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SUSTENANCEI realized as I was writing this that the late lunch sitting beside my computer included salmon and other types of sushi.J. Raloff

If eating meat in place of other proteins hogs natural resources and spews an overabundance of greenhouse gases into the atmosphere (see last blog), wouldn’t fish be a climate-friendlier menu selection? Usually, but not always. Or so panelists pointed out this morning at the American Association for the Advancement of Science annual meeting, here in Chicago. Focusing on salmon, they showed that fish consumption’s carbon footprint depends on what a fish has eaten, how it has been caught and stored, and how it’s transported to market.

There were some real eye openers within these assessments.

Peter Tyedmers of Dalhousie University in Halifax, Nova Scotia, focused on greenhouse gas — aka GHG  — assessments of fish production downstream of food-processing plants. In other words, how fish are reared and caught.

He started by focusing on the big North Atlantic and Chilean sources in Norway, Scotland, Canada and Chile. For every ton of fish harvested, there is a substantial GHG cost measured in terms of the amount of carbon dioxide that would produce equivalent warming. For production of Norwegian fish it’s 1,750 kilograms of CO₂ equivalents, 2,250 kg for Chilean salmon, 2,500 kg for the Canadian fish, and 3,300 kg for Scottish farmed stock.

The difference in the warming potential largely traces to what the finned populations have been fed, Tyedmers explains. Scottish farmers feed their salmon the highest proportion of fish meal — almost 70 percent, on average. Those fishy diets account for 85 percent of the greenhouse-gas emissions associated producing Scottish salmon, his team calculated. Elsewhere, fish farming operations tend to substitute plant-based meals and oil or meat byproducts for a share of that fish meal.

Not surprisingly, the higher the proportion of plant sources in a farmed fish’s diet, the lower the climate impacts associated with its rearing.

So why are Scottish salmon fed so much fish? Some markets — particularly France — put a premium on salmon that were reared on fish, arguing that it makes the farmed animals more “natural” than those fed rapeseed or other plant products. Yet clearly, Tyedmers said, if the goal is to reduce our food supply’s carbon footprint, rearing salmon on plant-based feed is a promising tactic.

But just substituting any plant constituent for fish in a salmon’s diet will not always prove beneficial, he noted — at least from a climate standpoint. Some fish are fed fishmeal derived from capelin, which doesn’t have a large GHG contribution. If wheat gluten or even palm oil (which isn’t yet a normal ingredient in fishmeal) were substituted for the capelin, the carbon footprint of the salmon could jump substantially, Tyedmers team calculated.

Data from another assessment, this one in wild fish, showed that fuel use associated with harvesting gear could greatly impact GHG emissions associated with salmon. Purse seining contributed 180 kilograms of CO₂ equivalent to the carbon footprint associated with a ton of salmon, gillnetting about 380 kg, and trolling a whopping 1,700 kg. So, do you know how your fish was caught?

Astrid Scholz, a food-production economist at Ecotrust in Portland, Ore., is part of an international consortium that is calculating GHG costs associated with getting salmon to market, independent of how they were raised. Again, there are some big eye openers here in the numbers that her team just crunched in the days leading up to this meeting.

Three-quarters of the world’s harvested salmon comes from three major markets: the Northeast Pacific (including Alaska and British Columbia), the Northeast Atlantic (mostly Norway and Scotland) and Chile. It turns out, her team finds, that the big climate costs for these fish trace to how they reach their designated market — by air, by container ships, or by truck.

And what determines the transport choice in most cases is whether the fish must arrive fresh (i.e. almost immediately), or whether it can arrive frozen at any point over many days or weeks.

In practical terms, for Chicagoans wanting fresh salmon, farmed fish trucked in from British Columbia will always have a smaller carbon footprint than salmon caught anywhere else — because all other fresh salmon must be flown into to the lower 48 states, especially inland cities.

If frozen salmon is acceptable, wild seine-caught Alaskan salmon will invariably prove the most climate-friendly choice at costs of 1 kg CO₂ per kg of delivered fish, Scholz says. Although this fish has to travel nearly the same distance to market as will fish from Canada, the Alaskans’ wild foraging means there are no feed costs, which jack up the GHG costs associated with aquaculture.

Where frozen wild, seined salmon is not available, a climate-friendly alternative will be frozen farm-raised Norwegian salmon. Its carbon footprint: just 1.8 kg CO₂ per kg of fish.

On a per dollar value, Chilean fish are usually the cheapest salmon in northern markets. But these monetary costs tend to disguise the high climate costs associated with moving South American salmon half-way round the world. There are 3 kg CO₂ costs associated with each kg of frozen salmon brought to North America from Chile, and 5.5 times that GHG cost for fresh Chilean salmon flown into the Northern Hemisphere. Meanwhile, British Columbia salmon can be trucked in fresh or frozen for 3 kg of CO₂ per kg of fish.

The problem for consumers, all of this morning’s speakers conceded, is that they don’t know any better than to choose their fish on the basis of dollar-cost or fresh-vs-frozen considerations. They certainly have no way of knowing how their fish were pulled from the water or what they might have been fed.This could be remedied by labeling, several of the speakers noted. Indeed, this approach to identifying climate costs associated with our diets is already being explored in a few European countries.

THIS WAS THE SECOND OF TWO PARTS: First part is at: http://www.sciencenews.org/view/generic/id/40934/title/AAAS_Climate-friendly_dining_%E2%80%A6_meats

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