Every year more than 170 billion pounds (77.9 million metric tons) of wild fish and shellfish are caught in the oceans—roughly three times the weight of every man, woman, and child in the United States. Fisheries managers call this overwhelming quantity of mass-hunted wildlife the world catch, and many maintain that this harvest has been relatively stable over the past decade. But an ongoing study conducted by Daniel Pauly, a fisheries scientist at the University of British Columbia, in conjunction with Enric Sala, a National Geographic fellow, suggests that the world catch is neither stable nor fairly divided among the nations of the world. In the study, called SeafoodPrint and supported by the Pew Charitable Trusts and National Geographic, the researchers point the way to what they believe must be done to save the seas.
They hope the study will start by correcting a common misperception. The public imagines a nation's impact on the sea in terms of the raw tonnage of fish it catches. But that turns out to give a skewed picture of its real impact, or seafood print, on marine life. "The problem is, every fish is different," says Pauly. "A pound of tuna represents roughly a hundred times the footprint of a pound of sardines."
The reason for this discrepancy is that tuna are apex predators, meaning that they feed at the very top of the food chain. The largest tuna eat enormous amounts of fish, including intermediate-level predators like mackerel, which in turn feed on fish like anchovies, which prey on microscopic copepods. A large tuna must eat the equivalent of its body weight every ten days to stay alive, so a single thousand-pound tuna might need to eat as many as 15,000 smaller fish in a year. Such food chains are present throughout the world's ocean ecosystems, each with its own apex animal. Any large fish—a Pacific swordfish, an Atlantic mako shark, an Alaska king salmon, a Chilean sea bass—is likely to depend on several levels of a food chain.
To gain an accurate picture of how different nations have been using the resources of the sea, the SeafoodPrint researchers needed a way to compare all types of fish caught. They decided to do this by measuring the amount of "primary production"—those microscopic organisms at the bottom of the marine food web—required to make a pound of a given type of fish. They found that a pound of bluefin tuna, for example, might require a thousand pounds or more of primary production.