The volume of seafood caught around the world is on a decline, but carbon dioxide emissions from fishing continue to rise, hitting levels that are much higher than previously thought. That’s according to new research from the University of British which tracked trends from fuel combustion in marine fisheries from 1950 through 2016.

The study, which was published in the journal Marine Policy last week, suggests that overall emissions from the world’s marine fisheries more than quadrupled between 1950 to 2016 to 207 million tons, about 30 per cent higher than previously estimated. That’s despite the fact that global catches have been dropping since the 1990s.

The scientists looked at each boat in fleets around the world and used their engine capacity to calculate how much carbon dioxide they release by burning fossils fuels.

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About three-quarters of the CO2 released in 2016 came from commercial fishing operations, according to the new paper. The remainder came from small-scale, artisanal and subsistence fishing.

A surprising finding was that over the 66 years included in the study, the carbon dioxide emissions burned by commercial boats per unit of fish caught dropped by about 10 per cent. But for small-scale operations, that emissions intensity actually increased by a factor of 2.3.

The researchers said the overall increase in emissions from the fishing industry may have something to do with the fact that vessels have to travel farther offshore to find good catches.

Past research has suggested that increasing appetites for seafood like lobster and shrimp, which require more fuel to harvest, also is driving the rise in CO2 emissions.

The study concluded that fishing operations are increasing emissions despite the availability of more efficient technology.

A finding from another study revealed that commercial fishing operations now cover 55 percent of the Earth’s waters – four times the area devoted to landbased agriculture.