Researchers based in Marquette have potentially grave news for Michigan anglers: Hundreds of shallow basins dug into riverbeds to collect trout- and salmon-harming sediment might be more like fish coffins than protectors.
After two reportedly successful experiments in the 1980s, sand traps were constructed worldwide in an attempt to save fish populations hurt by excessive sand in freshwater streams. Michigan has more than 250.
Fast-flowing rivers like this one in the Keweenaw Bay can destabilized river banks where sand traps were installed. (Great Lakes Echo photo)
Fish like the brook trout need coarse riverbeds for successful spawning. (Aaron Gustafson photos)
But now, researchers from the Department of Natural Resources say they doubt whether these measures have had any benefit. In some cases, sand traps could even harm river ecosystems, experts say.
Popular species like salmon and brook trout need coarse riverbeds of gravel or small pebbles. Too much sand suffocates fish embryos, leading to a decline in the number of fish, and faced with excessive sand, an eventual ecosystem change.
That means different species would take up residence, which is bad news for anglers expecting salmon.
"Once the population of the stream is degraded, people will lose interest in fishing there," said Troy Zorn, who led the new DNR study.
"If they're not fishing there, they may not buy a license, lures, hotel expenses or gas to get there. It would add up," Zorn said.
Fast-flowing rivers like those along the snow-melt-fueled north shore of the Upper Peninsula during the spring could have destabilized river banks if sand traps were installed, causing "potentially catastrophic effects" on the amount of sediment, Zorn said.
That would mean fewer trout and salmon in the U.P.
Zorn and DNR colleague Todd Wills examined 65 sediment traps. They concluded that it was doubtful that most of them had any positive effect on fish populations.
A 2005 report examining 126 sediment traps dubbed them "successful," which the new report said was a "subjective" evaluation. That led to the traps' widespread adaptation, Zorn said.
Jill Leonard, a fish biologist at Northern Michigan University, said she hasn't seen the newest study but is familiar with the methods used to reduce sediment. She agreed that traps could have a potentially negative impact on sport fishing.
She said many Michigan riverbeds are naturally sandy, but factors like logging, lack of vegetation along the side of a river and "improper land use" all contributed to make rivers even sandier.
"People are really very well-meaning. They want to do something to help," she said.
"But sometimes these are really long-term commitments. It's hard to make that judgment in the beginning, and as always, the best way to prevent damage to the stream is to prevent it in the first place."
Sand traps require a lot of maintenance, she said, and upkeep requires money that isn't there.
Leonard said the problem with habitat modifications is that they may work in one stream but not in another.
But she said people do want to help river ecosystems. "People are desperate to fix it. It sounds really trite, but it's the truth."
Wills, who works at DNR's Lake St. Clair Research Fisheries Station near Mt. Clemens, said the department has scaled back construction of the traps, and discussions have been held to dissuade nonprofits and public agencies from wasting money on ineffective techniques.
Wills said the more than 250 traps across the state will naturally disappear over time.
"They're going to eventually fill in, and there won't ever be any indication that they were ever there."