Nearly 2,000 years ago, the Roman emperor Vespasian taxed the sale of human urine, then a valuable industrial ingredient. When his son objected, the emperor held a gold coin to his nose, asking whether it smelled—arguing that money was money, no matter its source.

Perhaps Vespasian would have been a great marine biologist; when it comes to nutrients in coral reefs, he’s absolutely right. In fact, coral reefs wouldn’t be stunning havens for biodiversity without one key nutrient source: fish urine.

The trouble is, humans like to eat reefs’ best recyclers: the biggest, and biggest bladdered, fish atop the food chain. And unsustainable pursuit of that protein comes at a cost. A study published on Tuesday in Nature Communications