Big fish get bumped
Human impact and natural weather cycles are shortening the food chain in rivers and bumping big fish such as salmon off the top 

Large fish in rivers and streams are used to dining at the top of the food chain. But human impact and natural weather cycles are making changes in the global network of waterways that are turning their feast to famine and threatening the availability of salmon and other big fish for human consumption.

Assaults on rivers and streams have come from multiple directions, including land development, dams, agriculture, floods and droughts. But the end result is the same whether rivers overflow their banks or slow to a trickle: no place in the food chain for big fish, according to research co-authored by Jacques Finlay, associate professor of ecology, evolution and behavior, and published in the October 15 issue of Science Express online.

“We learned that anomalous high flow events [i.e. l arge, unpredictable floods] simplify the food web by reducing populations of intermediate species, which forces big fish to eat lower on the chain,” says Finlay, who led the effort to quantify food web structure and resource availability for the study. “On the other hand, droughts eliminate big fish altogether because many can’t tolerate low oxygen and high temperatures as a stream starts drying out.”

Climate change will likely take a greater toll on the Earth’s rivers and their food chains in the coming years. While some rivers near the equator will have less water, those at higher latitudes may have more.  And since drought increases demand for water to irrigate crops, rivers (and fish) in dryer areas will be further stressed. Food chains recover within a year of a major flood, but recovery from a drought takes much longer.

The study illustrates how competing users of river water – such as farmers and fishermen – will need to work out short-term solutions to share the water and long-term solutions to restore and protect rivers and streams, Finlay says. A recent global assessment published in Nature indicates that nearly 80 percent of the world's population live in areas where the quantity and quality of water in rivers, for both humans and fish, are at risk.” *

“Humans will need to make some really hard decisions about how to allocate water for agriculture and other human uses, but still leave enough in the rivers to sustain food webs,” Finlay says. “Some river fish, like salmon in the U.S., are important commercially; globally, river fisheries represent a major source of protein – and jobs” 

The researchers studied the food webs in 36 U.S. rivers and streams, ranging in size from the Mississippi and Colorado Rivers, to small tributaries, all of which provide water to large cities such as New York City, Minneapolis, Phoenix, Las Vegas and Los Angeles.

Naturally occurring stable isotopes of nitrogen were used to measure the position of predators in food chains because the heavy isotope accumulates in living organisms and increases by ~3.4 parts per thousand with each link in the chain.

Finlay’s research focuses on the ecology of freshwater ecosystems and their interaction with natural and altered landscapes that surround them. Other projects in his lab include investigations of mercury transfer through river food webs, and a collaboration with EEB Professor Bob Sterner to understand the mechanisms behind long-term nitrogen increases in the Laurentian Great Lakes. Finlay is also principal investigator for the National Center for Earth-surface Dynamics, which is based at the Saint Anthony Falls Laboratory.

John Sabo, Arizona State University, was lead author for the study. In addition to Finlay, co-authors included Theodore Kennedy, a former EEB Graduate Student now with the U.S. Geological Survey, Southwest Biological Science Center, Flagstaff, Arizona; and David Post, Yale University.

– Peggy Rinard