A new resource for monitoring reef ecosystems: The background of recreational diver photographs contains valuable habitat data

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Abstract Vast numbers of nature photographs are shared online on various social media and citizen science platforms, providing an important and growing source of ecological data. In addition to the focal subject, which provides species occurrence data, photographs often contain secondary information such as habitat captured incidentally in the background. Here, we test whether the background of underwater photographs taken by recreational divers can be used to monitor reef habitat composition by comparing them to photoquadrats from standardised scientific surveys at three locations in Sydney, Australia. Furthermore, we assessed the effect of different photographic styles (wide‐angle vs. close‐up) and photographic subjects (fish vs. gastropods) on the habitat captured. Over 90% of underwater photographs displayed habitat in the background demonstrating the considerable amount of data available within this unutilised resource. Overall, the habitat composition captured in the background of photographs was broadly similar to that recorded by standardised survey quadrats. Although significant differences between photographs and photoquadrats were detected for most habitats, the differences were small. Furthermore, we found the difference between the two methods could be reduced by using subsets of photographs; for example, fish photographs detected kelp habitat at a similar rate to photoquadrats. Synthesis and applications: The background of underwater photographs from recreational divers contains valuable data that can be used to monitor habitat taxa that are typically under‐represented. To reduce the potential for bias we recommend restricting photographs to a single photographic subject (e.g. fish), using wide‐angle photographs only and obtaining at least 50 photographs per site from different divers when using recreational photographs to assess changes in habitat‐forming species.