Global fisheries have expanded so rapidly over the past half-century that the world is running out of places to catch wild fish, according to a new study conducted by researchers in Canada, the United States and Australia.

The findings, published Thursday evening in the online journal PLoS ONE, are the first to examine how marine fisheries have expanded geographically over time. Looking at fleets’ movements between 1950 and 2005, the five researchers charted how fishing has been expanding southward into less exploited seas at roughly one degree latitude each year in order to compensate for the fact that humans have depleted fish stocks closer to their shores in the Northern Hemisphere.

During that same period, the world’s fish catch increased fivefold from 19 million metric tons in 1950 to a peak of 90 million in the late 1980s, before declining to 87 million tons in 2005. It was 79.5 million tons in 2008, according to the U.N. Food and Agriculture Organization, the most recent year for which figures are available.

Daniel Pauly, a co-author who serves as principal investigator of the Sea Around Us Project at the University of British Columbia Fisheries Centre, said the global seafood catch is now dropping “because there’s essentially nowhere to go.” The fact that fish catches rose for so many decades “looks like sustainability but it is actually expansion driven. That is frightening, because the accounting is coming now.”

The authors — including lead author and UBC doctoral student Wilf Swartz and National Geographic Society ocean fellow Enric Sala — write that this relentless pursuit for seafood has left “only unproductive waters of high seas, and relatively inaccessible waters in the Arctic and Antarctic as the last remaining ‘frontiers.’”

“The focus should move from looking for something new to looking at what we have and making the most sustainable use out of it,” Swartz said in an interview.

Although the new analysis largely confirms what researchers, activists and policymakers know, the way it quantifies this global trend could provide new ammunition to those who are seeking to curb fishing of some of the world’s most imperiled species. Last week, for example, negotiators at the International Commission for the Conservation of Atlantic Tunas — which oversees dozens of fish stocks in the Atlantic Ocean — imposed new restrictions on vulnerable species such as oceanic whitetip and hammerhead sharks but stopped short of making deep cuts in the annual catch of imperiled bluefin tuna in either the eastern or western Atlantic.