Rogue Shark

Tragically, there have been two fatal shark attacks off WA in the past couple of weeks. The families of the two victims have my deepest sympathy and my heart goes out to them.

Predictably the media and politicians are jumping on the populist bandwagon with the usual cries of "Rogue Shark" and promises to hunt the maneater down and kill it or install shark nets on beaches that are completely unsuited to them. They appear to be oblivious to the fact that it's incredibly unlikely that it was the same shark and even if it was, these animals are an endangered species and protected in Australian waters. You can't have it both ways. Healthy oceans need top end predators. You can't protect them and hunt them down.

So why don't two attacks in 12 days automatically mean a rogue shark on the loose. Fairly simple really. Firstly, humans offer a very poor return on investment in terms of energy. Great whites that have eaten humans usually regurgitate us shortly afterwards. They really don't want to tie up their digestive system for several days with a meal that has such a low calorific value compared to a seal or dolphin. These animals are also smarter than your average fish. Having made the mistake once, they probably have the smarts not to make it again unless truly desperate.  You might say that energetically speaking, they can't afford to eat us.

This is also why young great whites generally swap from hunting fish to marine mammals at around the three to three and a half metre mark. The energy expended chasing fish isn't recovered in the size of the meal. Once they get big enough they would starve to death eating fish and are forced to go for larger fattier prey.

So lets have a look at the two recent attacks and see if we can learn anything. The more recent attack occured 500 metres off the coast of Rottnest Island, where a diver was attacked by a three and a half metre great white shark.  While media articles have talked about great whites following the whale migration there is a far more likely seasonal food source in this instance.

Fur seals are a favourite food for great white sharks and they can show up off seal colonies at any time of year, however they will travel long distances to get to colonies when the pups are weaning. The young pups are inexperienced and easier prey. Fur seals breed on Rottnest Island pupping in December and the pups wean nine to ten months later, just about now in other words. This is likely to mean more sharks in the area.

The water was reportedly murky. For a visual predator like the great white shark this makes prey harder to catch and they're more likely to go in hard and hit something that they weren't actually targetting, such as a diver. This is probably particularly true for a three and a half metre shark that is still learning to hunt larger prey and may not be as good at identifying prey as an older individual. it may also be hungrier and consequently more aggressive, especially if it has travelled a long distance to get to the colony.

The earlier attack was on a swimmer four hundred metres off Cottesloe Beach. He was out for his regular morning swim. It was early and the water was murky. Great white sharks are known to hunt most actively at dawn and dusk. It is thought that this behaviour takes advantage of the low angle of light falling on the water which causes glare and makes it much harder for animals at the surface to see a shark below the water.

In both of these cases the victims were exceedingly unlucky. However both were in the water at times or places that are known to be associated with a significantly increased risk of shark attack.

So why do these animals improve the health of our oceans. Why shouldn't we eradicate them completely ?

Well lets look at a terrestrial example, the big bad wolf. The wolf was pretty much eradicated in the continental United States although there were still some populations in Canada and Alaska. They were reintroduced to much outcry from ranchers and hunters in a few selected locations such as Yellowstone National Park. In Yellowstone the wolves primarily feed on elk, but will also take deer and occasionally they will take livestock. Elk have for many years been over grazing the park and deer and bison numbers have dropped. They have destroyed vegetation along rivers resulting in erosion and shallower, warmer water with less dissolved oxygen which supports fewer fish. With no riverine vegetation there were no sticks and beaver numbers had dropped dramatically.

When the wolves were reintroduced they went to work doing what they do best, eating elk. Bison and deer numbers recovered, along with other small grazing animals. The riverine vegetation recovered, shading the rivers and providing habitat for fish, aquatic invertebrates, insects, birds, reptiles, amphibians and all the species feeding on these things. Beavers came back and their dams provide deeper, cooler pools with more fish in them and which provide shelter for a range of other smaller creatures which coexist with the beavers. The richness and diversity of the ecosystem changed dramatically with the return of a top end predator.

So what about the livestock ? Well the government does have to compensate the odd farmer for a lost animal. However researchers have observed wolves stalking right through livestock to get to the elk on the other side. Like sharks, these animals know what they like and only deviate from their usual prey in very lean times. So the simple fact is that we need top end predators, without them we don't get a full range of biodiversity and without biodiversity we don't tend to get the stable and sustainable ecosystems that provide us with our habitat.

It seems that both Jaws and the Big Bad Wolf have had more bad press than they deserve. Are these animals potentially dangerous ? Yes they are, but we can coexist with them and if we learn more about them we can reduce our risk of being dinner.