Sustainable tuna fishing is bad for climate – here's why

What’s good for the ocean might be bad for the planet. Fishing boats that target specific species, leaving others free to swim away, use more fuel than vessels intent on simply scooping up all the fish in their vicinity.

Eco-label initiatives and programmes like Monterey Bay Aquarium’s Seafood Watch, meant to help hungry diners quickly select sustainably caught seafood, have been gathering public support in recent years, says Brandi McKuin at the University of California Merced.

While those guides are helpful, their standards focus mainly on fishing-based factors, like leaving enough fish in the ocean to avoid decimating the population, and reducing the number of accidently caught fish, or bycatch, McKuin says. Other impacts, including the greenhouse gas emissions generated by using different types of fishing gear, are often overlooked.

“If we’re including climate change in the sustainability criteria, it changes things,” McKuin says.

By combing through published papers, reports, and online catch databases, McKuin and her colleague Elliott Campbell determined that tuna vessels using more sustainable methods such as troll and pole line fishing or longline fishing consume about three to four times as much fuel as boats that employ a large net called a purse seine.

Net losses

That’s because purse seining is more efficient, McKuin says – more fish can be gathered in a shorter amount of time – but it’s also less sustainable than selective methods because other species get swept into the net, too.

Thanks to a drop in purse seining in the US tuna fishery since about 1990, the team estimates that catching a tonne of tuna takes about three times as much fuel today as it did 25 years ago. The results were presented at the American Geophysical Union fall meeting in San Francisco in December.

The researchers also compared the climate impact of tuna to terrestrial sources of protein, like tofu, pork and beef. They calculated the warming effect of tuna fishing based on the different rates of fuel use, and the effects of sulphur-reducing fuel regulations, which improve air quality but can increase warming.

Sustainably caught tuna had a larger climate effect than any other protein considered, except beef, for which climate warming emissions are five times that of tuna per unit of weight.

And a complete accounting of the seafood supply chain could reveal even larger climate consequences, McKuin says, due to factors like the energy needed to freeze fish on its way to the supermarket.

To understand the full environmental impact of shipping vessels, including fishing vessels, we need to include all sources in our emission estimates, says Melanie Bläsing at the University of Bonn in Germany.

“Nobody thinks about small shipping vessels or inland navigation vessels,” she says, but the emissions of even small ships can add up.

Read more: Beware of ‘bluewash’: Which fish should you buy?