OFF THE SOUTH FLORIDA COAST — Even as experts played down the threat of a large-scale exodus from Cuba if Fidel Castro dies, law enforcement agencies staged mock exercises yesterday to be prepared just in case.

During one simulation, nearly a dozen government vessels maneuvered off Fort Lauderdale to stop a craft supposedly carrying armed smugglers headed to Cuba to pick up migrants.

That simulation began hours after a real Border Patrol mission picked up more than 40 Spanish-speaking migrants who happened to arrive along Miami-Dade beaches. Arrivals like those occur often in South Florida and involve mostly people from Cuba.

But Cuba experts said they don’t expect massive waves of migrants reminiscent of the 1980 Mariel boatlift — even after the death of Fidel Castro, who transferred power to his brother Raul Castro last July because of ill health.

More than 124,000 people were stopped at sea in a six-month period during the Mariel crisis, which was triggered when Fidel Castro said anyone who wanted off the communist island could leave.

“Forget it. It ain’t gonna happen,” said Jaime Suchliki, a University of Miami professor and the author of “Cuba from Columbus to Castro.”

“Raul would have to say, ‘Anyone who wants to go, go,'” Mr. Suchliki said. Such a move would destabilize the Cuban government and cause another major crisis with America. Raul Castro wants neither, he said.

Also, he said, the region doesn’t have enough vessels to transport half a million people out of Cuba. A more likely scenario would be thousands attempting to get into the American naval base at Guantanamo Bay, Cuba. “What would the U.S. do in that scenario?” Mr. Suchliki asked.