A few years ago Paolo Guidetti was leafing through a book on ancient art when he came across a Roman mosaic showing a man's legs dangling from the mouth of an enormous fish. Struck by the picture, Guidetti, a biologist at the University of Salento in Italy, recognized the fish as one that he studies: the dusky grouper.

Today fishers would be hard-pressed to find a dusky grouper that large and so close to the sea's surface. The fish, found throughout the Mediterranean, are endangered. While they can grow to a length of more than four feet and a weight of 100 pounds, most are much smaller, and at sites where fishing pressures are highest they occupy waters too deep to leap out and eat anyone.

To help groupers recover, fisheries managers have established marine reserves throughout the Mediterranean. Evidence suggests the reserves are working: groupers are becoming more common at a variety of depths, and they are generally larger. Yet Guidetti and Fiorenza Micheli, a biologist at Stanford University, wanted to have a more accurate sense of the grouper's historical abundance. To determine just how far recovery efforts had to go, it would be better to know how the fish had fared thousands of years ago.

But how to do that? Thousand-year-old data are hard to come by, so Guidetti and Micheli turned to the mosaics that had first caught Guidetti's eye. They looked in museums and books. They spread the word among other biologists. It took a couple of years, but they were able to cobble together 73 Etruscan, Greek and Roman mosaics from between the first and fifth centuries that showed fish or fishing scenes. Of those, 23 had groupers.

“Using the mosaics, we were able to assess how big groupers used to be and how they were caught,” Micheli says. What they saw surprised them. In some mosaics, fishers used nets and harpoons to catch groupers at the water's surface—techniques that would never work today. Others showed groupers so large that they more than justified their historical reputation as sea monsters. “It suggested that groupers were considerably more common and accessible than anyone had thought,” says Micheli, whose findings were published late last year in Frontiers in Ecology and the Environment.

Artistic license and a fisher's penchant for exaggeration aside, Guidetti and Micheli argue that older, extrascientific sources can aid in calibrating conservation and management aims. “For these types of questions, we must be willing to consider the importance of less quantitative, more anecdotal evidence,” Micheli says. “We wanted to emphasize art as a form of information.”

COMMENT AT ScientificAmerican.com/oct2012