June 30, 2010

How can a single Reef Check survey be considered sufficient to indicate coral reef health?

Each month, Reef Check will answer a technical question regarding the monitoring protocol of our coral reef or rocky reef programs.

Reef Check Tropical — How can a single Reef Check survey be considered sufficient to indicate coral reef health?

A standard Reef Check (RC) survey involves both transect and manta tow surveys over a large area of reef. The transects cover 800 square meters of reef twice (once for invertebrates and once for fish) and point-sampling of 160 points along 80m of reef to determine the substrate composition. As ecological surveys go, this is not a small unit of sample and because we focus on a small, carefully selected set of organisms, the results are relatively robust. Many coral reef monitoring programs use much smaller sampling units. However, a single RC survey may be adequate to estimate some parameters but not others, and adequacy will depend on several factors, especially the density of target organisms.

Reef Check was designed using a unit-based approach (four x 100 square meter transects) so that with proper sampling design it can be applied to measuring coral reef health at individual sites and on a regional and even global scale. Just as the right tools are required to build a house (you wouldn’t use a hammer to try to saw wood), it is important to use the right sampling design to achieve a particular goal with respect to producing reliable results.

For results covering a large area (scale), a single RC survey replicated at many sites over this area is sufficient to produce reliable results on reef health. For example, at the regional or global scale, a sample of say, 500 sites, spread over the area provides a reliable estimate of average conditions in the area. Conclusions regarding parameters such as mean coral cover and abundance of most indicators are likely to be accurate.

Replication of surveys is also important to obtain reliable results on a local scale e.g. a single reef. While corals do not move, fish do. So while a single survey may be sufficient to estimate coral cover on a certain reef, it will likely be insufficient to estimate abundance of rare organisms. Four or five surveys might be needed to obtain a reliable estimate of the abundance of common fish, and still might be insufficient to provide a reliable estimate of the abundance of rare fish e.g. the humphead wrasse. That being said, if zero humphead wrasse are recorded after five surveys on a reef, that “zero” has meaning! While we cannot estimate humphead abundance, we can say that humphead wrasse are very rare – usually because they have been fished out. This is very useful information both at the local and regional levels for those interested in coral reef management. Before Reef Check existed some scientists claimed that there were still lots of fish, we just weren’t looking in the right places! Because of the widespread RC monitoring program we now know that this is not true.