Shark Declines Affect Shellfish
Coastal Ecosystem Collapse Possible as Shark Numbers Decline
The potential for an ecosystem to collapse when top predators are removed has been demonstrated on land. New studies conclude that oceans may face the same danger
Recent news that shellfish population declines on the Eastern Atlantic coast of the US and Canada may be related to dramatic declines in great shark populations seems to indicate that coastal ecosystems are subject to the same danger of collapse as land based ecosystems.
It is known that intense fishing has caused the collapse of some fish populations, most notably the Atlantic cod fishery. But the knock-on effect found here hurts more than just the shark fishery.
Surveys of shark populations along the eastern US and Canadian coasts have shown frightening declines. Scalloped hammerheads (Sphyrna lewini) and tiger sharks (Galeocerdo cuvier) have declined by 97% since 1970, while bull sharks (Carcharhinus leucas), dusky sharks (Carcharinhus obscurus) and smooth hammerheads (Sphyrna zygaema) have all declined by 99%. The popularity of shark fin soup and trophy shark fishing are named as major causes of these declines.
When this data was compared with information from studies on how cow-nosed ray (Rhinoptera bonasus) foraging affects shellfish the researchers noted a connection.
Here’s how it works:
- Great sharks prey on the rays, skates and smaller sharks
- Rays, skates and smaller sharks prey on the shellfish.
- Loss of great sharks means shellfish face many more ray, skate and small shark predators and their numbers dwindle.
Without great sharks to keep the rays in check, they are decimating the eastern Atlantic bay scallop beds. Many areas are closed to scallop fishing. When the scallops are gone the rays are expected to turn to other shellfish, continuing the loss of biodiversity.
While not discounting the effects of poor water quality, loss of habitat-particularly seagrass beds, and even HABs, the predator situation is certainly a significant contributor to the scallop decline. This mimics land based studies in areas such as Yellowstone National Park where the return of the grey wolf has had a controlling effect on coyote populations allowing fox numbers to recover. Wolf predation on elk has also resulted in increases in healthy stands of aspen.
The damage done to complex ecosystems by removal of important predators is just beginning to be understood. In the oceans of the world where migratory patterns of many species are not known, this damage is going to be difficult to assess until situations reach crisis level, which is what is happening along the Atlantic.
