After nearly four decades as a fishing guide on the Great Lakes, Pat Chrysler has seen enough damage from invasive species to fear what giant, ravenous Asian carp could do to the nation's largest bodies of freshwater."It's like introducing piranhas to the Great Lakes," Chrysler said from South Bass Island in Lake Erie, which teems with walleye, perch and other fish that draw anglers from near and far.Federal and state officials are mounting a desperate, last-ditch effort to prevent the marauding carp from breaching an electrical barrier and slipping into the Great Lakes from the Mississippi River. Michigan is drawing up a lawsuit demanding the closing of shipping locks between the lakes and the Mississippi. And last week, Illinois officials poisoned a 6-mile stretch of a canal to wipe out any of the carp.The prospect of a carp invasion alarms environmentalists and people whose livelihoods depend on a strong fishing and tourism economy, from charter boat skippers to those who sell bait and tackle, rent personal watercraft and operate lakefront restaurants and motels. The Great Lakes fishing industry alone is valued at $7 billion a year."I'm afraid they can wipe us out in a hurry," said Jim Conder, a charter boat operator on Michigan's St. Joseph River, which flows into Lake Michigan. "We need to spend all we can to keep them out."Over the years, parasitic sea lampreys, zebra mussels and other invasive species have killed trout and birds, left prized salmon and whitefish skinnier, and done other damage to the lakes.Now, many fear that the despised Asian carp, which can reach 4 feet long and weigh up to 100 pounds, will wreak havoc, too — not by attacking native fish, but starving them out by gobbling up plankton.The carp were imported from Asia to cleanse fish ponds and sewage lagoons in the Deep South but escaped into the Mississippi and have been working their way north since the 1970s.Much is unknown about what will happen — and how quickly — if they conquer the Great Lakes. But the carp's ability to take over is evident in places like the Illinois River, where it has caused native fish such as gizzard shad and bigmouth buffalo to go hungry.