Stereo‐imaging AUV detects trends in sea urchin abundance on deep overgrazed reefs

Abstract

AbstractRemote underwater vehicles are cutting‐edge tools for mapping benthic habitats, yet their reliability for detecting patterns in abundance of benthic species remains largely unexplored. Here, we use a stereo‐imaging AUV to investigate changes in abundance of the overgrazing sea urchin, Centrostephanus rodgersii, which has undergone climate driven range‐extension to Tasmania. As part of ongoing bi‐yearly monitoring of urchin impacts (June 2009–2013), the benthic stereo‐imaging AUV (Sirius) surveyed fixed geo‐referenced 25 × 25 m plots (625 m2) on deep “urchin barrens” (25–30 m depth) and shallow barren/kelp transition zones (8–16 m) at two sites at St. Helens, northeast Tasmania (−41.25; 148.34). Coincident with initial AUV deployments, urchin abundance was also estimated in the same reef plots using conventional SCUBA diver belt‐transects; with comparison of AUV and diver sampling showing AUV‐derived estimates to be ∼ 40% lower; while additional AUV sampling at night (high risk for divers in deep water) detected abundances only ∼16% lower than that measured by daytime divers, demonstrating strong nocturnal emergence of C. rodgersii. Importantly, patterns in C. rodgersii abundance across reefs and depths were similar between methods; and long‐term population trends were concordant between diver and AUV methods. At finer‐scales, AUV detections were compromised where remnant kelp canopies obscured urchins, indicating divers to be superior for detecting early‐warning of population increases within intact kelp beds. Comparison of C. rodgersii with two other macro‐invertebrates (sea cucumber Australostichopus mollis and sea urchin Heliocidaris erythrogramma) revealed that while stereo‐imaging AUVs can detect space/time variability in macro‐invertebrate abundance, detectability is highly dependent on local ecologies and species‐specific behaviours.