There are many ways to measure the status of coral reefs. One of the most common is to use percent hard coral cover as an ‘indicator’ of reef condition because it describes the abundance of a critical ecosystem engineer on coral reefs. This measure describes the proportion of the seafloor that is covered in live hard coral. Percent hard coral cover is widely used by scientists worldwide and is a standard measure that applies to all locations. While it does not tell us anything about the diversity or composition of coral assemblages, it provides a simple and robust measure of reef health.

Percent hard coral cover can be estimated using various techniques. The technique used for this report is manta tow surveys, which are visual estimates of percent hard coral cover over the area covered by an observer during one 2-minute tow (~2000m²). The percent hard coral cover for a reef is then estimated as the average of the estimates from all tows around a reef and reported as broad categories (e.g., 0 = 0%, >0% – 10%, >10% – 30%, >30% – 50%, >50% – 75% and >75% – 100%; See video below).

A coral reef consists of more than just hard coral and contains a diverse array of other corals, sponges, algae, sand, rock and invertebrates. It is relatively rare for GBR reefs to have 75% to 100% hard coral cover and AIMS defines >30% – 50% hard coral cover as a high value, based on historical surveys across the GBR.

Other techniques for determining percent hard coral cover involve counting the number of points within sampling units (quadrats, photos), as used by LTMP in fixed site surveys, or the linear distance along a tape measure (line-intercept) that intersect live hard coral colonies. Adding up the total number of points of live hard coral cover and then expressing this as a percentage of the total number of points within a sample yields the estimates of hard coral cover. Data from both the fixed site and manta tow surveys conducted by the LTMP are highly correlated and show the same trends in hard coral cover estimates. However, manta tow estimates are generally lower than those obtained from fixed site surveys as they encompass the entire reef, including sandy back reef habitats that have low coral cover.