Maryland will impose a moratorium Jan. 1 on fishing in state waters for striped bass, or rockfish, the most valuable commercial fin fish in the Chesapeake Bay, the state secretary of natural resources announced today.

The ban, which will apply to both commercial and sport fishing, and which officials predict will last a minimum of four years, is the most stringent measure taken in Maryland to reverse the precipitous decline of what once was the Chesapeake Bay's premier game and table fish.

"Maryland must take immediate steps to . . . increase the survival rate" of its striped bass, said Secretary Torrey C. Brown in announcing the moratorium and his department's designation of the fish as a threatened species.

"We plan to keep the moratorium in effect until there is clearly a response in the Chesapeake Bay, and acceptable levels of reproduction," Brown said.

As head of the state department of natural resources, Brown has the authority to impose the fishing moratorium unilaterally and does not need legislative approval.

Brown, who said that the ban has the full support of Gov. Harry Hughes, also announced the formation of a task force that will be charged with devising mechanisms to compensate commercial fishermen for their losses.

Commercial catches of striped bass were worth about $1.7 million at dockside last year but the industry, extending from charter boat to restaurant table, is worth many times that.

Brown estimated the cost of the compensation program at between $1 million and $2 million per year.

The state's decision to halt all striped bass fishing immediately was denounced by the head of the Maryland Watermen's Association, who predicted that the ban would have a devastating effect on commercial fishermen in this area.

"It's a knee-jerk reaction that is typical of this administration," said Larry Simns. "They are incapable of managing the fishery."

Jim Price, a spokesman for the 280-member Maryland Charter Boat Operators Association, predicted that the ban would put "about 30" captains in the upper bay out of business.

"Those fishermen will be upset," although generally members recognize the need for the ban, Price said.

William C. Baker, director of the Chesapeake Bay Foundation, an environmental group that has spearheaded efforts to clean up pollution of the waterway, said that the foundation will support the moratorium even though it represents "a sad day for the bay."

The ban comes after more than a decade of steady decline of the species in the bay, which historically has accounted for about 90 percent of all the rockfish that is hatched in the east.

Since 1970, commercial catches in the bay have fallen from more than 5 million pounds a year to less than 400,000 pounds.

Brown said today that his department's annual summer survey of juvenile fish indicates that 1984 was another disastrous year for rockfish, the fourth poor season in the past five years.

Conservation measures enacted by the General Assembly last year, designed to cut the rockfish catch by 55 percent, proved insufficient to overcome such environmental factors as acid rain and disease, Brown said.

He added that the continued poor survival rate for young rockfish "adds to our concern," and said that a program proposed by the Atlantic States Marine Fisheries Commission to cut catches by 55 percent in all its member states "will not provide enough protection."

A committee in the House of Representatives recently approved enacting federal legislation to impose bans on striped bass fishing on any states that did not agree to reduce their catches by 55 percent.