The removal of top predators has been called “humankind’s most pervasive influence on nature,” and it is as detrimental in the sea as it is on land. Consumers prefer predatory fish like grouper, tuna, swordfish and sharks to species lower on the food chain such as anchovies and sardines, providing strong incentives for fishermen to catch the bigger fish. Going after the more valuable predators first, fishing them until there aren’t enough left to support a fishery and then moving on to species lower in the food chain, a pattern sometimes observed in global fisheries, has been called “fishing down the food web.”

New research (pdf) by the team that coined the term attempts to determine how severely predatory fish populations have declined worldwide since the start of industrial fishing. Scientists analyzed more than 200 published food-web (interacting food chains) models from all over the world, which included more than 3,000 ocean species. Their results show that in the 20th century humans reduced the biomass of predatory fishes by more than two thirds and that most of this alarming decline has occurred since the 1970s.

Many of these predatory fish species are known to be in trouble. The International Union for Conservation of Nature Red List of Threatened Species considers 12 percent of grouper, 11 percent of tuna and billfish and 24 percent of shark and ray species to be threatened with extinction. These population declines have implications far beyond a sustainable supply of fish that consumers like to eat. Predators keep prey populations in balance, and the loss of predators can cause trophic (nutritional) cascades through food webs that affect entire ocean ecosystems. For example, kelp forests, home to many unique and economically important species, have been destroyed by a growing population of herbivorous sea urchins that resulted from the loss of urchin predators like sea otters. “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 U.S. this gloomy picture has started to improve, thanks to science-based fisheries management. Thirty-four fish stocks have been declared rebuilt since the year 2000 and more than 90 percent of U.S. fish stocks are not considered overfished. Worldwide, though, the Food and Agriculture Organization of the United Nations considers almost 30 percent of fish stocks to be overfished. “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 exploiting far too much. More needs to be done by the international counterparts of the U.S.’s National Marine Fisheries Service as well as regional fisheries management organizations such as the International Commission for the Conservation of Atlantic Tunas. Fishing quotas for many species need to be reduced and existing quotas need to be enforced to ensure that we have healthy populations of these fish—and healthy oceans—in the future.