LONDON — The number of fish and other species in the sea has been almost halved since 1970, according to a new report from the World Wildlife Fund (WWF) and the Zoological Society of London (ZSL).

Numbers dropped some 49% between 1970 and 2012, the report says, with global population sizes of the Scombridae family of food fish that includes tunas, mackerel and bonitos falling by 74%.

The study, which hasn't been peer reviewed, tracked 5,829 populations of 1,234 species, which its authors say makes up a data set almost twice as large as previous studies.

They identify overfishing as the main threat to ocean biodiversity, but also conclude that climate change is causing the ocean to change faster than it has for millions of years. An increase in temperatures and increased acidity levels caused by carbon dioxide are compounding problems brought about from overfishing and pollution, they say.

Plummeting numbers of bluefin and yellowfin tuna, both of which are highly prized species at sushi restaurants, are highlighted as a serious cause for concern, as is a decrease in leatherback turtles and porbeagle sharks.

"The ocean works hard in the background to keep us alive, generating half of the world’s oxygen and absorbing almost a third of the carbon dioxide produced from burning fossil fuels," Ken Norris, Director of Science at ZSL, said in a press release.

"It also feeds billions of people around the globe, some of whom rely solely on the oceans to survive. These devastating figures reveal how quickly human beings are changing the wildlife in our oceans and are a stark warning of the problems we might face as a result."

WWF's recommendations for addressing the issue include creating Marine Protected Areas to help wildlife and habitats recover, and addressing climate change at a government level, and looking for sustainably sourced food at a consumer level.

There is a global movement toward establishing more protected areas off-limits to fishing, with the U.K. and the U.S. both establishing huge new protected areas in the past year alone.

Mashable Science Editor Andrew Freedman contributed reporting.