Measuring coral size-frequency distribution using stereo video technology, a comparison with in situ measurements Joseph A. Turner & Nicholas V. C. Polunin & Stuart N. Field & Shaun K. Wilson Received: 26 July 2014 /Accepted: 12 March 2015 # Springer International Publishing Switzerland 2015 Abstract Coral colony size-frequency distribution data offer valuable information about the ecological status of coral reefs. Such data are usually collected by divers in situ, but stereo video is being increasingly used for monitoring benthic marine communities and may be used to collect size information for coral colonies. This study compared the size-frequency distributions of coral colonies obtained by divers measuring colonies ‘in situ’ with digital video imagery collected using stereo video and later processed using computer software. The size- frequency distributions of the two methods were similar for corymbose colonies, although distributions were different for massive, branching and all colonies com- bined. The differences are mainly driven by greater abundance of colonies >50 cm and fewer colonies <10 cm recorded when using the in situ method. The stereo video method detected 93 % of marked colonies >5 cm and was able to record measurements on 87 % of the colonies detected. However, stereo video only de- tected 57 % of marked colonies <5 cm, suggesting that this method may be unsuitable for assessing abundance of coral recruits. Estimates of colony size made with the stereo video were smaller than the in situ technique for all growth forms, particularly for massive morphologies. Despite differences in size distributions, community assessments, which incorporated genera, growth forms and size, were similar between the two techniques. Stereo video is suitable for monitoring coral community demographics and provided data similar to in situ mea- sure for corymbose corals, but the ability to accurately measure massive and branching coral morphologies appeared to decline with increasing colony size. Keywords Coral size-frequency distribution . Stereo video . Digital imaging techniques . Reef monitoring . Coral population demographics Introduction Coral reefs are facing pressure globally from a range of stresses including climate change, overfishing and hab- itat loss (e.g. Carpenter et al. 2008). Management of these natural resources is critical to conserve Environ Monit Assess (2015) 187:234 DOI 10.1007/s10661-015-4431-8 J. A. Turner (*) : N. V. C. Polunin School of Marine Science and Technology, University of Newcastle upon Tyne, Ridley Building, Newcastle upon Tyne NE1 7RU, UK e-mail: joe.turner@jncc.gov.uk J. A. Turner Joint Nature Conservation Committee, Monkstone House, Peterborough PE1 1JY, UK S. N. Field : S. K. Wilson Marine Science Program, Department of Parks and Wildlife, Kensington, WA 6151, Australia S. N. Field : S. K. Wilson Oceans Institute, University of Western Australia, Crawley, WA 6009, Australia Present Address: J. A. Turner Joint Nature Conservation Committee, Monkstone House, Peterborough PE1 1JY, UK