How much sampling does it take to detect trends in coral-reef habitat using photoquadrat surveys?

  1. Coral-reef managers must detect and reverse collapses in habitat and evaluate the success of such interventions. Since these responsibilities must be met with limited time and resources,methods used should balance statistical power with practical and logistical constraints.
  2. Photoquadrat analysis is a commonly used method to survey coral habitats. This method, which involves photographing substratum along transects and digitally analysing habitat at points on the ‘photoquadrats’, affords efficiency in the field but is costly and requires extensive desk-based analysis. It remains unclear what is the optimal combination of sampling units (points, photoquadrats and transects) needed to detect important trends in coral habitat.
  3. Here, a dataset on Philippine coral-reef habitats, collected using intensive photoquadrat surveys, was used to explore the reliability of using different numbers of points per photoquadrat, photoquadrats per transect and transects per site to detect spatial differences in habitat.
  4. Results of leave-some-out analyses were compared with analysis of the complete dataset. Using fewer points per photoquadrat and fewer photoquadrats per transect caused little decline in ability to detect key trends, and lessened desk-based time; reducing the number of photoquadrats also lessened field time. Using fewer transects reduced time requirements but at the expense of statistical reliability.
  5. Prospective power analyses revealed that common rates of coral recovery could not be detected using even the most intensive photoquadrat protocols. This result implies that coral recoveries within protected areas might go undetected using standard surveying techniques.
  6. Using fixed rather than randomly placed photoquadrats, or more sensitive indicators of habitat recovery than coral cover (e.g. coral surface area) may improve power to detect coral recoveries. Finally, protocols that minimize desk time rarely also minimize field time and vice versa, which highlights the need to prioritize different logistical constraints when designing methods.

Molloy, P.P., Evanson, M., Nellas, A.C., Rist, J.L., Marcus, J.E., Koldewey H.J. & A.C.J Vincent (2013). How much sampling does it take to detect trends in coral-reef habitat using photoquadrat surveys? Aquatic Conservation: Marine and Freshwater Ecosystems 23(6):820-837.