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Discarded plastic, already blamed for the deaths of hundreds of thousands of birds and fish in the oceans and along seashores, is spreading to the Great Lakes.

The virtually indestructible material is becoming not just an eyesore but a potential threat to wildlife in the world's largest freshwater system, according to researchers and lay observers.Jim Mattis of Benzonia, Mich., has operated fishing charter boats for 15 years in Lake Michigan. He said he's found plastic debris floating up to 10 miles offshore, some of it in "scum lines" of garbage a mile long.

"There's more plastic bags and pieces of Visquine (a trade name for plastic sheeting) in the lake than I've ever seen," Mattis said. "It's gotten worse in the last three or four years.

"I've heard a lot of people say, `It's just a plastic bag.' They don't realize the damage it can do."

Most legislation, research and enforcement has focused on a different kind of Great Lakes pollution - toxic chemicals that threaten widespread areas and can be traced to specific sources.

"There're about 1,000 different types of hazardous substances that are known to exist in the Great Lakes," said Cameron Davis, a research analyst with the Chicago-based Lake Michigan Federation. "Right now, toxics (and) contaminated sediments are the No. 1 thing."

Plastic pollution is less fully understood, although its potential for harm is clear. A single shard of plastic sheeting, six-pack beverage carrier or strand of fishing line can float undetected until it strangles fish or birds, binds a boat propeller or jams an engine intake.

"We see the pictures every year; we know it happens," said Glen Dudderar, a Michigan State University biologist. "But we don't have any research on to what extent."

The Michigan Department of Natural Resources estimated that one bird in 100 observed by biologists was entangled in plastic fishing line, according to a 1987 report by the Washington-based Center for Environmental Education. Entanglement can also mean ingestion of the plastic.

Little is known, too, about another synthetic material intentionally put into the lakes: fishing nets that are loosed accidentally but continue trapping their prey.

"Ghost fishing" has been documented mainly along the Atlantic and Pacific coasts. But it also occurs in Great Lakes waters where gill-net fishing is done, either by state-regulated commercial interests or by Indians working under terms of federal treaties, officials said.

"From the fisheries' standpoint, the worst thing we've got is gill nets that break away," said Asa Wright, manager of the Great Lakes Program of the state DNR's Fisheries Division.

Just last month, authorities retrieved a gill net containing more than 600 pounds of rotting lake trout. The 1,000-foot-long, 44-inch-wide net probably had been adrift several months in southern Lake Superior, investigators said.

Bill Eger, a biologist who works with Michigan's Indian fishermen, said gill nets occasionally are abandoned or torn from buoys in storms or by passing freighters.

But the approximately 350 Indians who fish in the treaty-protected waters of northwestern Lake Huron, northern Lake Michigan and south central Lake Superior usually set and retrieve their gill nets daily, Eger said.

Since gill nets cost about $1 a yard, they're seldom abandoned, he said. Those that aren't retrieved tend to "ball up" and sink, bearing only the fish trapped on the first day or two after being set, he said.

"It does not continue to catch fish at an exponential rate, as Asa Wright would like to have you believe," Eger said.

Charter skipper Mattis said plastic refuse poses a threat to navigation.

"My son last year blew out an engine when a plastic bag got sucked up in the intake," he said. "I had a piece two years ago . . . an 8-foot piece of Visquine that got around my prop. It wouldn't come off, it knotted up so bad. Luckily a shaft didn't break."

The governor has signed legislation that will make Michigan the 13th state - and the second in the Great Lakes region after Wisconsin - to require that six-pack beverage carriers be made of plastic material that degrades within 360 days of exposure to sunlight. The bill awaits action in the state Senate.

Enforcement of anti-dumping laws is isolated, conceded Capt. Larry Murdock, chief of the Marine Safety Division of the U.S. Coast Guard's district office in Cleveland.

Disposal of solid waste anywhere in the Great Lakes has been illegal for years, Murdock said. Freighters and other large commercial vessels are required to store or incinerate their trash rather than dump it overboard, a practice common among oceangoing ships and blamed for at least part of the plastic pollution at sea.

"For the pleasure boaters, I don't think you're going to find any enforcement," Murdock said. "We haven't apprehended anyone. . . . You're talking about an individual-type problem."

Department of Natural Resources biologists have estimated that Michigan's recreational boaters produce up to 26,000 tons of garbage a year. An unknown percentage of that total finds its way into the lakes.

The plastics industry has joined government and environmental groups in conducting public education campaigns about plastic pollution in the Great Lakes and oceans alike.

Brochures describing the harm done to marine life by discarded plastic have been jointly distributed by the Center for Environmental Education, the Marine Entanglement Research Program of the National Marine Fisheries Service and The Society of the Plastics Industry Inc., an industry group.

The Fisheries Service and Defenders of Wildlife, another Washington-based environmental group, have distributed similar brochures encouraging boaters and beach visitors to dispose of trash properly.

Individuals can do the most to turn back the invasion of plastic flotsam, said Tom Martin, director of Michigan's Office of the Great Lakes: "The absolute key thing is people's individual responsibility, that it has consequences for the environment."