The Reagan administration, still trying to farm out the Census Bureau under its plan to remodel the Commerce Department, has hit upon a prospective new home that pleases almost no one outside of the White House.

The plan is to move Census in with the Treasury Department.

Private statisticians were horrified, and at a meeting last month with presidential counselor Edwin Meese III, a group of them laid down their bottom line: Under no circumstances should the confidential data supplied to the Census Bureau get close to the prying eye of the Internal Revenue Service.

Business, it appears, feels the same way. In a recent "Dear Dave" letter to Office of Management and Budget director David A. Stockman, the head of the National Association of Manufacturers warned that bedding down the nation's data-gatherer with its tax collector "could seriously jeopardize the bureau's reputation."

NAM president Alexander B. Trowbridge told Stockman that business support for a new Department of International Trade and Industry could go down the drain if Census links up with Treasury.

The OMB has asssured Trowbridge that Treasury would respect the confidentiality of Census data, but opponents of the proposed cohabitation are skeptical.

"If everything is in one department, it might become difficult to uphold restraints," said Katherine K. Wallman, director of the Council of Professional Associations on Federal Statistics. "Everybody would be reporting to the same secretary."

The NAM would like the Census Bureau to stay with the new trade department, but that would more than double the size of what the administration envisions as a light, quick-on-its-feet agency with a mere 7,500 employes.

Statisticians and demographers would be pleased to see the Census Bureau as an independent agency. But the adminstration doesn't want yet another independent agency, which are notoriously difficult to control.

So far the administration hasn't budged. Legislation for the new department, scheduled for presentation to Congress on Sept. 21, is expected to propose moving Census to Treasury. ***

A FISH STORY . . . Speaking of statistics and censuses, the General Accounting Office has concluded that the government is doing a fine job of counting coho salmon. That no doubt will come as welcome news to the National Marine Fisheries Service, but it is not likely to please Oregon salmon fishermen.

The population of coho, a superb game fish that once abounded off the West Coast, plummeted in the mid-1970s from about 4.1 million to about 1.1 million.

Biologists aren't sure what combination of factors caused the decline, but they are reasonably certain overfishing hasn't helped.

As a result, the fisheries service and state wildlife officials started using a mathematical formula called the Oregon Production Index to gauge coho populations and set annual quotas and fishing seasons off the Oregon coast.

Commercial and recreational salmon fishermen, however, thought the index was downright fishy. Last year, they decided they were fed up with scaled-back quotas and short seasons, and accused federal and state officials of seriously undercounting the coho.

A lot of the complaints fell on the ears of Rep. Les AuCoin (D-Ore.), whose constituents include not only commercial fishermen but also the motel, restaurant, marina and charterboat owners who make up the state's lucrative sport-fishing industry. AuCoin, being both an attentive congressman and an ardent conservationist, tossed the problem to the GAO.

The GAO's assessment: The index is biologically sound and more accurate than salmon predictors in use in any other state. It can be improved, the GAO said, but only by adding staff and equipment at a cost that might not be balanced by the value of the extra fish that might be found.

In the meantime, the report said, no degree of accuracy in the coho count will solve the problem of the salmon fishermen. "The fishing capacity of domestic fishermen," it said simply, "now far exceeds the number of fish available."