Commercial fishing operations in the past 40 years have precipitated a dramatic change in ocean fish stocks, with tuna and other big predators declining and small fish like anchovies and sardines surging. That’s the conclusion of the most ambitious study ever completed of fish populations in the Earth’s oceans, conducted by Villy Christensen of the University of British Columbia's Fisheries Centre.
In the past 100 years, 80% of the biomass of fish in the world’s oceans has been lost, Christensen says in a AAAS video  that coincided with a symposium at the Annual Meeting. “Just in the last 40 years, we have lost 60% of the biomass,” he explained. “So we’ve seen some very serious declines, and there’s no doubt about what the cause is: We’re talking about overfishing—overfishing at the global scale.”
Only a few studies have tried to assess the future of fish populations in the ocean, and they range widely, from complete collapse of marine fish populations by 2048 to improvement in populations. But effective management strategies will be required to achieve the latter outcome, he said.
The story received extensive news coverage internationally.
Christensen’s team of scientists based their conclusions on more than 200 marine ecosystem models and more than 68,000 estimates of fish biomass from 1880 to 2007, the Vancouver Sun reported , citing a University of British Columbia news release.
David Derbyshire, writing for the UK’s Daily Mail, focused in part on the simple mechanism that’s thrown the oceans out of balance.
“By removing the large, predatory species from the ocean, small forage fish have been left to thrive,” Christensen said in Derbyshire’s article . “If you take out the lions, then the number of antelope go up—and that's what's been happening in the oceans.”
Bryan Walsh, writing in Time magazine’s  Ecocentric blog, explained that the researcher sees overfishing as the result of the global spread of industrial fisheries.
“Christensen says that we're essentially fishing down the food web—removing the large predatory fish, which allows the smaller ones to proliferate undisturbed,” Walsh wrote. “But if we keep fishing down that web, eventually using the small fish to feed growing aquaculture operations, that could mean the end of the wild ocean.”
Already, "it's a very different ocean," Christensen says in an account  by Science staff writer Erik Stokstad in ScienceNOW. Stokstad continues: “In many places, these smaller fish are suitable for eating, but off southwestern Africa and other places, the predators were replaced by undesirable fish.”
For people concerned about the oceans, consuming more herring and sardines and less tuna, salmon, and grouper is one way to take pressure off the populations of predator fish and encourage more sustainable fishing, wrote Alok Jha, science correspondent for the Guardian. “How can we prevent the collapse of wild fisheries?” Walsh asked. “Pretty much the same way we might try to balance the budget—cut our consumption until we're no longer taking more fish than the sea is providing. So it is for most of our environmental challenges—we need to get out of the red and get into the black.”
Stokstand offers a caution from Michael Hirshfield, chief scientist of the advocacy group Oceana, based in Washington, D.C.: Populations of small fish tend to boom and bust, he said, and moreso when the ranks of predators have been diminished.