Sharks – who needs them?

Sharks scare the hell out of most people – they’re terrifyingly efficient feeding machines. Everyone has read the stories of shark attacks, some fatal, some not. Some survivors, such as Rodney Fox, spend the rest of their lives attempting to debunk shark myths. Others call for ‘culls’ or ruthless hunts.

So why shouldn’t we just wipe them out? There is at least one very good reason: we need them. Sharks are apex predators, which means they sit at the top of the food chain, and removal (or drastic reduction in numbers) of apex predators has dramatic results down the whole food chain.

A team of Canadian and US ecologists studied populations of various shark species over a period of more than 20 years, and found that depletion of shark numbers led to the destruction of a scallop fishery trade that had been thriving for over 100 years. This happened because the sharks, when plentiful, had preyed on smaller predators, keeping their numbers limited. When the shark numbers plummeted, the smaller predators multiplied enormously, dining on the bay scallops as they went. Goodbye sharks, hello cownose rays, goodbye scallops.

But it’s not just the loss of a century-old bivalve trade that’s at stake. Other areas have lost entire fish stocks due to depletion of sharks, either through overfishing for fins or meat, or through losses as bycatch in tuna and swordfish catches, or trophy hunting. Sharks tend to eat the weak or sick individuals, so when the sharks aren’t removing these, the fish populations can plummet or become extinct.

It goes on: most shark species feed on smaller fish, which mostly feed on phytoplankton. As these photosynthesising microorganisms are responsible for at least half the replenishment of oxygen in our atmosphere, when the shark numbers decrease and the fish they feed on increase, the phytoplankton decrease, and so does the amount of oxygen they produce. Since our own lives are dependent on oxygen, we need phytoplankton, and so we need healthy shark populations in our oceans.

So the next time you hear someone demanding that sharks be killed, let them know that without sharks, they might need to learn to do without oxygen.

 

[Featured image: Great white shark at Isla Guadalupe, Mexico, August 2006. Shot with Nikon D70s in Ikelite housing, in natural light. Animal estimated at 11-12 feet (3.3 to 3.6 m) in length, age unknown. Photo by Pterantula (Terry Goss) at en.wikipedia. Licensed under the GNU Free Documentation License version 1.2 or later.]