Optimising stereo baited underwater video for sampling fish and invertebrates in temperate coastal habitats

Stereo Baited Remote Underwater Video systems (BRUVs) provide a quantitative sampling method for assessing mobile fauna in a range of habitat types and environments. This non-destructive and cost-effective technique is increasingly used for assessing estuarine, coastal and marine fauna. Although single camera systems have been used to assess the relative abundance (Nmax) of marine fish and invertebrates in low visibility North Atlantic near shore and coastal waters, stereo systems remain in their infancy within such environments and their use requires validation. Stereo systems that measure fish and invertebrate size enable size-frequency analysis of populations, providing critical information about life history and population status. We investigated the performance and efficacy of stereo BRUV systems within Welsh shallow coastal waters to test the effectiveness of this method. Data are presented on the capacity of the system to measure fish and invertebrate size under the constraints of low visibility (<2 m). We also present data on the required duration of deployment required to effectively sample relative fish abundance and diversity. Our study recorded 19 species of mobile fauna with species-time accumulation curves providing evidence that a minimum deployment time of 1 h is required to sample 66% of the fish species, and 2 h to sample 83%. Results suggest that the efficiency of surveys for target species (e.g. gadoids) could be maximised by modifying the sampling duration between 1 h and 2 h. The study also demonstrates that measurements of fish and invertebrate length could be taken on 41% of individuals (of those counted in relative abundance measures using Nmax), a statistic that varies between species. This indicates a need to modify the construction of the equipment for improved fish assemblage size frequency analysis. This is particularly relevant in areas and habitats where seine netting and trawling are not possible. We describe how the system can be modified for optimised usage within low visibility waters and suggest potential uses of the system.