DRUMNADROCHIT, Scotland (AP) _ A scaled-down effort to find the Loch Ness monster with sonar got a signal Monday indicating a very large ''fish-like'' form at 450 feet, the spokesman reported.

A clear, arched blip was registered near the southern end of the lake on the sonar aboard one of five vessels still searching for signs of Nessie after a three-day sweep of the 23-mile-long lake by a score of motorboats, said Guy Pearse, spokesman for Operation Deepscan.

The boat scanned the same area again, but the object had gone, he said.

''Because we couldn't find it when we came back, it would appear to be moving,'' Pearse said. ''The contact shows a fish-like arch. The sonar boom would indicate it is something very large indeed.''

The expedition involving a simultaneous sweep of the 754-foot-deep lake by 20 motorboats ended Sunday. Pearse said two or three vessels will continue scanning the lake Tuesday.

Adrian Shine, who organized the search, sid the latest contact and three less-clear contacts Friday, ''will be thoroughly investigated through the use of underwater television equipment and further sonar scans.''

He speculated Friday that Nessie might be a big fish or a 20-foot-long eel.

More than 4,000 people have reported sighting the monster in modern times and searchers have have used everything from breadcrumbs to submarines in trying to track Nessie.