The message said "Fish safely until further notice." At midnight yesterday it was Morse-flashed 600 miles north of here to the Japanese trawlers fishing the sub-zero, wind-shrieking darkness of the Sea of Okhotsk.

The Soviet Union put a 200-mile fishing zone on its Arctic and Pacific coasts into effect today, and the Japanese fishing crew harvesting Alaskan pollack from the rich spawning grounds well inside that limit were apprehensive. They feared arrest, confiscation of catch, maybe worse. The message that they could continue for a while at least was relayed from Moscow, where their fisheries minister, Zenko Suzuki, was negotiating on deadlinewith his Soviet counterpart, Alexander Ishkov.

The latest international fishing tangle over the sporadic advance of 200-mile zoning rocked this hard-bitten fishing city of 210,000 in the remore northern island of Hokkaido. The city lives on the 900,000 tons of fish brought home by the 1,400 boats based here and when the supply of fish is threatened Kushiro's future is uncertain. Shock went down the main street like the wind-blown grit and into the snug homes and salty waterfront bars when the Soviet Union announced the 200-mile limit last Friday, allowing only five day's warning.

Fears, real and imagined, are sweeping the town. Rumors say that 40,000 to 50,000 people will have to leave, that some big factories are already close to bankruptcy.

Mayor Tetsuo Yamaguchi says it was "a tremendous shock." At town meetings in recent months, people directly and indirectly dependent on the fishing industry - "everyone from barbers to fish-processing workers" - have anxiously sought his opinions. "Now what we fear has happened," he said today.

Although highly unlikely, an outright ban on fishing Soviet waters would cost Kushiro something like 20,000 jobs and 28 per cent of the city's annual product. Of Japan's total fish haul, 10.6 million tons, 2.65 million tons is caught off the Russian shoreline.

Some of the anxiety voiced in Kushiro is syntheysized for tactical purposes and some of the fears are clearly exaggerated. The city's leaders are genuinely concerned, however, that they may be trapped between a rock and a hard place because the fishing problem is linked with an emotion laden territorial dispute.

The Soviet Union has declared the its 200-mile limit applies to the four islands off Hokkaido's northeastern coast. Japan has long claimed ownership of the Soviet-occupied islands, [TEXT OMITTED FROM SOURCE] in Kushiro fear that the dispute may boil down to renouncing the claim to the islands or delaying a new fishery agreement.

When the 200-mile zone took effect today, about 150 Japanese boats - 50 or so from Kushiro - had their nets down inside it. Government fishery officials and industry executives stayed at their office phones late into the night, prepared to send a signal to withdraw if necessary.

When the good news came through, some trawlers were cruising on the boundary of the Soviet zone. Eight that were in Kushiro's harbor dropped their moorings and headed north through the bobbing ice floes.

After years of hauling fish from the world's high seas unhindered, the Japanese are adjusting slowly and with a sense of grievance and insecurity to the international move to protection of marine resources. The United States, residents here said frequently today, is to blame for setting the trend with a unilateral declaration of its 200-mile zone.

Word that the Russians were not going to police their zone immediately was greeted here as a stay of execution. There is a gloomy expectation that the Russians will demand high fees or sharply reduced catches.

The 349-ton Hokutou Maru 3 was fresh back in port from the frigid waters of the Soviet Union's Kamkhatka Peninsula, and her crew in rubber slickers, boots and helmets were chipping tons of ice from the deck and the trawler's rust-streaked super-structure. Ruddy from the cold, they worked cheerfully. Half a deckhand's $8,500 in average yearly income is paid in bonus, and they had 480 tons of pallock in the holds.

In the wheelhouse, surrounded by modern navigation and fish-detection gear, Capt. Nobukuni Seto looked grimly at hisnew 200-mile charts: "It's a huge problem. We have no jobs on the land. We work and live on the sea 10 months a year. It's our life."

While the temporary agreement with the Soviet Union allows fishing on the old generous terms, Capt. Seto is wasting no time. He sailed later today for the pollak grounds 35 miles off Kamchatka.