Development of low-cost image mosaics of hard-bottom sessile communities using SCUBA: comparisons of optical media and of proxy measures of community structure

Abstract

Underwater image-based sampling procedures, using SCUBA, are compared using imagery collected from a temperate hard-substratum community. The effectiveness of a low-budget, high-resolution image mosaicing technique is assessed by comparing the relative efficiency of data collection, extraction and analysis among sampling procedures. In addition, a manipulative experiment tested whether the sampling procedures could detect the physical removal of 10% of the reef community. Overall, four factors were explored within the data: data collection media (stills and video), cover and community composition estimation techniques (visual cover estimation, frequency of occurrence and point extraction), change detection (pre- and post-impact) and depth (8, 14, 18 and 22 m). Stills imagery sampled the reef community at a higher image resolution than the video imagery, which enabled identification of more species and less-conspicuous benthic categories. Using the visual cover estimation technique, the stills imagery also had the greatest benefit in terms of efficiency and species identification. However, the experimental impact was detected using only the point extraction technique. The recommendations are that: (1) the image mosaicing technique is applied to fixed-station monitoring; (2) the point extraction technique be used for efficient and cost-effective monitoring at coarse taxonomic resolutions; and (3) survey depths remain constant over the duration of hard-substratum community monitoring.

Development of benthic monitoring methods using photoquadrats and scuba on heterogeneous hard‐substrata: a boulder‐slope community case study

“…A problem arises, however, where the diver cannot identify a specimen in situ . Multi‐layered communities, such as those dominated by macroalgae, present additional difficulties for photoquadrat sampling, where canopy taxa may obscure those of the understorey, which affects the estimation of the true diversity of an area (Pech et al , ; van Rein et al , ). However, this problem is unavoidable when working with optical imagery in the sublittoral (Leonard and Clarke, ), where solutions developed for littoral habitats are not practical (Meese and Tomich, ; Dethier et al , ).…”

“…(A) Purpose‐built photoquadrat used to collect all samples in this study. The aluminium frame was constructed around a Nikon digital single‐lens reflex (DSLR) camera in an Ikelite underwater housing, with a single Ikelite strobe, based on the recommendations of van Rein et al (). (B) Photoquadrat sample image (replicate 7, dive transect 3) collected from 30 m depth at Damicornis Bay, in 2010.…”

“…video and stills cameras) collect data quickly, generate permanent survey records, achieve greater sampling objectivity than traditional diver observations and have a low environmental impact in sensitive areas of conservation importance (Bohnsack, ; Bullimore and Hiscock, ; Preskitt et al , ; Moore et al , ; Barrett et al , ; Cabaitan et al , ; Leujak and Ormond, ). Optical sampling using photoquadrats offers additional advantages: fully quantitative data can be extracted from the imagery; imagery is typically collected at a higher resolution than video; and parallax is reduced by the rigid frame (Preskitt et al , ; Leujak and Ormond, ; van Rein et al , ). Accordingly, photoquadrats have become widely used in hard‐substratum community studies, where in particular they are recommended for rapid ecological assessments (Goldberg and Foster, ; Preskitt et al , ; Garrison et al , ; Vroom et al , ; Page et al , ; Coles and Brown, ; Walker et al , ).…”

Fixed‐station monitoring of a harbour wall community: the utility of low‐cost photomosaics and scuba on hard‐substrata

“…The negative effects of water column turbidity (i.e. scatter) can be limited by collecting imagery closer to the substratum, although this reduces the overall sample area, additional imagery can be collected from the surrounding area and mosaicked to increase the final sample size (Burton et al , ; van Rein et al , ). Indeed, image mosaics encompassing 400 m² of sea bed have been used to assess coral reef communities in the tropics (Lirman et al , ; Gleason et al , ).…”

“…The recommendations of van Rein et al () are closely followed to do so: a study in which the effectiveness of a low‐budget, high‐resolution image mosaicking technique was assessed by comparing the relative efficiency of data collection, extraction and analysis methods in a multifactoral design. The final photomosaic method recommended by van Rein et al () is trialled in a harbour setting, as harbours are generally considered easy marine systems to sample (Hewitt and Thrush, ). The following two objectives were tested: To quantify the established benthic community growing on a quay wall and to describe community changes over the seasons. To estimate the biological parameters (i.e.…”