Loss of Viking-era herring may be a warning to today’s fishers

Fatty and abundant, herring have fed humans for centuries—and perhaps even longer than that. A new genetic study of modern and ancient herring bones suggests humans have been trading the fish across long distances since Viking times—and we’ve been overfishing them for nearly as long.

This is the first research to comprehensively show how herring populations in the Baltic Sea have changed through time, says Poul Holm, a marine environmental historian at Trinity College Dublin who was not involved with the work. The study also provides a lesson for today’s fishery managers, he says: Important stocks of this common fish can vanish.

Herring—a silvery, rolling pin–size fish—have been prominent across the globe for so long that many places regard them as a cultural icon. They are easy to catch, as they assemble into schools many millions strong to spawn. In the Baltic, Atlantic herring (Clupea harengus) “supported one of the most important trades in medieval Europe,” fueling the rise of Amsterdam, Copenhagen, and other cities, says James Barrett, an archaeologist at the Norwegian University of Science and Technology.

Herring are still common, but fish ecologists worry about the viability of certain populations. To better understand humans’ impact on these fish, marine historical ecologist Lane Atmore and evolutionary biologist Bastiaan Star, both at the University of Oslo, along with Barrett and other colleagues carried out genetic studies of both ancient and modern herring. Atmore obtained DNA from herring bones unearthed at seven archaeological sites in Poland, Denmark, and Estonia that dated between 750 C.E. and 1600 C.E. The team sequenced ancient DNA from these 40 samples, as well as DNA from 53 fish collected between 2002 and 2010.

By combining these modern genomes with already published herring genomes from 22 other places, the researchers had fish DNA from throughout the Baltic Sea. “The scale and range of data of this study is very impressive,” says Iain McKechnie, an archaeologist at the University of Victoria who has studied the historical use of herring in the U.S. Pacific Northwest but who was not involved with the work.

From the modern genomes, the researchers divided Baltic herring into four groups depending on where and when they spawn. Two groups reproduce in the fall in the center of the Baltic or in the western Baltic. The other two spawn in the spring in either the central Baltic or its gulfs farther north.

The researchers then compared modern and ancient genomes to determine which group dominated at each archaeological site. Bones at the oldest site, Truso in Poland—dated to between 800 C.E. and 850 C.E. during the age of the Vikings—correspond to fall spawners from the western Baltic at least 400 kilometers away, the team reported last week in the Proceedings of the National Academy of Sciences.

Herring spoils quickly if not salted or cured, so fish sales need to be big enough to make processing and transporting the fish worth it, Atmore explains. Finding herring that have been traded long distance as early as 800 to 850 C.E. is really surprising,” she says. That predates the urbanization and population booms that some researchers think encouraged long-distance trading of herring.

Because the scientists had DNA from bones from archaeological sites that spanned many centuries, they were able to tell how fishing practices changed over that time. The data suggest the two fall spawning populations declined one after the other over the next 8 centuries, even though the cooler climate of the Little Ice Age (1300 to 1850 C.E.) should have favored an increase in their numbers, Atmore says. Today only spring spawners are numerous enough to make commercial fishing for Baltic herring viable. (Spring spawners are smaller fish, and therefore considered less desirable.)

Herring are the poster child of a seemingly inexhaustible marine resource,says Thorsten Reusch, a marine ecologist at the GEOMAR Helmholtz Centre for Ocean Research Kiel who was not involved with the work. But the new work shows the more valued stocks were in fact exhausted some hundreds of years ago.

A few researchers question the conclusions. Holm points out that royal courts in regions that now include Denmark and Poland were in contact during the Viking period. Its possible that those earliest herring bones arrived at Truso not so much as commercial trade as such, but part of a gift exchange between high-level social strata,” he says. We need more data,” adds Leif Andersson, a geneticist at Uppsala University.

Others take these results as a bellwether. Today governments tend to set their own management plans, but McKechnie thinks it would be better to manage the genetically distinct populations even if they cross jurisdictional boundaries.

Modern commercial activities suggest spring spawners in the central Baltic have also been depleted; today, fisheries largely target the herring population that spawns in the northernmost reaches of the Baltic in the spring. Thus, Reusch says: Extra precaution should be taken not to make the same errors and exhaust that one, too.

source: sciencemag.org