According to a story "Asian clams found at national hatchery, By Journal staff | Sunday, August 16, 2009" in the Rapid City Journal, Rapid City, SD, "Asian clams, foreign troublemakers in waters throughout 39 states, have turned up at the federal Gavins Point National Fish Hatchery at Lewis and Clark Lake in eastern South Dakota."

The story goes on to relate further details. However, fans of this particular famous folksinger's website maybe interested in two comments provided to the paper by two readers. The first is a question: "Question wrote on Aug 15, 2009 9:17 AM: 'How do these clams get here? Are they introduced by humans?'"
The second comment is a response: "Reply wrote on Aug 15, 2009 10:26 AM: 'According to Arlo Guthrie, clams have tiny feet.'"

The story and the comments can be found at http://www.rapidcityjournal.com/article ... commentdiv