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Fill 'er up. Trawling for tiger prawns can burn an enormous amount of fuel, but better management of the stock has increased efficiency.

Fill 'er up. Trawling for tiger prawns can burn an enormous amount of fuel, but better management of the stock has increased efficiency.

© Australian Fisheries Management Authority

What seafood guzzles the most gas?

Most of us don’t think about fuel when we eat seafood. But diesel is the single largest expense for the fishing industry and its biggest source of greenhouse gases. Not all fish have the same carbon finprint, however, and a new study reveals which ones take the most fuel to catch.

Robert Parker, a Ph.D. student at the University of Tasmania, Hobart, in Australia, and Peter Tyedmers, an ecological economist at Dalhousie University in Halifax, Canada, analyzed more than 1600 records of fuel use by fishing fleets worldwide. They added up the fuel required to catch and bring various types of fish and seafood to port, which they reported online this month in Fish and Fisheries.

Parker and Tyedmers didn’t consider the energy required to process the catch and transport it to consumers, but other studies indicate this is usually a smaller fraction. Nor did they look at environmental impacts that depend on the type of fishing gear, such as habitat destruction and the accidental killing of turtles, birds, and dolphins.

Here’s the upshot, ranked by average amount of fuel required to land a metric ton:

7. Sardines: 71 liters

Abundant forage fish like these tend to school close to shore, and it’s fairly quick work to surround them with an enormous net. Icelandic herring and Peruvian anchovies are the least fuel-intensive industrial fisheries known, caught with just 8 liters of fuel per ton of fish.

6. Skipjack tuna: 434 liters

Like forage fish, these tuna and other kinds of open-water finfish are caught en masse in a net called a purse seine. But the vessels must travel farther to find the fish, hence the bigger gas bill.

5. Scallops: 525 liters

Bottom-dwelling mollusks are scooped up with heavy steel dredges.

4. North American salmon: 886 liters

Salmon are typically caught in rivers and bays with gill nets or purse seines. Catching them by hook and line takes more fuel.

3. Pacific albacore: 1612 liters

Trolling takes more fuel than using nets does. After dropping long lines with baited hooks, vessels race to keep up with the speedy predators.

2. Sole: 2827 liters

To catch flatfish, a boat drags a heavy metal beam across the sea floor with a net attached. This is hard work for the engines.

1. Shrimp and lobster: 2923 liters

Although it takes just 783 liters of fuel to fetch a ton of Maine lobsters from traps, Asian tiger prawns (a type of shrimp) from Australia required 7000 liters of fuel per ton in 2010, and Norway lobster from the North Sea has taken as much as 17,000. These two species are small and relatively scarce, so boats must pull a fine net for long distances.

How does wild seafood compare with other kinds of animal protein? The median fuel use in the fisheries is 639 liters per ton. In terms of climate impact, that’s equivalent to a bit more than 2 kilograms of carbon dioxide emitted for each kilogram of seafood landed. Chicken and farmed salmon and trout are roughly the same, but beef is significantly higher at 10 kg of carbon dioxide per kg of live animal. “If you’re looking at having a green diet, you want to transition away from beef,” Parker says.

One implication of the study is that a lot of fuel has been wasted due to mismanagement of fisheries. In past decades, government subsidies led to bigger and more powerful boats that could catch even more fish. But as stocks became depleted, crews had to fish longer and farther away from shore. Fuel use appears to have declined over the past decade. The most important factors in this decrease, Tyedmers and Parker say, are the recovery of fish stocks and the reduction in the size of fleets; the remaining vessels don’t have to travel as far.

Tyedmers and Parker are working with the Monterey Bay Aquarium in California to determine if fuel use can be incorporated into the Seafood Watch program, which evaluates the sustainability of fisheries. But people probably shouldn’t get too hung up on the fuel numbers, says Christopher Costello, an environmental economist at the University of California, Santa Barbara, who wasn’t connected to the study. Fish consumption is a tiny part of the carbon footprint of most Americans—probably less than a half a percent of the carbon output of driving, he estimates. Still, Parker says that changing your diet, unlike changing your means of transportation, can be a relatively easy thing to do.