Spend enough time in the world of fisheries science, and you're likely to hear an adage, coined by a researcher named John Shepherd, that neatly encapsulates the field's uncertainty. Counting fish, the saying goes, is not unlike counting trees — if the trees constantly moved around the forest, and also you had to somehow tally them blindfolded. The point is clear: Few natural resources so confound measurement, let alone management, as fish.

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