The Living Blue Planet Report, just released by World Wildlife Fund (WWF) in collaboration with the Zoological Society of London, documents the extraordinary losses, which occurred from 1970 to 2012. Alarmingly, some fish species were found to have declined by almost 75 per cent.
The authors of the report attribute the dramatic population drops to human-driven climate change as well as to habitat loss, overexploitation and pollution.
"In less than a human generation, we can see dramatic losses in ocean wildlife -- they have declined by half -- and their habitats have been degraded and destroyed," said Mr Brad Ack, senior vice president for oceans at WWF.
"Driving all these trends are humans actions: from overfishing and resource depletion, to coastal development and pollution, to the greenhouse gas emissions causing ocean acidification and warming."
The findings were determined after researchers surveyed more than 10,000 populations of 3,038 marine species, including fish, birds, mammals and reptiles. The report estimates that close to one-third of the world's fish stocks are overfished, and one in four species of sharks, rays and skates are threatened with extinction.
Several shark species "have declined dramatically around the world due to overfishing," and other human-driven causes, said Professor Mahmood Shivji, director of the Guy Harvey Research Institute at Nova Southeastern University, who was not involved with the study.
Three-quarters of world's coral reefs threatened
Yet another key finding from the report is that three-quarters of the world's coral reefs are currently threatened. At current projected levels of warming and ocean acidification, all coral reefs are projected to be lost by the year 2050.
"Corals are suffering from poor water quality due to deforestation and coastal agriculture, along with increasing fishing pressure on beneficial animals that help them stay healthy, such as reef fish," Mr Ack said.
"Ocean warming from climate change and ocean acidification from excess carbon absorption significantly threaten reefs over the long-term.
"At current rates of temperature rise, oceans will become too warm for coral reefs by 2050, resulting in the loss of the world's most biologically diverse marine ecosystem."
Currently some coral reefs are able to bounce back from disturbances, such as bleaching events, cyclones and outbreaks of damaging crown-of-thorns starfish. Ocean acidification, however, slows down the ability of corals to recover from these and other threats, adding to the downward trend.
Call for action to curb decline
Mr Ack and his colleagues call for immediate measures to curb the decline, such as ending illegal fishing and protecting coral reefs, mangroves and other critical ocean habitats.
This month, world leaders are meeting at a United Nations summit in New York to develop goals for global sustainable development.
Mr Ack said the meeting would provide "an opportunity for the international, concerted action we need for ocean conservation." He also said the United Nations Conference on Climate Change, to be held later this year in Paris, was an opportunity to "slash the emissions contributing to the climate and ocean crisis."
In the report, Mr Marco Lambertini, Director General of WWF International, wrote that "humanity is collectively mismanaging the ocean to the brink of collapse."
"Considering the ocean's vital role in our economies and its essential contribution to food security -- particularly for poor, coastal communities -- that's simply unacceptable," he added.
"Could the economic implications of the collapse of the ocean's ecosystems trigger the next global recession or undermine the progress we have made on eradicating poverty?
"We can and we must correct the current course now."
http://www.abc.net.au/
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