Losing Nemo – bleaching on the Great Barrier Reef


Bleaching of the world’s coral reefs puts many fish species in danger. Image supplied by Tory Chase

The relationship between coral colonies and fish is one of the greatest love stories of the marine world. The only other marine celebrity couples that could compete are anemones and clownfish, which attain stardom akin to ‘Beyoncé and Jay Z’ status. Mutualisms, a form of symbiotic relationships, are when both partners benefit from being in a relationship together (think ‘best buddies’). And like all symbiotic relationships, the one between schooling fish and coral takes many forms. It can include many partners and is constantly in flux with the environment, human impacts, and other organisms trying to get in on this sweet mutualism deal.

That’s where my PhD research comes in. I’m looking at how schooling damselfish (close cousins to Nemo), engaged in mutualistic relationships with corals, impact coral growth, physiology, and health. A great place to look at this is Lizard Island, a tiny island in the northern Great Barrier Reef. Many species of fish, like damselfish, spend their entire lives around a single coral colony, swimming through its branches, defending the coral from predators, and sleeping among the corals’ branches (quite a loving relationship, in my opinion). These fish excrete nutrients (poop!) which the corals love (think manure on farm crops) – the corals can take it up to help growth. Also, the movement of fish around a coral colony enhances water flow in and around its branches, leading to further growth of the coral colony. OK, so what do the fish get out of this relationship, you may ask? They get a cute little home, with lots of space for their mates, access to food that floats by, and a place to hide from predators. Like I said, it’s a sweet deal.

Wait, wait – what does this have to do with coral bleaching, climate change, and other human impacts? Well, as you might know, coral reefs around the world are threatened by climate change through warmer water temperatures associated human activity, increased CO2 in the atmosphere and seawater and El Niño weather patterns. When corals are exposed to elevated seawater temperatures for a long time, they ‘bleach’, meaning they expel their algae symbionts (zooxanthellae), thus temporarily ending their mutualistic relationship with them. Bleached corals are not dead! Bleached corals can continue to survive for several weeks and when the seawater temperature decreases, they can reabsorb zooxanthellae from seawater and regain their healthy status. However, if the sea temperature remains high, bleached corals can get covered in filamentous algae and die.

Bleached reefs can recover, but after a certain period of time, they will die.

Bleached reefs can recover, but after a certain period of time, they will die.

One of my PhD research projects investigates how the mutualism between schooling fish and corals fares during bleaching (high temperature) events. Here’s the theory: slight increases in nutrients and water flow can help corals grow. During bleaching events, however, corals exposed to slightly higher levels of nutrients and water flow will survive better than corals without these two things. Now, here is where the schooling fish come in: we know that fish increase the nutrients around coral colonies, primarily through their poop, and increase the water flow, through their swimming and fin movement. So as a result, the hypothesis is that corals with associated schooling fish will survive better during bleaching events than their fishless counterparts.


Do reefs associated with schooling fish cope better with bleaching events?

Unfortunately, even if schooling fish are able to come to the rescue of some corals, it won’t be enough for the Great Barrier Reef. Currently the Great Barrier Reef is experiencing the worst bleaching in history: 93% of the GBR is affected by bleaching. Constant threats, such as the addition and expansion of coal ports, overfishing, increased industrialisation, and pollution in the forms of sediment and nutrients continue to attack the health of the Great Barrier Reef. (Also of note: the Australian government has reapproved the construction of the Carmichael megamine in Queensland, Australia’s biggest coal mine. This will cause higher emissions than New York City each year; the mine has been decried as an economic and environmental disaster. – S.)


If coral becomes covered in filamentous algae, as in this image, it can die.

On a recent expedition as part of the National Coral Bleaching Task Force, headed by the ARC Center of Excellence for Coral Reef Studies, we surveyed GBR reefs from Townsville to Thursday Island (~2/3 total GBR) and completed in-water bleaching surveys on scuba dives. We continually came up from our dives with tears in our eyes, as we watched vast sections of the reef succumb to bleaching and die. Oh, and this current bleaching is not limited to the GBR – it’s happening in western Australia and across the Pacific to the Maldives. So, is there anything we can do to help bleached corals? Yes – all the usual things we can do (but a majority of people don’t and the current Australian political administrations actively ignores) to help reduce the effects of pollution and climate change. The current government even implemented vicious cuts to climate research lately, so, you know, the future’s bright all round.

So what does this all mean? The GBR, one of the iconic seven wonders of the natural world, will be irreversibly damaged as huge swathes of it die. Tourism and livelihoods dependent upon the GBR will be badly hurt. And the next Pixar movie will most likely be entitled “Losing Nemo”. And for my PhD, it’s time to look at fish behaviour, in relationship to their bleached coral homes to determine if these are ‘damselfish in distress’, how different fish utilise corals, and what happens in environments high in sediments and nutrients.

ToryTory Chase is a marine biology PhD student at James Cook University and ARC Center of Excellence for Coral Reef Studies, in Australia. He’s a United-statesian (the term American is so egocentric) with a passion for hiking, scuba diving, skiing, sailing, teaching, and singing in the shower. His PhD focuses on relationships between schooling fish and corals, and the impacts on coral growth, physiology, and health. Learn more about him here, on ResearchGate or connect with him on Twitter (@Cuttothechase6)

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