Large-river specialist fishes—from giant species like paddlefish and blue catfish, to tiny crystal darters and silver chub – are in danger.
According to a new study, in the U.S. 60 out of 68 species, or 88% of fish species found exclusively in large-river ecosystems like the Mississippi, Missouri and Ohio rivers, are of state, federal or international conservation concern. The report is in the April issue of the journal, Frontiers in Ecology and the Environment and was authored by Center for Limnology postdoctoral researcher, Brenda Pracheil, faculty member, Pete McIntyre, and Wisconsin DNR fish biologist, John Lyons (also a CFL alumnus).
What makes the findings especially worrying, is that conservation opportunities in America’s largest rivers are scarce.
“If I’m [trying to conserve species in] Wisconsin inland lakes,” explains Pete McIntyre, “I have 8,000 lakes to choose from. The lakes are fundamentally similar in their fauna, flora and other ecological processes. But, if you care about big-river specialist fishes, n equals one. There’s only one main trunk of the Mississippi.”
That main trunk is the nation’s shipping, power production, and flood control backbone. Locks, dams and levees in the Mississippi River Basin have destroyed fish habitat by blocking off migration pathways and changing the annual flood-cycles species need to spawn. Unlike smaller rivers and streams (for example, the CFL study site Big Spring Creek), where removal of dams and other structures is often feasible, undoing alterations responsible for habitat loss in the Mississippi Basin’s big rivers would cripple infrastructure necessary for shipping goods, generating electricity, and controling devastating floods.
Yet, despite this reality, many large-river fish conservation efforts are focused on replicating or restoring habitat in the big rivers themselves, even though removing these obstacles is economically and politically unfeasible.
That doesn’t mean that there’s nowhere for conservationists to turn. In fact, says Brenda Pracheil, the study’s lead author, we’re underestimating the importance of slightly smaller tributaries. Pracheil and her collaborators say that, for large-river specialist species, it’s not all or nothing. Some rivers are just big enough. And the threshold for supporting these fish is surprisingly well defined.
“I remember when [Pracheil] came in to my office with the graph [showing flow rate and large-river specialist fish abundance]” recalls McIntyre. “I thought it was wrong and sent her to get more data. I thought there was no way it was such a sharp threshold.”
But the numbers weren’t lying. For any river in the Mississippi Basin with a flow rate of less than 166 cubic meters of water per second, the researchers found virtually no large-river specialist fishes. But, in each river that even slightly exceeded that threshold, 80% of large-river species were present.
Mississippi tributaries about the size of the Wisconsin River and larger are providing crucial habitat for large-river fishes. When coupled with conservation efforts in the large-rivers themselves, these rivers may present important opportunities for saving the species.
“Talk to any large-river fish biologist, and they will tell you how important tributaries are to big river fish,” says Pracheil, who has been working to understand how tributaries benefit large river restoration since her PhD work showing a tributary of the Missouri River was crucial for paddlefish reproduction. “When you look through the scientific literature, you’ll find lots of examples discussing [this]. But, until now, we’ve not really understood which rivers are most important. Our study tackles that and shows which tributaries in the Mississippi River Basin show the most promise for conservation of large-river fishes.”
However, Pracheil says, current policies governing large river restoration projects are funded largely through the US Army Corps of Engineers, which requires that funds be spent on mainstems – or the big rivers themselves. Pracheil’s study suggests spending some of that money on tributary restoration projects like the passageway at the Prairie du Sac Dam on the Wisconsin River, might do more conservation good for fish and make more effective habitat restoration per dollar spent.
What’s more, keeping the connectivity between tributaries and the Mississippi is crucial. Historically these fish populations have been connected over a vast range and many of them are migratory and swim hundreds or even thousands of miles to spawn.
“Tributaries may be one of our last chances to preserve large-river fish habitat,” Pracheil says. “Even though the dam building era is all but over in this country, it’s just starting on rivers like the Mekong and Amazon— places that are hotspots for freshwater fish diversity. While tributaries cannot offer a one-to-one replacement of main river habitats, our work suggests that tributaries provide important refuges for large-river fishes and that both main rivers and their tributaries should be considered in conservation plans.”
The researchers hope that their study can point big-river fisheries managers to areas where both time and money will go further to protecting these unique species. By “diversifying” the portfolio of suitable large-river fish habitat, they’re hoping to up the odds for the long-term survival of big-river fish in our big rivers.
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