Exploration

Clean-up crew

In April, the Australian autumn of 2022, Western Australia’s Ningaloo Reef witnessed an annual coral spawning event that didn't go to plan. Sharks emerged as the silent heroes.

Words and photographs by Lewis Burnett

A severe coral bleaching event and subsequent fish kill left thousands of dead fish floating on the surface of the inner lagoon of Bateman’s Bay. It threatened to overload the ecosystem’s carrying capacity for nutrients and cause another die off.

Over the span of two weeks, we were able to watch the event unfold and gain a deeper insight into the roles that our ocean inhabitants play within their ecosystem. It highlighted not only the importance of sharks and their position at the top of the food chain but also what an ocean might look like without them. It was a period of reflection for me, an event that highlighted both the extreme fragility and the stoic power of mother nature. Although I’m deeply sad that this occurred to our reef, I will be forever grateful for the opportunity to observe what I had read about for years. I was able to gain a deeper understanding of how our ocean works and learn more about the importance of keystone species.

When I was younger, I often went snorkelling at a local reef off Perth. It filled me with joy to see everything living in perfect harmony. The golden strands of kelp hypnotically swaying in the waves, light rays dancing through the water scattered over schools of bullseyes and buffalo bream, and octopuses hidden deep within dark caves. Spending time over summer there, I was able to observe individual fish species playing their part in the web of life, helping maintain a fine balance within the ecosystem around them. It was always in the back of my mind that I might encounter something at the top of the food chain, but I only ever had brief glimpses of sharks. At the time, Western Australia was filled with hysteria surrounding sharks. The saying “the only good shark is a dead shark” was heard often. It wasn’t until a visit to Coral Bay on a family holiday that I had my first good look at a shark underwater. I was scuba diving with a family friend at one of the outer reef sites when we came across three beautiful grey reef sharks swimming slowly into the current. I can vividly remember the feeling of awe as these graceful animals moved effortlessly through the current.

As life moved on and I began my career working in dive shops in South-East Asia, I couldn’t take my mind off the beauty of what I had seen on that visit to the Ningaloo Reef. So, when I had the opportunity to take an underwater photography job in Coral Bay, I jumped at the chance. Sharing the water with the charismatic marine megafauna that is found in abundance there, I began to learn about sharks and their roles in our ocean.

Admittedly, coral is something that up until very recently I had overlooked. I spent the majority of my time trying to photograph the larger animals of the reef and neglected the very building blocks that allowed life to exist there. Over millions of years the coral species have developed a survival technique to outsmart fish that live within them. Their reproductive cycle is timed in unison with neighbouring coral colonies. That way, when they release their eggs into the water column, enough survive the onslaught of hungry fish and get evenly distributed throughout the surrounding reef, allowing new life to emerge and regeneration to occur.

This spawning event usually occurs within a week of the first full moon of autumn, a time where traditionally light offshore winds help disperse the floating coral spawn across the lagoons and into the south-flowing Leeuwin current through which the coral spawn can sink and begin its crucial job of growing new reefs.

In 2022, the coral spawned as expected. On a night of magic on the reef, the water column filled with millions of pinky-orange eggs suspended in the inky black waters, slowly floating towards the surface. Unseasonal onshore winds were blowing that night due to a cyclone that was sitting off the Ningaloo coast. Unfortunately, the weather continued for several days, and the onshore winds trapped the floating spawn mass near the beach in Bills Bay. This event, coupled with very little swell, made for the perfect storm as the reef was unable to flush out the nutrient load in time. Eventually, the coral spawn sunk to the floor of the bay and began to rot. This turned the water anoxic, suffocating thousands of fish and rendering the water a dark muddy brown colour. The first night after the spawn, more than 16,000 dead fish washed up on the shores of Coral Bay.

For the next couple of days, the weather pattern stayed the same. Wave after wave, dead fish washed up on shore having suffocated in the stagnant waters. It wasn’t until almost ten days later that the water had settled enough that could see through the water to the corals below. Although expected, it was gut wrenching to see the destruction that this event had caused, killing off almost every single coral colony within a two kilometre stretch of the bay. A couple of the larger Porites stony coral species had managed to cling to their lives but the remainder of the reef was bleached beyond repair.

Corals are complex. A singular coral is made up of a colony of thousands of small individual animals called polyps. They are able to gather up to 20% of their food by filter feeding in the water column, grabbing whatever floats their way with small tentacles. They mostly catch plankton and the microscopic larvae of small fish and crustaceans but I’ve often seen shrimp in their grasp big enough to see with the naked eye. These individual polyps form the ‘stony’ part of the coral by building up a calcium carbonate skeleton over time. It’s this skeleton that makes the coral so susceptible to ocean acidification. It’s also the reason that corals grow so slowly.

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Issue 28
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This feature appears in ISSUE 28: SEA FORESTS of Oceanographic Magazine

Issue 28
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