Shark-hunting harms animals at bottom of the food chain

Blogging on Peer-Reviewed ResearchOn the surface, plummeting populations of sharks do not seem like much cause for concern for humans or, for that matter, other sea life. But this simple viewpoint relies on splitting animals into two groups - predators and prey. In practice, this distinction is far too crude. Too put it bluntly, there are predators and there are predators. Those at the top kill those in the middle, and stop them in turn, from killing those at the bottom. As the old saying goes, the enemy of my enemy is my friend.

i-3323542457f92cf9c4c93f31f90d3327-Tigershark.jpgThe rise in shark fishing is mainly driven by a growing market for their fins. Shark fins soup is a delicacy in China, which is utterly ludicrous given that the fins themselves are tasteless and merely add texture. China's strong economy has put this expensive treat in the range of the expanding middle classes and the world's sharks are paying the price for it.

Ransom Myers from Dalhousie University, Halifax, decided to study the effects of declining shark numbers by analysing a uniquely comprehensive shark census taken over the last 30 years on the American eastern seaboard. In these waters, several shark species have all but vanished since 1972, including 99% of the bull, dusky and smooth hammerhead populations. And because fishing expeditions tend to catch larger individuals, the average size of the survivors has plummeted. Mighty animals like the tiger and black-tip sharks are now up to half as long as they used to be.

Unsuprisingly, as the sharks declined, their prey benefited. Great sharks mainly hunt smaller predators, including their close relatives skates, rays, and indeed, smaller sharks, whose numbers surged in their absence. The cownose ray, for example, is now ten times more common than it was in the mid-70s. And here's where the domino effects begin.

It turns out that large sharks inadvertently carry out a sort of protection racket for animals at the bottom of the food chain. By taking out the mid-level predators, they prevented these lesser hunters from decimating stocks of small fish and invertebrates.

i-8b92971264e0915b9b8ea9cbc2a17046-Shark-fin-soup.jpgThe cownose ray feeds mainly on shellfish like scallops, clams and oysters. In the 80s, their small numbers made little dent on the local scallop population which sustained the economies and stomachs of local seaside towns. But in 1996, the ray explosion started to spell the end for the scallops. By 2004, the local North Carolina scallop fisheries which had thrived for centuries were forced to close and remain closed to this day. Little did the locals imagine that the disappearance of dangerous sharks from their waters could have such strongly felt economic consequences.

This is far from an isolated incident. In Ariake Sound off Japan, shark fishing is particularly intense, and a booming population of long-headed eagle rays has decimated shellfish populations just like their cownose cousins in the Atlantic.

This is one of the first times that the removal of apex predators has been so thoroughly studied in the ocean. On land, the consequences are well-known. Just last year, Australian scientists found that in some areas, persecution of the local top dog - the dingo - has allowed introduced predators like cats and rats to kill off up to two thirds of ground-dwelling mammal species.

As for the great sharks, the responsibility for preserving these great animals lies with the only predators they themselves face - the fishermen who kill them, and the Asian restaurant-goers who create the demand for their fins. Surely, the price of irrevocably altering an ecosystem is too high to pay for the right to eat textured soup?

Reference: Myers, Baum, Shepherd, Powers & Peterson. 2007. Cascading effects of the loss of apex predatory sharks from a coastal ocean. Science 315: 1846-1850.

Images: by Terry Goss and Arthur Hungry


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Out of curiosity (I'm not a biologist) - what does population biology predict the long-term effect would be? If for instance the apex predator sharks were to go extinct: would the predator-prey dynamics oscillate back and forth lower down the food chain, until they restabilize into some new but sane equilibrium, or would it completely fall part? In other words, are we more likely to see entire extinctions, or moderately reduced populations with less genetic diversity? Is such a system closer to being 'robust' or 'fragile'?

Because as far as I can reason, a state with an excess of predators and a shortage of prey can not persist very long.

By anonymous (not verified) on 08 Nov 2008 #permalink

So I am a biologist. There is no easy way to predict what will happen when a top predator dies, but I can give you an example of what happened in another aquatic ecosystem. Killer whales normally eat seals, but as seal populations have declined killer whales have started eating smaller prey items, such as otters. Among other things otters eat a lot of sea urchins, which eat kelp. With the decline of otters came the rise of the urchin populations and because sea urchins are rather aggressive they ate all of the kelp in the area. Once once was described as a "kelp forest" became a barren urchin zone. I say barren because the kelp supplied food and shelter for many other species that zones covered only in sea urchins don't supply. However, these were only local extinctions meaning that the species still exist just not in those areas.

The decline of sharks may paint a different picture because sharks migrate much more than otters do. This could mean that the effect would also be more widespread.

And as a last point, there are several stable systems in which population numbers of predators and prey osculate.

Are ecosystems knocked out of balance, they are also constantly evolving, in a way that often makes it impossible to return to the original state. That happened with the Cod.

Hope the fast food giants like McDonald's and Starbucks change the taste buds of the Chinese. At one time the Chinese had to eat anything they could offer nutrition, but why eat shark fin now if it's tasteless? I had bird nest soup once, it was horrible. With 1.5 billion Chinese it is easy to overwhelm any food supply and now that they are getting wealthier they need to think more about their actions on a global scale. That applies to all of us of course.
I am an old guy and have seen many changes in the local marine environments over the last five decades. It's sad. Hope our next president will spend more money for the sciences.

In the case of wolves, more like 50 years ago.

By Blind Squirrel FCD (not verified) on 29 Nov 2008 #permalink