"Fish soup?" asked the incredulous waiter in the Breton seaport restaurant. "Never mind what the menu says. You need fish to make fish soup. Don't you know what's going on?"

That conversation, a real one, could have taken place anywhere along the French coast this summer as the staple entree on sea resort menus, served with grated cheese and croutons, disappeared after the fishermen went on strike and blockaded the ports -- except, of course, where restauranteurs were willing to cheat with canned or frozen substitutes.

When ill-advised entrepreneurs tried to run the blockade in its early days with frozen foreign fish, they wound up with their cargoes dumped on the docks. w

Anger and fisticuffs were not confined to the fishers or the blocked British tourists whose plight prompted angry British headlines about the Hundreds Years' War and the battle of the beaches of Dunkerque.French pleasure boat owners, who number 120,000 as opposed to the 50,000 fishermen, also felt aggrieved and sometimes came to blows with the blockaders of the major ports.

In a strike movement that started building two months ago, climaxed in August and is still not altogether finished, Franc's seagoing professional fishers presented a crazy quilt of demands that added up to a cry for the preservation of a romantic profession that seems to be endangered by foreign competition, overfishing in European waters, ancient practices and outdated technology.

The fishermen's trump card was their spectacular ability to block French ports at the height of the tourist season. The government had a relatively easy time countering them by refusing to accept industry-wide negotiations, arguining against them because of the diversity of interests and demands of the crews of coastal trawlers, ships that go out for giant crayfish off West Africa and the industrial boats that are out of port for months at a time dragging the herring banks of the North Sea or Newfoundland.

"There is not a single but many problems of fishing, one Cabinet minister said. "While solidarity is morally worthy of respect it could be economically disastrous."

The French government apparently decided that mid-August, while the vacationing French were largely occupied trying to take advantage of the country's first long run of sunshine since Easter and to ignore the economic gloomsayers, was a good time to demonstrate authority and try to head off the expected autumn strike wave whose avant garde the fishermen seemed to be.

So Joel Le Theule, the transport minister, announced that the fishermen's grievances were none of the government's business -- a remarkable assertion in one of the most regulated economies outside the communist world -- and that the fishermen were highly overpaid besides. He offered as a typical example the $120,000 earned by the captain of one fishing sloop in 1978, an example that turned out to involve the highest paid captain in the best recent fishing year.

Anyhow, the minister's office let it be known that the real problem of French fisheries is not the fishermen's complaint that the price of fish has only doubled during the same period that the fuel to operate at sea has gone up six times, but because of the distribution system needed to satisfy the peculiarly individualistic tastes of the French fish-eater. Eighty percent of the British market is satisfied by four varieties of fish, it was noted, while it takes 20 species to meet the refined demands of the same proportion of the French.

In the face of such government stonewalling, the fishermen's exasperation quickly spread from the confines of the original strike movement in Normandy to all the other major fishing regions, including the most important of all, Britanny, even though there is traditionally no love lost and occasional clashes at sea between Breton and Norman fishers. Le Theule managed to create temporary unity between the boat owners and captains on one side and the heavily unionized seaman on the other. A sacred union to defend the future of a hard and noble profession was formed among the Nationaly Assembly deputies representing the fishing regions including Giscardists and Communists alike.

Prime Minister Raymond Barre's entourage offered his traditional arguments that the French economy will never be efficient unless there is an end to honoring the demands of lobbies and special interest groups. This angered fishermen's representatives into replying that when the far more humerous and electorally powerful peasants go out en masse, the government listens, talks and does something.

By then, Le Theule, dubbed "Pontius Pilate" by the fishermen, was admitting that it was more than just a private problem, but, he said, redress of grievances was not to be found in Paris but at Common Market headquarters in Brussels.

The Communist-led union, number one in the country but holding a minority following among the fishermen, jumped on the occasion to outbid the industry's dominant pro-Socialist union by pressing for increasingly extreme actions.

When it came to ending the strike, pending negotiations last weekend, there was general stupefaction that the seamen in the two main Norman ports where the strike movement had started rejected their union's recommendations to return to work and followed the communists' lead to stay on strike. By then, however, the movement had peaked.

Government strategists hope to head off the national strike wave that seemed to be building up. If it does not engage in some very fancy footwork, however, the result could be the opposite as the unions complete with each other to incite strikes to prove that they are better defendors of worker interests than their rivals.

After a shaky start, the government seems very aware of this danger and appears to have maneuvered well.

In the fishermen's strike, the government's stress on the conflicting interests of captains and seamen, between regions and between seamen fishing different types of catches succeeded in breaking the back of the strike.

The government's most spectacular contribution to the unaccustomed break in the August holiday truce was to call out the Navy to break the blockades of the oil ports of Fos near Marseilles in the Mediterranean and Le Havre in Normandy.

The unequal contests that ensued, with fishing boats rammed by naval vessels and mock-heroic battles taking place in full sight of the ports, earned Barre more amused cartoons of the prime minister in 19th century admiral's uniform than outrage.

Tactics similar to Le Theule's approach with the fishermen have surfaced in the last few days as Education Minister Christian Beullac has been warning schoolteachers not to start off the new school year on the wrong foot with a strike wave.

In what now seems in retrospect to have in part a maneuver to dissipate the impact of a strike by the powerful teacher's unions, the government decreed the start of school on a staggered, regional basis rather than all on the same day. Teachers' spokesmen have been saying that if the first day of school is going to be staggered, there is no reason that the traditional first-day strikes connot be staggered too.

Beullac then switched from lumping the teachers' union all together as irresponsible to attacking Communists in the unions separately as the root of the troubles. Teachers are one of the few groups in France whose unions are not divided along political lines. The noncommunist leaders rushed to the defense of their communist colleagues.

The teachers may find that the government has laid a trap for them. It can tell moderate voters that it is not attacking the noncommunists, but they have chosen to associate themselves with the communists.