THE WEEKEND SHOWDOWN on the seas off Nantucket, in which the Coast Guard seized a Soviet trawler for violating the United States' new 200-mile fishing limit, cheered East Coast fishermen immensely. The Russian fleet takes 59 per cent of the foreign catch. American fishermen tend to blame it, rathern than their own inefficiency and fishing practices, for their economic difficulties and for the depletion of many Atlantic species. They had feared that the State Department would, for diplomatic reasons, balk enforcement of the new law, which took effect March 1.

It was not an insupportable suspicion. The State Department ha dlong argued that the fishermen's remedy, an us-first grab of the world's richest fishing waters, would spur grabs by other nations, and undercut the United Nations Law of the Sea Conference. but the political pressures were overwhelming. President Ford declined to veto the 200-mile bill last year, and President Carter, after first applying less severe measures, finally decided to "draw the line" and ordered the Soviet trawler seized.

But the problem persists. Officially, the Kremlin, which has proclaimed its own fishing zone, says it's ready to obey the law. But the Russians own more than half of the world's big fishing ships and are buying more. Enactments of 200-mile zones have cost them free access to 60 per cent of their catch. Their captains are not likely to surrender their bread and butter without a prolonged fight.

A further dange is that the American fleet may either try to get rid of the quotas that the new law still allows to foreign fishermen or may engage in overfishing on its own. These possibilities arise because the fleet is inefficient and highly labor-intensive, which is to say, highly voter-intensive. These are pressures to be resisted. It would be foolish to fend off foreign fishermen with new measures of protectionism, and to fish out the protected waters, too.