Predatory Fish Have Declined by Two Thirds in the 20th Century

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 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.”

Source: Scientific American, October 20, 2014
Predatory Fish Have Declined by Two Thirds in the 20th Century