A study has uncovered the highest pesticide levels ever detected in California shellfish in a national marine sanctuary in Monterey Bay, the state Department of Fish and Game reported. It is a marshy estuary near Moss Landing. "At first we thought the measurements could be a fluke," said state water quality biologist Mark Stephenson. "Now we are certain we have real high levels." An investigation disclosed excessive levels of toxaphene, endosulfan and DDE, a breakdown product of outlawed DDT, in shellfish at Elkhorn Slough. The slough is a national preserve and breeding ground for thousands of shore birds, waterfowl and marine creatures. The Monterey County Health Department has posted warnings that the shellfish should not be taken and consumed by humans. The area has been popular for clam-digging.