Our “stop the beast’’ strategy of relying on dams and other barriers to block Asian carp has two very big drawbacks.
It doesn’t work, and it causes more harm than good, according to a river ecologist and researcher.
Over a century of experience with the common carp has demonstrated how ineffective any barrier is in stopping non-native species in Minnesota waters.
We have also learned that we harm the native species we need to control the invaders when we fragment a river, said Luther Aadland, Ph.D., a river ecologist with the Minnesota Department of Natural Resource’s stream restoration program.
“The best strategy in my view is to protect the native species and provide a functional system so they can compete with and prey on the non-native species,’’ said Aadland.
He spoke in Granite Falls the Friday before last about how the removal of the Minnesota Falls dam on the Minnesota River can benefit native species, and help us control invasive species.
Minnesota has roughly 1,300 dams on its larger waterways and another 2,500 on smaller streams.
Dams are one of the factors responsible for what some are calling the sixth mass extinction in the earth’s history. The extinction rate for freshwater species is five times that of the extinction rate for creatures on land.
“Dams are probably the biggest cause of extinction, period,’’ he said when speaking of the freshwater species being lost. “So if are really worried about native species, putting up barriers probably isn’t a good strategy.’’
Native fish depend on the ability to migrate. Once a river system is fragmented by a dam, native species lose access to spawning and other habitat. The mussel species that depend on specific species of fish and their free movement lose out too.
While we bemoan the extinction of creatures in faraway lands, we are hardly aware that species like freshwater mussels- critical to water quality and river health- are disappearing too.
In the Minnesota River, one-half of the 43 species of mussels native to it are no longer found in the main channel.
Dams create silt-filled reservoirs that concentrate nutrients and warm the water, creating the conditions for blue-green algae to flourish. Non-native species like Asian carp tolerate these harsh conditions far better than native species.
As for stopping Asian carp, there are 22 other ways that Asian carp can reach new waters besides upstream migration. On the Minnesota River, they will readily get by the dams now in place, he pointed out.
During flood conditions when these fish instinctively migrate upstream there are waters that bypass most dams.
The Granite Falls dam on the Minnesota River is 21-feet high, but during flood conditions there is only a five-foot elevation difference. Asian carp easily jump 10-feet high to reach new waters.
Page 2 of 2 - And during flood conditions, a glacial-era secondary channel carries onethird of the flow and connects the river by bypassing the dam.
The good news is that in healthy rivers, the native species do a very good job of controlling invasive species, according to Aadland.
The other good news is that nothing helps native fish communities re-establish themselves better than removing dams.
The evidence is right here, he said. The Pomme de Terre River reservoir below the Mill Pond dam in Appleton was dominated by bullheads and carp. With its removal after the 1997 flood, walleyes and channel catfish have returned and these game fish can now be caught all the way upstream to Morris.
Similar results are being seen in Dawson and Montevideo, where dams on the west branch of the Lac qui Parle and Chippewa Rivers have been removed.
Aadland said the removal of the Minnesota Falls dam this past winter will benefit shovelnose and lake sturgeon, paddlefish, walleye and sauger, and flathead catfish by opening up some of the best spawning habitat in the entire river system.
“This is a key area to not only the Minnesota River but also the Upper Mississippi because this is such unique habitat. These rapids and bedrock outcroppings are very important,’’ he said of the free-running river now restored.
The Minnesota River now runs free for 257 miles from Granite Falls to the Mississippi River, leading some to argue for at least a fish passage at the Granite Falls dam.
Del Wehrspann, a founder of CURE and river advocate, noted that the City of Granite Falls is in the process of replacing two small hydro-electric turbines in the municipal dam without so much as discussing the need for a fish passage. “With all we know…,’’ he said, “Why isn’t this even considered.''