The importance of the oceans, their wildlife and ecosystems

Under the heading “Boy, 7, is rushed to hospital in a serious condition after a suspected shark attack at a beach south of Melbourne”, the UK’s Daily Mail reported a “possible shark bite” in southern Victoria (Australia) just recently.

Anecdotal reports suggest it was nothing of the sort. It appears the boy might have stood on a small stingray (and is okay). But as usual, the incident stirred up emotions and for most internet-goers who saw the original post, it will probably remain set in their minds as a shark attack. Media are too consumed with breaking news these days to bother about anything as trivial as fact-checking.

I thought I’d use this as a chance to think about what occurred, because misrepresenting shark attacks can have significant economic impacts and cost lives.

I read once that getting bitten by a shark was less likely than photocopying accident. In any case, very unlikely! Australia is nonetheless renowned for its shark activity and it’s any wonder more isn’t done to discourage swimming when sharks are likely to be in full feeding mode. It would certainly be preferable to the indiscriminate killing of ocean animals and sharks by drum lines and the like.

I personally would feel very uncomfortable swimming on any Australian beach morning or evening and would never put my children in that situation. Anyone who dives, knows the ocean wakes up at that time. As the sun lowers, it gets darker and there is less contrast underwater, which makes it harder for fish to see looming predators. When surf conditions are up (the wind blows offshore), pelagic sharks come in to prospect for prey behind the breakers. The sediment and bubbles thrown up, provide even better conditions for these ambush animals.

Sharks are extremely fast and impulsive, grabbing at anything they home in on, as potential food, moving in and out between the confusion and the clear water beyond. When they bite, they are temporarily blinded. When you’re chasing incredibly fast food, it helps to have a kill strategy that is pretty swift and decisive.

It’s the same as a big cats hunting in African grassland. You can do walking trips in Africa among big game but everyone is back safe behind doors or fencing, before the sun gets low. There’s a reason we get a heightened sense of perception at those times and why dusk and twilight are associated with human taboos, like ghosts and hauntings. It’s a survival instinct we have built into our psyche from a time when we were predated by megafaunaThe largest animals that represent the top of the trophic pyramid. These are the final building blocks in ecosystem structures for maximum entropy production. Megafauna can be measured at any spatial scale. The largest animal that ever lived on Earth is the Blue Whale. In a grassland, spiders could be considered megafauna The term is generally reserved for animals larger More.

We could leverage this understanding and these feelings to teach Australians how to better live among sharks.

Seeing a shark while diving or snorkelling is simply amazing. Generally, diving or snorkelling when the visibility is anything less than about 6-8m* hampers our vision, and is more than enough to remove any advantage a predator might have. Further, sharks aren’t looking to eat people … if they can see what you are, they are quite shy and will flee as soon as they see you.

The majority of dive-related incidents are spear-fishers carrying bait in hip pockets – no need to explain why that’s a bad idea.

When ocean swimmers are bitten (an extremely rare event) it’s often morning/evening or in turbid / rough conditions. I don’t like using the word “attack” as it isn’t what the sharks are doing. Sharks follow routine cycles of behaviour and to some extent, this predicts where and when they are more likely to occur. It’s an adaptation to be the most likely to surviveDarwin’s theory how species are formed, where those that stand in closest competition with those undergoing beneficial modification and improvement, will go extinct faster. Natural selection is by survival of the likeliest, not survival of the fittest. The fittest are only likely to survive because they happen to be most suited to the environment into which they are born. The More in modern ecosystemsHow ecosystems function An ecosystem is a community of lifeforms that interact in such an optimal way that how ecosystems function best, is when all components (including humans and other animals) can persist and live alongside each other for the longest time possible. Ecosystems are fuelled by the energy created by plants (primary producers) that convert the Sun's heat energy More and gives us a clue when it’s best to avoid entering the water.

Some incidents in the Whitsundays recently, where people sadly lost their lives, were in areas notably sediment-rich. After rainfall, the water becomes very dirty. Near the mouths of creeks and rivers, this load swirls around, creating corridors of clear water, through which sharks can seek an advantage and search for the tell-tale shapes of potential prey through the landward murk. It’s unwise to swim anywhere off Australia, where freshwater runs into the sea and creates what ecologists call “fronts”. It’s along these edges, that prey concentrates. It’s the perfect storm of conditions that increase the sharks’ probability of hunting successfully. Why would you want to put yourself in among that?

Learning to live with sharks is essential for stable ecosystems and campaigns to alert newcomers to our oceans, of how to avoid dangerous conditions, would almost certainly help save the lives of some unfortunate few. Just as importantly, many many sharks who get a bad wrap for doing what comes naturally, might be saved.

My personal rules to minimise unsuspecting shark encounters are:

1. Never swim near river mouths or where muddy water empties into the ocean.
2. Avoid swimming in water where the underwater visibility is less than about 6-8m*.
3. Never enter high risk locations in the morning or evening, particularly within an hour or so, of dusk and dawn. This is when animals are actively hunting.
4. Especially in the morning or evening, avoid swimming near the back side of breaking waves (although this is where surfers generally hang out and they tend to be very aware of the risks).

These rules aren’t perfect but they will greatly minimise the risk of encounter. Of course, it will also reduce the time you can spend underwater.

*Underwater visibility cues for predators

I once did a study on the effect of underwater visibility reduction on penguin distribution in Port Phillip Bay. Variation in surface secchi depth in Port Phillip Bay (Melbourne) was 7.33m, (±0.9m 95%CI; n=67; range = 1.4 –14.6m). Little penguins were recorded mostly in areas with 6.18m (4.3‐8.1m) underwater visibility. This range of visibility appears to provide reasonable parity between the ability of the predator to find prey and the ability of prey to escape predation. In other words, beyond about 8m, the penguins lose their ability to hunt, as they do below about 4m. Sharks, like most animals, use a suite of cues to find prey and vision is essential in these tens of metres. Because visibility is limited by sediment load, it is likely to be the same, irrespective of the animal. I suspect this limiting distance applies to most marine vertebrate species.