Fairfax County Sheriff James D. Swinson, saying his critics should "fish or cut bait," yesterday asked the county's Board of Supervisors to increase the size of the medical staff at the county jail he oversees.

Appearing at the board's regular meeting, Swinson requested that a registered nurse supervisor and two physicians' assistants be added immediately to the jail's staff of five medics.

The board, which three weeks ago ordered a management study of Swinson's procedures in running the jail, referred the sheriff's request to its staff for further study, arousing Swinson's ire.

"I would like some commitment from the board that something will be done," once the study is completed, Swinson told the board.

"You can rest assured of that," replied Supervisor Warren I. Cikins (D-Mount Vernon).

Swinson recently became the center of controversy after three black prisoners died in the last six months after being in custody in the county's year-old, $4.6 million detention center.

An investigation by Fairfax prosecutor Robert F. Horan Jr., carried out at the board's request, found that the third prisoner, Donald L. Ferguson, died Dec. 8 of kidney failure following a "failure to monitor his situation in the jail during the time he was virtually unable to take care of himself," Horan said.

Swinson admitted to the board last month that "some mistakes" were made in caring for Ferguson.

Swinson yesterday said he was basing his request on the advice of the jail physician, Dr. David M. Abbot. "I don't have the expertise or know-how to say what medical care is needed. I rely on what the doctor says," Swinson told a reporter outside the board room.

"If the board is going to demand a higher standard of medical treatment, they have to do something about it. They can either fish or cut bait," he added.

Asked what would happen to medical care at the jail while the county staff analyzes his request, Swinson said, "It isn't going to get any better. But it isn't going to get any worse, either."

County officials said that the three positions would cost a total of $45,000 in salaries. Half of that amount would be provided by the county and half by the State Department of Corrections, they said.

Swinson also told the board the jail has become so overcrowded that he may have to open the old jail in the Fairfax courthouse to house prisoners. The new jail, which has the capacity of 198 prisoners, yesterday was holding 225 prisoners, he said, Swinson said 68 other prisoners were farmed out to jails elsewhere in the state.