HUDSON, Wis. (AP) -- A return of zebra mussels in the Lower St. Croix River is causing concern that the clams could clog water intake systems and threaten the river's native mussels.
Nick Rowse, a U.S. Fish and Wildlife Service biologist, and two divers from the National Park Service last week searched the bottom of the river for the small, non-native clams.
The divers found 20 of them from Stillwater, Minn., south to Prescott. Despite their worst fears, they found no reproducing population. Had they done so, it would have been a big blow to protectors of the national scenic river on the Minnesota-Wisconsin border.
Even so, what the divers found is cause for concern.
''This is worrisome to me that we've found this number,'' Rowse said. ''I don't know what this means. We've got to figure it out.''
Zebra mussels, a Eurasian species accidentally introduced into the Great Lakes in 1988, can clog water intake systems and displace native mussels, outcompeting them for food and encrusting and killing them.
That's a key concern in the St. Croix, a river prized for its diverse ecosystem. The river, part of the National Wild and Scenic River system, contains 38 native mussel species, including two -- the Higgins' eye and the winged mapleleaf -- that are on the federal endangered species list.
''I would say if zebra mussels get in the St. Croix, it is going to cost millions (of dollars) to taxpayers,'' Rowse said. ''You've got cooling plants, power plants, city intake pipes. And then you've got the biological issues, with the native mussels.''
When the divers entered the river for the first of this year's three weeks of exploratory dives, they weren't sure what awaited.
Last month, divers from the Minnesota Department of Natural Resources found 10 ''zebes'' -- as zebra mussels are commonly called -- in a bed of relocated native mussels north of the Interstate 94 bridge near Hudson. Federal divers found 14 more in a second dive.
Last week's plan was to check the relocation bed thoroughly, as well as other mussel beds, marinas, buoys and bridge supports that might yield evidence of the exotic invader. In all, the divers hit 20 sites, and checked 300 boats.
All but two of the zebes found this year were the same size and were scattered, an indication that no reproductive colonies have been established, as has occurred in the Mississippi River. There, zebes have been found as far north as Minneapolis, and are especially thick from Lake Pepin to the south.
Unlike other rivers, the St. Croix still has as many mussel species as it had centuries ago, said Byron Karns, a Park Service diver and biologist. ''That's an indication of the water quality here,'' Karns said.
Zebes were discovered in the St. Croix in 1994 on the hulls of boats entering the river from the Mississippi.
In the early- and mid-1990s, federal and state officials imposed a series of measures to hold off an infestation.
In addition to aggressive education efforts along the river, the Park Service set up a houseboat at the Arcola sandbar north of Stillwater and does not allow unclean boats north if they've been in infested waters. Laws also were passed making it illegal to transport zebes into uncontaminated waters.
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