In 2011, as though with the porpoise in mind, the journal Trends in Ecology and Evolution published a spirited exchange between two groups of researchers on the conundrum at the heart of conservation biology: When a species is swirling down the extinction vortex, how do you decide —quickly and accurately—what to do? The debate focused on the so-called 50/500 rule, first proposed in the 1980s, which says that in order for a species to survive, it must have at least 50 breeding-age individuals in the short term and 500 in the long term. Intended as a sort of back-of-the-envelope calculation, the rule had a couple of limitations: It took account only of genetics and inbreeding, excluding all the other threats a species might face, and it aimed to apply a universal standard to creatures as different as a gorilla and a condor.

One group of researchers, made up of Australians and Brits, had recently proposed that the long-term number be revised upward, to 5,000. It wasn't a perfect system, they wrote, but it was better than no rule of thumb at all. “Conservation biology is a crisis discipline akin to cancer biology, where one must act in a timely manner on the best information available,” they wrote. As the climate crisis worsened, so would the need for quick decisions. A second group, consisting mostly of American researchers, was having none of it. Each species, they wrote, deserved its own case-by-case analysis. It was a sin to use scientific guesswork to decide whether an animal “should be tossed from the ark.”

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In 2015, at Rojas-Bracho's urging, former Mexican president Enrique Peña-Nieto banned most gill net fishing in the Upper Gulf, a cataclysmic prohibition in a region whose economy is at least 80 percent fishing-related. That stick came with a carrot in the form of a compensation plan to pay local fishermen not to fish. The problem was, all the money was given to the local fishing bosses, who owned the boats and held the permits, for them to distribute. You can probably guess what happened next: Many fishermen didn't get any money at all, and because their profession was essentially illegal anyway, they kept hunting totoabas, sometimes encouraged—and equipped—by their bosses.

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Sea Shepherd, a conservation nonprofit, dispatched several decommissioned US Coast Guard cutters to San Felipe to pull up nets, but the crews were periodically harassed, attacked, and even shot at by fishermen while the Mexican navy stood by. On San Felipe's promenade, fishing leaders burned a skiff in effigy. They emblazoned the names of their opponents on the hull, narco-banner style.

The vaquita's foes remain as entrenched as always. In late 2018, a few months before the United Nations announced that a million plants and animals face extinction this century, I went to San Felipe in search of the answer to a simple question: How many fishermen had been arrested for illegal gill-netting? A well-dressed junior officer at the local police station—a stark little building on the outskirts of town—phoned his boss, then told me the local army base could help. A guard at the gate there told me to go to the navy base, where a media person told me to email the general-inquiries inbox in Mexico City. When I returned to the police station, the same officer kindly led me to the city administration building to talk to a municipal delegate—the man I should have asked all along, apparently. He told me to talk to the navy.

In my many conversations with Taylor, she had repeated a kind of mantra: “People are always looking for excuses not to do the hard thing.”