Populations of numerous migratory fish species – those that move between freshwater and saltwater during the course of their lives – have declined by more than 95 percent in the North Atlantic . This threatens food supplies and economic systems, and is worse than was thought as people have tended to compare numbers over too short time scales, according to a paper published this month 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 report that a 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 10 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.”
Declines were seen in all but two of the fish populations studied. Striped bass, already the subject of protective measures, increased in North America, and lampreys were found to be more abundant in some rivers in France.
The analysis showed that the once-abundant allis shad, a member of the herring family that lives most of its life in coastal waters but migrates into rivers to spawn, plummeted by 99.9 percent in the Rhine River in the Netherlands between 1886 and 1933; the same species dropped by 99.4 percent in the Minho River in Portugal between 1925 and 1988. The European eel’s population plunged 95.4 percent in the Ems River, which flows through the Netherlands and Germany, and in the Vidå River in Denmark between 1960 and 1997; it decreased by 99.5 percent in the Yser River in Belgium between 1974 and 2004
When viewed as a group, the magnitude of the migratory species’ declines appears even more serious than that of marine predatory fishes, which has received far more attention, Limburg said.
She said the study highlights the importance of a relatively new school of thought in the scientific community: ecosystem services.
“We want to put this in the context of the new way many ecologists are now thinking, to say that ecosystems have a value by themselves,” Limburg said.
In particular, she said, the study highlights the interwoven relationship between marine and freshwater ecosystems. The two are linked in the North Atlantic by the 24 species of fish whose populations were analyzed; they are migratory fish that move between freshwater and saltwater during the course of their lives. “Sadly, the links are largely broken today because of the enormity of declines in abundances,” she said.
The findings were reported after Limburg and Waldman analyzed data formerly reported in scholarly literature and collected over the years by government agencies and management organizations in nations whose waters flow into the North Atlantic basin, which includes the Atlantic Ocean north of the equator, plus the Caribbean Sea and the Gulf of Mexico.
SUNY College of Environmental Science and Forestry
What do you think of this news item? Join a discussion.