CAPE TOWN — Some of the elephant corpses had begun to decay, their skins dried stiff over bony carcasses. Others appeared to have been freshly killed, partly covered by bushes in an attempt to hide them from view.

The tusks of 87 animals, which were counted during aerial surveys over the past few months in Botswana, had been chopped off — evidence of what conservationists are calling one of the biggest slaughters in recent years.

Michael Chase, director of the charity Elephants Without Borders, which conducted the surveys, said he had never seen so many dead elephants in one go. After counting 48 during a single flight in August, Mr. Chase wrote in a report that the tally was “indicative of a poaching frenzy which has been ongoing in the same area for a long time.”

The numbers were expected to climb as the survey continues. But the results so far already signal a major escalation in elephant poaching, said Tom Milliken, a program manager at Traffic, a nonprofit organization that monitors wildlife smuggling.