WASHINGTON (AP) _ The Commerce Department issued a preliminary ruling Friday that certain imports of fresh fish from Canada were being illegally subsidized.

The department put the amount of the subsidy at 6.85 percent of the value of the fish and said it would require importers to begin posting bonds equal to that amount.

The ruling involves imports of fresh whole and fresh fillets of cod, haddock, pollack, hake and flatfish.

The bonds will remain in effect until the department issues its final decision in the case on March 18. If the department determines that its initial finding is correct, then the 6.85 percent tariff, known as a countervailing duty, will remain in place.

Countervailing duties are imposed in order to offset the price advantage subsidized imports have over domestic products which are not subsidized.

The Commerce Department investigation was begun last August after the North Atlantic Fisheries Task Force filed a petition on behalf of the U.S. fishing industry.

The department's preliminary ruling found that 19 programs administered by the Canadian government and 20 programs administered by the provinces of Nova Scotia, New Brunswick, Prince Edward Island, Newfoundland and Quebec were providing subsidies to the Canadian fishing industry.

These programs included grants for vessel constuction, preferential government financing and development grants for new fishing firms.

Imports of fresh Atlantic groundfish from Canada in 1984 were valued at $54 million.