“The whale,” wrote Herman Melville in Moby-Dick, “would by all hands be considered a noble dish, were there not so much of him.” Nineteenth-century whalers, of course, weren’t put off by bulk—quite the opposite. On their long oceanic voyages, where fresh meat was scarce, their kills were also sources of much-needed food. And so, as Melville describes in Chapter 65 of his magnum opus, they would cook biscuits that had been soaked in whale oil, fry flour-coated sperm-whale brains, and guzzle down whale “fritters”— moldy scraps of fat that had gone brown and crisp after being left ashore for too long.

Humans had been catching and eating whales for millennia, but America came to to dominate the world of cetacean slaughter in the 19th century. With hundreds of powerful ships, gun-loaded harpoons, bountiful coasts, and a strong seafaring tradition, the Yankee whalers were incredibly successful. It is estimated that they harpooned over 100,000 sperm, right, bowhead, humpback, and pilot whales for their oil and whalebone, killing more of these animals than in the previous four centuries combined.

But whales weren’t the only animals they killed.

The ships spent months or years at sea, returning to port only when their holds were heaving with whale remains. With little besides hard biscuits and preserved meats to eat, the crews leapt at any chance to get fresh meat. Whale meat, sure, but that became rarer over time as the whalers became victims of their own success. Fish, of course, but ships that ventured far north would encounter oceans that were covered in ice. So, they often sent away teams onto islands to plunder the local wildlife.

For the first time, Joshua Drew from Columbia University has quantified this collateral damage. By studying dozens of logbooks from that era, he and his students showed that the whalers slaughtered their way through walruses, ducks, cod, and many other animals, including surprising species like polar bears, reindeer, and even a couple of kangaroos. The legacy of these other kills still affects the ecosystems of the Arctic and the health of its indigenous people.

Drew is a historical ecologist, who uses often unorthodox sources of information to work out what wild ecosystems looked like before scientists began measuring and studying them in earnest. Case in point: In 2013, he worked out which sharks frequented the Pacific Gilbert Islands by studying the fearsome weapons that local islanders made from the sharks’ teeth.

“You have to be creative about looking back in time,” he says. “We were damaging the oceans long before we started collecting data on them. So if we want to look at what those ecosystems really look like, we have to use things like shark-tooth weapons.” Or, as it happens, logs from whaling vessels.

The New Bedford Whaling Museum had digitized 79 such logs representing 74 voyages between 1846 and 1901. Drew got a team of undergraduates to pore through them. The project began as a mid-term assignment, but the students were so enthralled that they kept at the project, eventually turning it into a proper scientific paper (to be published soon in Ecology and Evolution).

They showed that the whalers captured roughly 2.4 million kilograms of non-whale meat over more than 71,000 days at sea—roughly 34 kilograms per day. The vast majority of their prey were walruses, caught for both food and tusks. Between the 1850s and 1860s, the number of captured walruses rose by 500 times, before collapsing. This fits with other historical evidence showing that in the mid-to-late 19th century, American ships killed as many walruses as currently survive on the planet.

By the 1890s, the whalers had so heavily depleted the large mammals of Alaskan waters that they were forced to move even further north, to hunt bowhead whales on punishingly long voyages. From the oceans, they took cod, dolphins, porpoises, seals, turtles, and sunfish. But during these trips, they also became subsistence hunters, making landfall to capture ducks, reindeer, ptarmigan, rabbits, moose, and even a polar bear.

“The biggest surprise was just how much of an impact these things had on land,” says Drew. “The fact that they hunted fish ... well, yeah. But they were also targeting polar bears and caribou, and from very specific islands. We use those islands as baselines for how climate change is operating, because we thought of them as pristine. But our baseline is missing several thousand mouths.”

That’s why work like this matters. Humans were changing the world long before we realized it. To take the full measure of what we’ve done, to fully understand how far the natural world has fallen, and to appreciate how far we have to go to restore it, we need to rely on unconventional sources of data. The logbooks provide “almost laughably bare minimum estimates,” says Drew. But they are the best we’ve got.

“They give a glimpse of what ecosystems in the Arctic looked like in the 19th century, long before modern ecological sampling protocols and techniques existed,” says Sophie Monsarrat, another historical ecologist. “They can help to set relevant baselines for conservation.”

They might be relevant for understanding human health too. The whalers often commissioned indigenous people to help them hunt, and paid them in flour, molasses, and canned meats. “Think about it: you’re a nomadic person trying to find walruses and bowheads, and those get taken out. Simultaneously, someone sets up camp that sells food that’s delicious but incredibly unhealthy. And now there’s no incentive to move around foraging.” Those factors—less mobility and unhealthier food—portended the rising levels of heart disease and diabetes in the peoples of the high Arctic.