The world's oceans have lost too many top predators to fishing boats
Top predators are large animals, such as lions and great white sharks, that hunt smaller animals, but have no natural predators of their own — except humans. Their removal through overhunting and overfishing has been called “humankind’s most pervasive influence on nature.” It is as harmful in the sea as it is on land.
Top predators are at the top of food chains. Food chains can be thought of as a series of levels, with each level dependent on the next for food. An ocean food chain starts with many microscopic organisms and plant life. Tiny animals like shrimp eat those, small fish eat the shrimp, and bigger fish eat those smaller fish. By eating other fish, top predators keep their populations under control. This keeps ocean life in balance.
"Fishing Down The Food Web"
People generally prefer eating larger predatory fish like grouper, tuna, swordfish and sharks, rather than species lower on the food chain, such as anchovies and sardines. Their preference provides strong incentives for fishermen to catch the bigger fish. The result is something called "fishing down the food web." Fishermen go after the more valuable predators first, fish them until there aren’t enough left to support fishing and then move on to species lower in the food chain.
New research shows just how severely predatory fish populations have declined worldwide since the start of industrial fishing. Scientists analyzed food webs involving more than 3,000 ocean species. Their results show that since the beginning of the 20th century, humans have reduced predatory fish populations by more than two-thirds. Most of this alarming decline has occurred since the 1970s.
Many of these predatory fish species are known to be in trouble. Twelve percent of grouper, 11 percent of tuna and billfish and 24 percent of shark and ray species are considered to be threatened with extinction.
Damaging Ocean Ecosystems
These population declines have effects far beyond maintaining a supply of fish that consumers like to eat. Predators keep prey populations in balance, and the loss of predators can cause ripple effects that affect entire ocean ecosystems. For example, kelp-eating sea urchins have been growing in number due to the loss of urchin predators like sea otters. The result has been the destruction of kelp forests, which are home to many unique and economically important species.
“Predators are important for maintaining healthy ecosystems,” says professor Villy Christensen, lead author of the new research paper. “Also, where we have had collapses of the larger fish, it has taken many decades for them to rebuild.”
In the United States this gloomy picture has started to improve, thanks to careful fisheries management. Thirty-four fish stocks — local populations of fish of the same species — have been declared rebuilt since the year 2000 and more than 90 percent of U.S. fish stocks are not considered overfished. Worldwide, though, almost 30 percent of fish stocks are overfished.
Managing Fisheries More Effectively
“The main problem is really in the developing countries where we need more effective institutions for fisheries management,” says Christensen. “We need to get effective management introduced in all countries, or it will have dire consequences.”
The new study adds important information to the global debate on how many fish we should be taking from the oceans. It shows that for many species of economically and ecologically important predatory fish, we have been fishing far too heavily.
More needs to be done internationally to restore predatory fish populations. Fishing quotas for many species need to be reduced and existing quotas need to be enforced. Only by taking these steps can we ensure that we will have healthy populations of these fish — and healthy oceans — in the future.