A disturbing new report published by the World Wildlife Fund found that the world marine vertebrate population declined by 49 percent between 1970 and 2012.
The Living Blue Planet Report -- analyzed by the Zoological Society of London and issued as an update on our oceans' health -- also found that local and commercial fish populations have been cut in half, tropical reefs have lost nearly half of their reef-building coral, and there are 250,000 metric tons of plastic in our oceans.
"Global climate is one of the major drivers causing the ocean to change more rapidly than at any other point in millions of years," the WWF reported in a press release. "These findings coincide with the growing decline of marine habitats, where the deforestation rate of mangroves exceeds even the loss of forests by 3-5 times; coral reefs could be lost [to temperature rises] across the globe by 2050; and almost one-third of all seagrasses have been lost."
Populations of some commercial fish stocks, such as a group including tuna, mackerel and bonito, had fallen by almost 75 percent, according to the study.
Marco Lambertini, director general of WWF International, told Reuters mismanagement was pushing "the ocean to the brink of collapse.".
"There is a massive, massive decrease in species which are critical," both for the ocean ecosystem and food security for billions of people, he said. "The ocean is resilient but there is a limit."
The analysis said it tracked 5,829 populations of 1,234 species, such as seals, turtles and dolphins and sharks. It said the ZSL data sets were almost twice as large as past studies.
"This report suggests that billions of animals have been lost from the world's oceans in my lifetime alone," Ken Norris, director of science at the ZSL, said in a statement. "This is a terrible and dangerous legacy to leave to our grandchildren."
Damage to coral reefs and mangroves, which are nurseries for many fish, add to problems led by over-fishing. Other threats include coastal development, pollution and climate change, which is raising temperatures and making waters more acidic.
The study said the world's fishing fleets were too big and supported by subsidies totaling $14-35 billion a year.
World marine fish catches dipped to 79.7 million metric tons in 2012 from 82.6 million in 2011, according to the U.N.'s Food and Agriculture Organization. Safeguarding the oceans can help economic growth, curb poverty and raise food security, it says.
Reuters contributed to this report.