Technical details of coral cover stats, and context – Emerging with that?

By Peter Ridd – August 4, 2022

Since 1986, the Australian Institute of Marine Science (AIMS) has surveyed around 100 of the Great Barrier Reef’s 3000 reefs (GBR). Ridd has been using this data to build coral cover since 1986 (Figure 1) showing the GBR has a record high coral cover.

Figure 1: Coral cover since 1986: Coral cover in 2022 is very high.

What is coral cover? Coral cover is the percentage of the seafloor that is covered with coral. It is a measure of coral abundance. Coral cover decreases after major cyclones, when the number of coral-eating starfish increases, and after several bleaching events. Coral cover at a given location usually takes five to ten years to recover from these events. Coral cover on an individual reef can drop to just a few percent after a mass die-off event. It fluctuates naturally over time.

AIMS data: Raw data can be found at this link

https://www.aims.gov.au/docs/research/monitoring/reef/latest-surveys.html

The data for GBR is divided into three regions, Northern, Central and Southern. These regions are divided into ‘sectors’ with 3, 5 and 3 sectors in the Northern, Central and Southern regions respectively. For each of the 11 domains, approximately 5-10 individual reefs were surveyed. Surveys of each reef were carried out by dragging a diver around the perimeter of each reef. The diver observed the coral cover from a distance of 140 meters and recorded an estimate of the percentage. Each reef is many kilometers per mile around its circumference so there may be about 50 to 100 of the 140 m long cross-sections for each reef.

The coral cover of each reef is the composite of the coral cover of all 140 meter long sample points. The coral cover for each area is an aggregate of all coral reefs sampled in the area. Coral cover for key areas can be calculated by aggregating data from contributing industries. Coral cover from harvested GBRs can be calculated by aggregating data from all 11 fields (note: AIMS no longer performs this final calculation).

Industry data, as found on the AIMS website, is shown in table 1 below.

Table 1: Coral cover for each area of ​​the Great Barrier Reef in 2022. Uncertainty estimates vary, but typically range from 5% to 10% according to the AIMS graph for an individual area.

Using these data, coral cover for the entire reef could be calculated by averaging all sectors, and was found to be 33.9% with an uncertainty of about 4%. This assumes the density of each area is equal. AIMS no longer performs this final calculation to average the entire GBR (33.9%), i.e. AIMS no longer provides the final mean statistic of most interest. It shows data for individual reefs, sector data and area data, but does not show average/aggregated data for the entire GBR.

However, by 2016, AIMS do published averages for the Great Barrier Reef (e.g. see the report for 2016/17 found at the link above) as shown in figure 2 below.

Figure 2: Screenshot of the coral cover for the entire GBR from the AIMS website.

To generate the average GBR data from 1986 to 2022 (Figure 1) Ridd used the graph published by AIMS (Figure 2) for 1986 to 2017 and from 2017/8 to 2022, the data Sector data (as shown in Table 1 for 2022) are averaged (see Table 2).

you 2: Summary of AIMS data since 2017/18. The yellow items indicate the field was not surveyed and the results of the previous survey were used.

AIMS has effectively masked very good Reef news in 2022 by not publishing GBR average data since 2017. This is highly unusual for all three major regions and for most. All areas are above average. any time in time. For example, waves caused by a large tornado will often kill large amounts of coral over a large area, so some industries are often recovering from such an event and have low coral cover. . Only by looking at all the data aggregated as averages for the entire reef can the particular state of the coral cover be assessed. AIMS shows graphs for all three major regions, and all have very high coral cover – but none have hit a record high. Because there is about a 1/3 chance that an area has very high coral cover, there is only a 1 in 27 chance All three simultaneously high. 2022 is special because all three regions have very good coral cover at the same time.

What is surprising is that AIMS no longer provides average coral cover for the primary GBR as it has previously made sweeping claims about the poor condition of the GBR based on the GBR-wide average . For example, when coral cover reached a low level in 2011, after large cyclones destroyed large amounts of coral, the AIMS authors (De’ath et al., 2012)[1] wrote in a very famous article, widely cited in the world media, as follows:

Without significant changes to coral disturbance and growth rates, coral cover in the central and southern regions of the GBR could decline to 5–10% by 2022. The future of the GBR therefore depends on decisive action. Although governments around the world continue to debate the need to limit greenhouse gas emissions, reducing local and regional pressures is one way to strengthen the natural resilience of ecosystems (7, 9).

This 5-10% prediction for 2022 turned out to be incorrect as the average coral cover for all areas is currently over 30%. By no longer publishing the GBR average, it obscures the good data for 2022 and their inaccurate prediction of a decade ago.

Reason given for AIMS no longer providing GBR average data

One reason, informally stated, seems to be that the AIMS considers a single number (average) not to represent the full diversity of reef conditions. That is true, however the average is still an interesting statistic and data on area, area, reef and 140 m cross-section are available for a more detailed discussion of the data.

AIMS is inconsistent. It aggregates cross-sectional data to generate a unique number for each reef surveyed. It aggregates reef data to generate an average for an area. It aggregates reef/area data to generate an average for each area – so why doesn’t it aggregate reef/field/area data to generate an average for the whole reef set?

However, AIMS should be congratulated for having collected such a remarkable dataset over 3 decades. That’s a huge data set, and Ridd estimates that AIMS pulled a roughly equivalent diver around the world during this time.

Why the coral is not 100% on the reef:

Coral cover is the percentage of the seafloor that is covered by coral. It is generally assumed that coral cover should be 100% on a healthy reef. However, a coral reef is made up of many different ecosystems. These include coral sands made from broken coral, ancient dead coral ‘rocks’, soft corals, layers of algae and shell coralline algae, a hardy algae that helps bind dead coral together on a surface. Coral reef. Dead coral is like concrete – it doesn’t rot like wood. Corals grow on the corpses of their ancestors, and in doing so create ‘reefs’. Most of the GBR’s reefs have built up 50 to 100 meters above the surrounding seafloor over the past million years.

Last comment:

The latest data on GBR shows it is in good condition. It happens to have a lot of coral in 2022 as there have been very few major death events over the past 5 to 10 years. Three of the four catching events since 2016, which have been widely reported in the media, could not have killed many corals otherwise the 2022 statistics would not have been so good.

Data from 1986 show that every area, every area, and most reefs has had periods of very low coral cover. This is completely natural. Much of this information is often given out in the media. But the measure of a system’s health is its ability to recover from a major stressor. Weak systems will not recover. Strong system recovers well. It is similar to the good resilience of healthy people from inevitable diseases like covid19. Sick people are often killed by disease. GBR has proven to be a vibrant healthy ecosystem. This is not a surprise; there is minimal human pressure on the reef and it is well protected. It is also unreasonable to expect that the small increase in temperature over the past century (1^oC) will have many effects, especially given that most corals grow faster in warmer water.

Data collected by AIMS shows that the GBR is a robust system with rapidly fluctuating coral cover. We must expect that, at some point in the future, a sequence of events will halve the coral cover, as happened in 2011. Then, we must remember that this is almost certainly natural, and does not allow the perishable merchants to debilitate the children. .

[1] De’ath, G., Fabricius, KE, Sweatman, H. and Puotinen, M. (2012). The 27-year decline of coral cover on the Great Barrier Reef and its causes. Proceedings of the National Academy of Sciences109 (44), pp. 17995–17999.

For more on this topic, see here and here.

Peter Ridd Co. developed a series of world-first optical and electronic instruments for measuring environmental conditions near corals and other ecosystems.