What is a picture worth? The value of underwater photographs for tracking change in marine environments

We live in an era of global environmental change and our coastal ecosystems are under intense and increasing pressure from multiple anthropogenic stressors. The rapid pace and scale of change, combined with the resources required to monitor marine ecosystems, means it is infeasible to rely solely on conventional scientific survey techniques. Consequently, scientists are turning to new methods, technologies and data sources to assess these changes in an efficient and timely manner. Utilising people power, in the form of citizen science, is one approach that is increasingly being explored to cost-effectively gather vast amounts of biodiversity and environmental data. Underwater photographs are a potential source of valuable information on marine species occurrence and distribution. Despite their potential value, they are arguably underutilised, in part due to concerns about the potential for bias and selectivity. The objective of this thesis is to explore the valuable information contained in underwater photographs on both the current and past state of reef ecosystems. The thesis consists of a series of methodological studies assessing the data contained in opportunistic photographs including the subject of the observations and the habitat captured incidentally in the background. I found that collectively more fish species are captured by underwater photographers than are recorded by standardised survey transects. This included capturing rare, small and cryptic species which are often missed or overlooked during transect surveys done by divers. Further, I show that the habitat captured incidentally in the background of photographs was similar to that recorded by standardised photoquadrats. These results demonstrate the potential of underwater photographs to not only assess the richness of species inhabiting reefs but also to monitor changes in habitat composition. I also highlight the potential of old diving photographs as a source of information on both the inhabitants and conditions at dive sites in the past and for tracking change through time. Ultimately, much of the value of citizen science data comes from the vast quantities of observations available. I show the considerable potential of computer vision techniques to rapidly extract data from large numbers of underwater photographs. Overall, this thesis demonstrates the value of underwater photographs as a record of past state and current conditions of reef ecosystems and as a tool for rapidly and cost effectively monitoring future change.