This image dataset has been collected as part of a long-term study on the population dynamics of Giant Clams on the Great Barrier Reef by Coral Sea Foundation staff in collaboration with consultant clam biologist Dr Rick Braley
of Aquasearch (www.aquasearch.net.au).
The original survey sites for this study in the mid-1980’s were Lizard Island, Rachel Carson Reef, Michaelmas Reef, and Myrmidon reef. The first three sites were re-surveyed in 2007,
and Lizard Island has been surveyed again in 2017 and annually since.
The GBR, along with reefs all over the world, is suffering an increasing frequency of disturbance as the heat stress events become more common, adding to the existing load of cyclonic disturbances, reductions in water quality due to coastal agriculture and other development, and Crown of Thorns Seastar (COTS) outbreaks. The Great Barrier Reef has evolved in a setting where it is regularly hit by disturbances, and from the ground-breaking longterm research by Prof Joe Connell and others at Heron island on the southern GBR, we know that even under natural circumstances, the coral community rarely gets the 40-50 years it needs to reach an equilibrium composition – at any given reef, the average return time for cyclones on the GBR is about 10-15 years, and a similar frequency for COTS outbreaks.
The individual reefs that make up the system are effectively in a constant state of recovery from disturbance. As we now start factoring in the more frequent coral bleaching episodes, the big questions are now focusing on the dynamics of recovery; firstly, whether the reef is losing its ability to recover, and secondly, as the recovery occurs, are we seeing shifts to more stress-tolerant species in the coral community? In particular, we know that the beautiful branching corals from the genus Acropora are very susceptible to bleaching stress, and many researchers have suggested these corals will be the first to go extinct as the climate impacts keep rising.
The Lizard Island Case study – Lizard Island is one of the best studied reef systems on the GBR, through having had the Australian Museum’s Lizard Island Research Station operating there since the late 1970’s. We have really good data on the coral community dynamics at the location for several decades, and the whole island is within a Marine Protected Area. I have personally been visiting the island since 1991, and have detailed imagery of the coral community at multiple locations around the island dating back to about 2005. Furthermore, in collaboration with Dr Rick Braley from Aquasearch, we have been mapping the populations of giant clams at two sites at the island since the mid 1980’s.
In general, the coral cover around the island has been high to very high, with a COTS outbreak between 1993 and 1998 being the main disturbance event until the more recent cyclones and bleaching. Starting in April 2014, Lizard Island suffered 4 major disturbance events back-to-back over 4 years – Cyclone Ita (April 2014), Cyclone Nathan (March 2015), Strong Bleaching event (early 2016), mild bleaching event (early 2017). Both cyclones were category 4, and the storm eye’s passed close to the island, with winds above 200km/h. There was extensive damage at exposed sites, with delicate corals broken and even large multi-ton heads of porites rolled around on the reef slopes.
The 2016 bleaching event was the most severe recorded on the GBR since European occupation, and Lizard Island was right at the epicenter of the worst-hit region, with extensive coral mortality seen. The important point with relevance to this study, is that virtually all the adult colonies of branching Acroporid corals around Lizard Island, and indeed most of the nearby reefs within 100km, were killed. So from the perspective of reef resilience and reef recover, this situation provided us with an almost complete reset of the coral community, as only the toughest of the tough corals survived. This time series video of the reef at North Point will give you an idea of how extreme these changes were: https://youtu.be/UBQy3oxwgqU
So by mid-2016, some prominent coral reef scientists were suggesting that the ecology of the GBR had been fundamentally altered, and that coral community recovery at Lizard Island might be strongly delayed because of the lack of parent colonies within the nearby area. I went to the island in June-July 2016 on one of my regular teaching field trips and I noticed a couple of things. Yes, the coral cover around much of the island had been severely reduced. But at our regular research sites in Watson’s Bay in the lee of the island (eg. Clam gardens) I noticed there were still quite a few juvenile colonies of Acropora alive on the reef, and not only that, but they were in at least three different size classes, corresponding to corals spawned in 2013,14 and 15. So not only had these corals clearly survived the worst bleaching event ever seen on the GBR, some of them had survived both cyclones as well!
After more bleaching was seen on the GBR in early 2017, the statements about the decline of the GBR were
coming thick and fast, both in the scientific literature and the popular media. I returned to Lizard again in JuneJuly 2017, with Dr Braley, and at that time we completed another full census of the giant clams at our two study sites, one of which is the clam gardens. At that time I noticed there had been extensive recruitment of small Acroporid corals all over the clam gardens site, including onto the clam shells themselves, with hundreds of baby corals about the size of a 5c coin, along with the older ones that had been there the year before. This immediately raised a couple of very interesting questions – if the recovery potential of the reef around Lizard had been badly damaged, and virtually all adult colonies within a 100km radius killed off, then where had all these recruits come from, and were they from bleaching-tolerant parents?
So as we began the process of measuring and photographing all the clams at our study sites in 2017, I decided to also photograph every coral recruit that I could see on the clam shells. In the end we obtained images of about 350 juvenile corals on the clams, in at least 4 different size / year classes. I had a meeting with coral taxonomist Dr Jacqui Wolstenholme at JCU and we attempted to make some provisional identification of the specimens from the image dataset, and the final result was that there were at least 26 different species of Acropora represented at the clam gardens site.
So in other words, at a site that had been smashed the hardest by the cyclones and bleaching event, we were seeing clear evidence of reef recovery taking place, with new juvenile corals from a whole suite of species settling over at least the previous few years, and a big cohort from the late 2016-spawning event settling all over the reef at the clam gardens site. At this point (2017) I will note that the clam gardens site had the best juvenile coral settlement of the whole island, with most other sites fairly bare. The good news is that waves of settlement have taken place at other locations around the island since then, and especially on the exposed reef crests, which have been transformed like this:
Now that we have re-photographed (most of) the clams again in 2018 and 2019, we have a great opportunity to extract some really precise and useful information about the species composition, growth and survivorship of the cohort of Acroporid corals that have settled onto the clams at the clam gardens site. While several ongoing studies are looking at settlement of juvenile corals to artificial settlement media (ceramic tiles) around the island, there are far fewer studies looking at settlement of corals to the actual reef substratum, and there is also evidence that coral settlement to artificial tiles may not always be the same as settlement to natural space on the reef. (corals are smarter than we think – why settle to a weird tile when there is a nice piece of available reef real estate nearby?)
So the first step in this data analysis is identifying the clams in the yearly image sequences. Once we have that, then the next step will be to go through and identify the individual corals from each clam and estimate their growth rates, which as you can see are amazing!
Words By: teamnogal