Monday, 27 April 2015

What Lies Beneath

A Closer Look at a Gold Coast Predator.


To live on Queensland's Gold Coast is to live in a place obsessed with fitness. On any given day of the year, at any time of day, the beaches, parks and public spaces are packed with people undertaking their chosen fitness pursuits: running, swimming, surfing, weight-lifting, kayaking, power-walking and more. At 84 years of age, even Gold Coast resident Bob Purcell made sure to live a healthy lifestyle. Previously a champion lawn-bowler in the Commonwealth Games, Bob had always led an active life and had for many years enjoyed a regular morning swim in Burleigh Lake. Connected to the sea by a network of canals, this estuarine lake is sheltered from the pounding surf of the nearby beach, and perhaps this is why it was his Bob's favoured swimming location. Or perhaps it was the convenience: the highly regarded retiree lived in one of the houses surrounding the lake, in the affluent suburb of Burleigh Waters. Regardless of the reason, when Bob entered the lake on the morning of Saturday the 8th of February in 2003 for his daily swim, it would prove to be his very last--he was never seen alive again.


Burleigh Lake.

At some point on his morning swim, Bob had encountered another Burleigh Lake local: a two-metre long Bull Shark (Carcharhinus leucas). The great fish had struck his lower right leg so hard that the kayakers who found his body later that day noted that the limb looked almost severed. In his incapacitated state, Bob had been unable to reach the shore and died from blood loss right there in the water.

The story made headlines around the nation but was most notable for its similarity to an incident six weeks earlier, when 23-year-old Beau Martin was attacked and killed on a late night swim, less than one kilometre away in Lake Heron. The remains of Beau--a Doctor of philosophy and a youth ambassador-- were discovered by his father two days later on Wednesday 18th December 2002. His left leg had been bitten off below the knee, and it appeared that in total, the shark had made three lunges at the young man.

Unfortunately, it took these two incidents for Gold Coast residents to fully understand that their canals, rivers and connected lakes are simply not safe places to swim. That our recreational choices should still be at the mercy of nature, despite millions of dollars being spent on waterside development and lifestyle promotion, no doubt seems insulting to some. The Bull Shark belongs to a family of sharks that have been on this planet for approximately 50 million years however, and plainly they just don't seem to care about our lifestyle desires. Nevertheless, we have good reason to adapt to theirs.

Bull Sharks are unique among sharks in that they can tolerate both salt and freshwater. They are able to penetrate further into estuaries and up rivers than any of their ocean-going relatives, and thus find these protected waterways to be ideal nurseries for their young. The vast majority of sharks in the Gold Coast's 400-kilometre long canal system are juveniles, less than one metre long. Anglers regularly report seeing large numbers of them on certain occasions, with a trawler fisherman in the nearby Logan River catching 64 in one evening. Apart from humans, there's just one main predator that a young Bull Shark has to worry about: the adults of their kind.

Bull Shark. Photo by Albert Kok, courtesy of Wikipedia Commons.

The bulk of a Bull Shark's diet consists of smaller fish, and there are plenty of these in the Gold Coast canals. On a recent fish survey I carried out in Burleigh Lake, I encountered healthy schools of Yellowfin Bream (Acanthopagrus australis) and mature Snubnose Garfish (Arrhamphis sclerolepis), both potential prey items. The juvenile sharks in these waterways would also consume various crustaceans and molluscs, while the larger adults eventually expand their diet to include sea turtles, birds, dolphins, stingrays and other sharks.

Despite such a varied diet, humans do not seem to trigger a Bull Shark's appetite, even if at first glance, the evidence appears to suggest otherwise. In both of the shark attacks mentioned above, it is important to note the relative intactness of the human bodies recovered from the water, particularly in the case of Bob Purcell, where a single bite seems to have been his undoing. Scores of shark attacks in Sydney Harbour in the early 20th century feature similar trauma to limbs, but not wholesale consumption of humans the way that Tiger (Galeocerdo cuvier) and Great White Shark (Carcharodon carcharias) attacks sometimes do. There are two possible explanation for this: firstly a case of mistaken identity, where the flashing of pale palms and soles resembles a silver fish to these turbid water-dwellers, and secondly, that they are territorial animals which protect their hunting grounds. Both of these scenarios are equally conceivable explanations for the Gold Coast canal attacks, though personally I lean towards the latter. Bull Sharks live in all of the murky rivers along the South-east Queensland coastline, and yet fatal attacks have not occurred in these locations for almost one hundred years, despite their increasing recreational use in this time. Ironically, it is perhaps our artificial design of the canal systems--with their dead ends and lack of mangrove and seagrass cover-- that encourages such territorial behaviour in these sharks, as they can more easily herd schools of target fish species in this modified habitat. When they are forced to share the water with a large, disruptive mammal during their peak hunting period in prime territory, it is feasible that they would deal with this unknowing interloper in the way most familiar to them: with their teeth. 

So what does this mean for the fitness and lifestyle asperations of Gold Coast residents? Sage swimming advice would be to do so at one of the many patrolled ocean beaches in the area, always during daylight hours and preferably not during or shortly after large rainfall events. The canal systems would still be available for recreational activities such as kayaking, fishing and paddle-boarding, where contact with the water is minimal. 

Suggestions that the canals could somehow be depopulated of their sharks are ludicrous and flawed in their thinking. Comprehensive netting or underwater gates would potentially lock sharks into the waterways if not careful, and regular summer flooding would still grant sharks access anyway. Such proposals would also adversely affect the other forms of marine life often observed in the canals, including dolphins. A better solution would be to follow the example used in Sydney Harbour, where small swimming enclosures have been set up in various inlets so that people still have the opportunity for bathing in an otherwise dangerous location.

Ultimately, Bull Sharks are fascinating animals that belong to South-east Queensland as much as you or I, and they have an important role to play in our environment. Just like all predators at the top of the food chain (except humans, arguably), they function as an essential selection process, weeding out the sick, diseased, old and genetically weak organisms around them. This ensures a healthy ecosystem that is beneficial to us in more ways than we will ever know, and around which entire economies, lifestyles and communities are based. A world without sharks is a world out of balance, so play safe and treat them with respect.


12 comments:

  1. i'd not have considered them to be in the canals. that would certainly keep me out of the water.

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    1. Me too! When I undertook my fish survey there, I had to wade in knee-deep and even that unsettled me! :)

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  2. You have written a very informative post Christian. Your analysis of the "problem" is eminently sensible and well balanced. In such issues we humans often forget we are but bit players in the grand scheme and often make wrong decisions based on selfish agendas.

    As you point out, there must be countless places in your part of the world where humans can practice their sports and pastimes without intruding upon and often damaging the natural world.

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    1. Thanks very much Phil, that is kind of you to say. I agree that there is room for all of us!

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  3. What a great post! I am surprised to hear they are in the canals and lakes..Providing a safe swimming area does sound like the answer.. I would not want to be in the water with these sharks.. Enjoy your day!

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    1. Thanks Eileen, yes it's not the first location you'd think would be 'sharkey'!

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  4. ,,, "Just when you thought it was safe to go back in the water." - Jaws

    Interesting read, I think there was a "River Monsters" episode on these Bull Sharks.

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    1. Haha, I did actually throw in a 'Jaws' reference in there, when I called one of the sharks "the great fish", as Peter Benchley does in his book.

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  5. Thanks for championing the sharks and defending their right to exist, despite sometimes negative interactions with people. You say that there haven't been mortalities in SEQ rivers, but I recall seeing a small collection of old newspaper reports about shark attacks since settlement - published by a local historical society I think - that included reports of fatalities in the Brisbane river. Perhaps these were never verified? Cheers, Paula

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    1. You're right Paula, there are actually historical records of fatal attacks, the most recent being in the 1920s. Thanks, I'll edit the post to reflect that!

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  6. Lake Kawana is man made and when it opened it was declared to be shark free and we used to have Surf Lifesaving nipper training there for extra training during the week. After a couple of years it was realised that young bull sharks were capable of finding their way in and growing once they had arrived in such an ideal location so training was stopped. I prefer to swim in the ocean any day!

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    1. I have always been suspicious of the Sunshine Coast waterways, Sri! I didn't know this, thank you! I get nervous in Currimundi Lake sometimes too, but I haven't dug up any shark reports from there yet. I prefer the ocean also - give me clear water in one of those lagoons behind the sand bar and I am set for the day! :)

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