Mussel spat rope can help small fish species navigate through culverts

Mussel Spat RopeMussel spat rope is an inexpensive option to aid fish passage in culverts. In a recent U of M study, researchers showed how low-cost mussel spat rope can help small fish species navigate through culverts by reducing current velocity and providing protected areas for fish to shelter and rest. Mussel spat rope is a “fuzzy” polyethylene rope with 2- to 3-inch fibers commonly used for mussel aquaculture, says research associate Jessica Kozarek.

Maintaining fish passage for Minnesota’s high-quality fish population is a concern for fish and wildlife organizations and for the Minnesota Department of Transportation (MnDOT), which must maintain culverts and construct new channels that do not create fish barriers. “Minnesota is a headwater state, and we have a responsibility to keep our fish population healthy,” says Petra DeWall, bridge waterway engineer with MnDOT’s Bridge Office.

MnDOT funded research by the U’s St. Anthony Falls Laboratory (SAFL) to investigate mussel spat rope as a possible low-cost, low-maintenance method for Minnesota’s culverts. Led by Kozarek, a SAFL team conducted experiments in the laboratory and in the field. Key observations from these investigations:

  • Mussel spat rope created small corridors (about 6 inches) of reduced velocity and turbulence along its length, which was sufficient to aid the passage of small fish. Sediment collected in, between, and beneath the ropes—an important finding, given that culvert floor sedimentation may assist fish passage.
  • The rope displayed wear over two years in the field, raising a concern about plastic microparticle release into streams. Sediment covered some ropes over time, suggesting a need for maintenance in some culverts.
  • In the laboratory flume, test fish swam near and between doubled rope lengths, apparently taking advantage of the reduced current near and beneath the ropes. While there was variation among species, most fish that swam upstream through the simulated box culvert ended their passage on the rope side, evidence that the rope provided cover and refuge from the current. (Only a few fish were observed at the field installations.)

“Overall, we believe that mussel spat rope is an inexpensive method that could be installed quickly in problem culverts, providing fish passage when other alternatives are infeasible,” Kozarek says.

MnDOT’s DeWall concurs: “Mussel spat rope will be one more effective tool in the toolbox of methods we have to assist fish passage through culverts.”

The final report includes guidance for installing the rope. The method will also be included in an upcoming guide for designing culverts that allow aquatic organism passage.

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