Once believed immutable due to their massive size and volume, today's oceans are no longer the same oceans of our grandparents. In just a few generations, human activity has radically transformed ocean ecosystems. Case in point: Recent research has found that populations of predatory fish around the world have declined by a shocking two-thirds in the last century alone, with most of the damage coming since the advent of industrialized fishing practices in the 1970s, reports Scientific American.

Although you might not think at first that fewer predators lurking in the oceans is such a bad thing, animals at the top of the food chain can be important indicators of ecological health. They are also often considered keystone species, and their disappearance can harm the ecosystem all the way down the food chain.

Furthermore, predatory fish like grouper, tuna, swordfish and sharks are typically the fish we most like to eat, which is actually a huge part of the problem to begin with. Fisherman target the biggest, tastiest fish first. After these stocks are depleted, they move on down the chain in a pattern sometimes called "fishing down the food web." It makes economic sense given the higher demand for large predatory fish, but the pattern has devastating consequences for marine environments. 

Scientists recently analyzed more than 200 published food-web (interacting food chains) models from all over the world, which included more than 3,000 ocean species. They found that humans have reduced the biomass of predatory fishes by more than two-thirds in the last century, with the steepest collapse occurring in the last 40 years, which correlates with the development of industrialized fishing practices. 

Some of this comes as no surprise. 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. But these new results put things in a much broader perspective, reflecting the overall impact of human activity on fish populations on the whole. Even for species not immediately threatened with extinction, a two-thirds population collapse is profound.

“Predators are important for maintaining healthy ecosystems,” said 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.”

Other research has shown that predators keep prey populations in balance, and the loss of predators can cause nutritional cascades throughout the food web.

“The main problem is really in the developing countries where we need more effective institutions for fisheries management,” added Christensen. “We need to get effective management introduced in all countries, or it will have dire consequences.”

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