediterranean Coastal Areas: "Problems and Measurement Techniques", celebrado en Livorno (Italia) entre el 14 y el 16 de junio de 2022.
Temperate coastal ecosystems dominated by macrophytes are diverse, ranging from macroalgal forests to seagrass meadows. They provide numerous ecosystem services and are highly valuable as nursery areas for fish. However, these habitats are under increasing pressure due to anthropogenic actions. In the last decades, the use of Diver Operated Stereo-Video (stereo-DOV) systems has become popular for making accurate, precise, and repeatable measurements of fish lengths. The objective of our study is to develop an efficient video-based methodology for studying the patterns of recruitment of temperate littoral fishes at various temporal and spatial scales. We aim to evaluate our methodology compared to other non-invasive techniques used for the study of juvenile fish assemblages. In this paper, we propose a combination of habitat mapping and stereo-DOV transect methodologies and test them in two case studies to investigate the link between juvenile fish and macrophytes, considering various scales of heterogeneity of littoral habitats. Data on species composition, abundance, and mean total length of the juvenile assemblages and canopy height from each habitat can be obtained and geo-referenced in the sampling sites, mapping their distribution in space and time to understand recruitment patterns. Some benefits of using stereo-DOV transects and habitat mapping for monitoring fish assemblages in comparison with traditional non-invasive methods used for fish counts, are a) videos are permanent, they can be reanalyzed; b) divers do not have to be experts in fish and macrophyte identification; c) inter-observer variability can be controlled through quality checks; d) high accuracy and precision in fish count and length measurements; and e) image data can be used for developments of artificial intelligence (AI). However, there are also some disadvantages: a) cameras have a limited field of view; b) video processing is time-consuming; and c) experts are needed for fish identification during image processing. In conclusion, the combination of stereo-DOV and habitat mapping is an effective methodology for monitoring fish recruitment patterns and habitats at different scales. It can be also advantageous for evaluating temporal changes in the habitat structure, and therefore, in the ecosystem functions provided for the fish community.