Ten years ago, it was impossible to walk into a U.S. supermarket and buy fresh Chilean salmon at any price.

Today, the prized fish can be had for around $6 a pound up and down the East Coast - from ethnic food stores in Miami to Safeway supermarkets in Washington, D.C.Clearly, Chilean aquaculturists are shipping and flying more salmon to the United States and other overseas markets than ever before. In 1989, export sales of fresh and frozen salmon brought Chile $45 million - up from $3.6 million in 1986. While still a small part of the country's $900 million fisheries industry, salmon has quickly become a fast-growing segment of Chile's export-based economy.

Fernando Klimpel, marketing manager of the Chilean Association of Salmon and Trout Producers, says continued strong demand for fresh salmon in the United States and for frozen salmon in Japan will push exports to around $72 million this year, despite sagging retail prices.

"Chile has a catch of over 5 million metric tons every year," he said. ''Consumption of seafood in Chile is rather small, so 90 percent of the catch is exported."

Some 55 percent of the Chilean fisheries industry consists of exports of fish meal, of which Chile is the world's largest producer. Fresh and frozen fish, including salmon, trout, hake and swordfish, comprise another 28 percent, while oil fish represent an additional 4 percent. Canned fish and seaweed make up the remaining 12 percent of seafood exports.

The salmon industry, concentrated around Puerto Montt and Chiloe Island, about 650 miles south of Santiago, relies on strong air links to rush the salmon to U.S. supermarkets within four days of harvesting.

"From Puerto Montt, we either truck or fly the fish to Santiago," said Jorge Mandiola, commercial manager of Ventisqueros SA, the country's second- largest salmon exporter. "From Santiago, we fly to the U.S. on cargo planes, but we have to use the passenger planes also, because it's not enough. There's a big demand for fruit, so we have to compete for space. We don't have many alternatives."

Mr. Mandiola said his company alone processed 600 tons of salmon last year. While most of that was frozen salmon intended for the Japanese market, about 60 tons was fresh salmon destined for the United States. This year, he expects to fly 150 tons of fish north. To get that fish to U.S. markets in time, he said, Ventisqueros uses one all-cargo carrier, Fast Air, and two passenger airlines, Lan Chile and Ladeco.

Frozen fish, by contrast, is sent by ship mainly to Japan, much of it on Chilean Line, known in Spanish as Sud Americana de Vapores. Chilean salmon is

marketed by a number of companies in Japan, where per capita consumption of fish is about 150 pounds a year, compared with 34 pounds in the United States.

"In the U.S. and Europe, Norway is our main competitor. In Japan, our main rivals are Alaska and Canada," said Mr. Mandiola.

"What we need is to diversify our markets. We cannot concentrate only on Miami, New York and Los Angeles," he said. "We are also looking to other markets. We want to be present in Europe, South America and the Far East."

According to the exporters' association, U.S. purchases of fresh salmon brought Chile $10 million last year, while Japanese purchases of the frozen variety came to $25 million. Chile also exports salmon to the Netherlands, France and Brazil. Total volume, which doubled from 4,208 tons in 1988 to 8,595 tons in 1989, is expected to reach 12,500 tons this year and 18,000 tons in 1991.