Sometimes it seems like buying fish is more trouble than it's worth. You have to consider its freshness, the concentration of mercury it's likely accumulated, the relative health of its worldwide population, its price and, if you're not a fan of strong tastes, its level of "fishiness." Oh, and, if you're at all worried about climate change, its relative carbon footprint—which, as a new study published in the journal Fish and Fisheries shows, can vary dramatically.

According to the paper, it takes an average of 2,100 to 2,600 gallons of fuel to catch one metric ton (that's 2,200 pounds) of shrimp or lobster, NPR reports. In contrast, it takes only about 5 gallons of fuel to catch the same amount of sardines or anchovies. Catching mackeral also requires relatively little fuel. It's a big difference.

The most energy-conscious choice should then be an easy one, the study implies. Small fish like sardines also contain high levels of omega-3 fatty acids, and since they are at the bottom of the food chain tend to be ecologically sound fish to eat. Small fish also generally contain lower concentrations of contaminants like mercury, as compared to larger long-lived species like swordfish. But, as a rule, these fish aren't very popular in the United States, although, increasingly, overfishing of small, oily fish is a problem, too. (Those fish oil supplements have to come from somewhere.)

The new study has limitations, the researchers said—the data comes mostly from Europe and Australia, with some information from North America. The team told NPR that they hope to soon add data from the developing world. The data the study presents, then, don't tell the whole story. For example, it takes only about 260 gallons of fuel to net a ton of lobster in the United States. That's about 10 times more efficient than the worldwide figure.

And overall, fisheries are not a major producer of greenhouse gases compared to other food sources, such as raising livestock for beef. In other words, ordering a summer lobster roll in Maine might not be a terrible choice—just steer clear of the surf and turf.