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In the year 1289, King Philip IV of France was worried about fish. “Each and every watershed of our realm,” he proclaimed, “large and small, yields nothing due to the evil of fishers.” Environmental change, expanding cities, and overfishing had sent aquatic populations into a tailspin. Because they were scarce, the fish, King Philip noted, “are much more costly than they used to be, which results in no moderate loss to the rich and poor of our realm.” This state of affairs could not stand. The king promulgated the country’s first fisheries ordinance.

In medieval Europe, an era stretching from about A.D. 500 to 1500, fish was a prestigious food. Chefs experimented with ways to disguise beef as fish: At least half a dozen cookbooks of the era include recipes for turning veal into imitation sturgeon for wealthy lords and ladies. Sturgeon was so rare in England and France that it was reserved for the monarchs, and the Cistercians, a Catholic religious order that used sign language to communicate, referred to it using the sign for fish and then the sign for pride.

People of all social classes, though, ate freshwater fish—trout, whitefish, pike, eel, lamprey, and shads. This taste started to have consequences. Today, fish populations around the world are rapidly declining; a millennium ago, Europeans faced similar challenges. Overfishing resulted in local extinctions, and popular food fish had to be domesticated through aquaculture. The population pressures created by humans may have even changed the size of fish.

Human appetites and needs are indisputably transforming ecosystems and wildlife in the modern world. But the more clues archaeologists uncover from the European past, the more they understand how dramatically these same influences have been shaping fish populations for hundreds of years.

Richard Hoffmann, an environmental historian, has been studying the complex interplay between humans and the aquatic environment for most of his career. He’s read a medico-dietary analysis of the Catholic saint Hildegard that names 37 fish taxa; he’s found tax records for the price of fish; and he’s reviewed zooarchaeological analyses on the rise and fall of fish populations across Europe. All these details help him reconstruct which fish were on the menu for different social classes, how big those fish grew, and when they disappeared.

Asking those questions often means confronting myths. “Some people think everyone in the past was rapacious,” Hoffmann says. “You also get the opposite myths of hyper-abundance.” One false tale that originated in the 17th century alleged that salmon and sturgeon were so abundant during the Middle Ages that servants had contracts stipulating they wouldn’t be served those fish more than a few times a week.

The reality is more complicated. In Europe, aquatic animals have been traded at least since the days of the Roman Empire. But it was during the early Middle Ages, with the arrival of widespread Christianity, that the animals became a popular source of protein. That’s partially due to the roughly 130 days a year when the faithful were exhorted not to eat meat, because fish didn’t count in that category.

At the same time, expanding agrarian populations were cutting down forests to create fields and diverting rivers to fill defensive moats around castles and towns, Hoffmann writes in one paper. From the ninth century A.D. to the 11th, the number of grain mills built along rivers in England exploded from about 200 to 5,624. Species that came into fresh water to spawn, such as salmon and sturgeon, began declining. New regulations, such as King Philip’s, were put into place to manage fish populations. A Scottish statute from 1214 required all dams to include an opening for fish and barrier nets to be lifted every Saturday, for instance. Soon highly sophisticated aquaculture ponds stocked with carp also provided regular access to fish for the landed elite.

This decline in freshwater populations coincided with a sudden, commercial-scale boom in sea fishing, which began around A.D. 1000 and is known as the “fish event horizon.” In one study, archaeologists collected cod bones in London from 95 Roman, medieval, and postmedieval sites. The number of bones jumped circa the year 1000, and isotopic sampling showed that in the following centuries, fish came from farther and farther away, indicating long-distance trade. In the southern English town of Southampton, the remains of marine species (such as cod) began to outnumber freshwater species (such as eel) by 1030.

That “fish event horizon” could have been caused by a number of forces. It came at a time of population growth, urbanism, new ship technology, and increased trade, says the archaeologist James Barrett, from the University of Cambridge. But, he adds, “I’ve argued consistently that this must also be about human impacts on freshwater and migratory fishes. The degree of vulnerability of fishes depends on how bounded the ecosystem they occupy is.”

In other words, because their habitat was smaller, freshwater fish were more likely to respond to human pressures sooner. When the reliable stocks of freshwater fish began dwindling, hungry Europeans turned to the much larger oceans. And while those populations had larger ranges, humans still had an impact.

In a recent paper for the Journal of Fish Biology comparing archaeological records and modern trawling surveys, Barrett notes that cod from the North Sea in the 11th and 12th centuries measured more than 31 inches in length. For fans of McDonald’s Filet-O-Fish sandwich (made with pollack, not cod), a 31-inch fish would sprawl across nine buns lined up side by side. By the 13th century, the average was somewhere between 20 and 31.5 inches (about five buns’ worth); today, the average is six to 12 inches. On the smaller end, the fish might just poke over the edges of a single bun.

Analyzing fish remains is incredibly labor-intensive. Around Britain alone, there are something like 350 species; each fish has more than 50 vertebrae. Unlike the mostly connected skull bones of mammals, fish skulls are an assemblage of dozens of loose bones. Because of the sheer amount of labor involved, zooarchaeological work on fish in Europe didn’t really find its footing until the 1970s, Barrett says. And before the labor-intensive practice of sieving sites with fine mesh began, researchers would turn up the remains of only large fish.

Since the 1970s, the field has seen an explosion of new technology. Now, by looking at varying levels of nitrogen and carbon preserved in fish bones, scientists can tell where the fish lived (and were likely caught) based on what they ate. A newer method, zooarchaeology by mass spectrometry, or ZooMS, uses collagen—the structural protein in bone—to identify the species based on an existing library of collagen fingerprints.

“You can potentially tell what species a bone is from about 10 milligrams of bone,” says David Orton, a zooarchaeologist at the University of York. For comparison, a teaspoon holds about three grams of flour, or 3,000 milligrams. One of his doctoral students will try to use ZooMS to distinguish the bones of two extremely closely related species. Trout live exclusively in fresh water, whereas salmon come to fresh water only to spawn, so distinguishing the two species could reveal more about medieval fishing practices.

At times, medieval fishery managers actually succeeded in course correcting. In the medieval Salzburg Alps, fishers paid the local archbishop 27,000 whitefish and 18 trout a year for the right to catch and sell even more of those fish; within one generation, whitefish populations collapsed, and the pike that were brought in to replace them ate nearly all the trout. The community decided to forbid fishing for three years, then set regulations for a limited season in a restricted area. “The rules were changed, there was proper enforcement, and it was restored and operated for hundreds more years,” Hoffmann says. But for all the efforts of King Philip and like-minded monarchs, sturgeon remains critically endangered across Europe.

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