Populations of numerous migratory fish species in the North Atlantic have declined by more than 95 percent, threatening not only food supplies and economic systems, but also the way humans perceive the health of the planet’s ecosystems, according to a paper published today (Dec. 1) in the journal BioScience.

“It’s shocking,” said Dr. Karin Limburg, a fisheries ecologist at the SUNY College of Environmental Science and Forestry in Syracuse, N.Y., who is the paper’s lead author.

Limburg and her co-author, Dr. John Waldman of Queens College of the City University of New York, report that a complex combination of habitat loss (caused largely by the construction of dams that prevent fish access to traditional spawning areas), urban sprawl, overfishing, pollution and climate change have led to the precipitous decline. Compounding the problem, they say, is the evolving knowledge of the humans who make decisions about how natural resources are managed.

“We’re looking at shifting baselines here,” Limburg said. “Every human generation gains knowledge about the world and establishes a baseline for what’s normal. But there is no institutional memory about how things used to be.”

As an example, Limburg pointed to a graph that depicts the status of the American shad between 1887 and 1997. It indicates the species was more than ten times as plentiful during most of the early years of that period as it was during the middle of the 20th century. But a second chart shows that the levels in the 1880s were just 10 percent of what they had been 50 years earlier.

“We can’t envision salmon being a thing of the past,” she said. “That was once the case with shad. It was the most important fish in U.S. fisheries, after cod.” In fact, the shad’s Latin name (Alosa sapidissima) reflects the species’ high status as a food fish: “sapidissima” means “most delicious.”

In their findings, the authors wrote: “Loss of historical baselines contributes to marginalization of the species, as social customs relating to bygone (collapsed) fisheries also perish, and ecosystems unravel at rates that go unnoticed.”