The European Economic Community opened negotiations here today with the Soviet Union on reciprocal fishing rights, an event tantamount to Soviet diplomatic recognition of the Common Market after 19 years of stubborn refusal.

Soviet Fisheries Minister Alexander Ishkov conveyed his country's wish to establish a licensing system permitting fishing access in each other's waters to his EEC counterpart, Finn Olav Gundelach.

Ishkov promised to reply Thursday to a Common Market offer that would allow 27 Soviet vessels to trawl EEC waters until the end of March. The Soviet Union has been issued an ultimatum to leave Common Market waters unless their boats are licensed.

European Community officials claim that Soviet boats have been fishing illegally off British, Danish and Irsh coasts since the Common Market's 200-mile sea limit came into effect Jan. 1.

The Soviet Union initially approached Britain and Denmark about signing bilateral fishing pacts but was rebuffed and told to negotiate through the Common Market. The British and Danish stand posed a sensitive diplomatic dilemma for the Soviets, who have refused to recognize - let alone negotiate with - the European Community.

While Common Market officials hailed the bargaining session as explicit evidence of Soviet recognition, Ishkov told reporters that "our attitude toward signing an agreement shows no change on the part of the Soviet Union toward the Common Market."

He refused to elaborate when prodded by reporters about the significance of today's talks. "It would take a long time to explain," he said.

But Gundelach, who is responsible for the EEC's fishing pacts with outside countries, emphasized that "the Russians made it clear they want to negotiate an agreement with the community."

Some observers here claim that Soviet fishing vessels are equipped with radar instruments used to track Western air and naval maneuvers. A sudden cutback in the number of Soviet ships in North Atlantic waters, these observers say, would adversely affect Soviet data gathering.

Soviet trawlers have been engaged in a massive effort to harvest fish from the North Atlantic seas in recent months before yielding to negotiations, The Soviet catch in European waters reached a record 600,000 tons during 1976.

The Common Market holds only marginal fishing interests in Soviet waters, focusing primarily on cod supplies in the Barents Sea. The catch amounts to only one-tenth of the fish the Soviets get from European waters.

Informed sources say the Soviet negotiators argued today that the European Community offer of 27 licenses is far too low and that they want as many as 70 boats to be allowed to trawl in Common Market waters.

The Soviets plan to issue their own licenses in the future, but because of the disparity in catches, the ECC obviously holds bargaining leverage.

Apart from licenses, the Common Market and the Soviet Union hope to frame an accord that would permit future discussions of quotas, similar to the pact signed between the EEC and the United States yesterday in Washington.

Both sides have stressed the need to conserve fish stocks, which have been depleted in North Atlantic waters in recent years.

Before leaving for his hotel, Ishkov said wearily, "I believe we will have an agreement and avoid a confrontation."