Seattle — The Pacific Ocean has become the newest competitive arena for US and foreign airline carriers, and this city, which likes to think of itself as the ''gateway'' to the Pacific Rim, is about to become part of the battle. The competition will begin in earnest Friday, when United Airlines begins daily flights to Tokyo and Hong Kong.
Overnight, the number of international flights arriving and departing from Seattle's ''Sea-Tac'' (Seattle-Tacoma) airport will more than double, adding 17 new international flights to the Pacific Rim each week.
This is the result of several years of tedious negotiations between the Japanese and United States governments which culminated last June in a three-year agreement allowing United its coveted route to Japan. This will be the first foray outside North America for the US's biggest domestic carrier.
In addition, Japan Air Lines will be allowed to fly to Seattle and on to Chicago, two large American cities it has long wanted to add to its American route structure.
Transpacific air travel is in a slump because of the worldwide recession, so United officials say their planes may be flying nearly half empty when the new service begins, yet they say they will make a profit even with a 50 percent occupancy rate on the new routes.
Airline officials concede that the Pacific Northwest market alone is not nearly large enough to fill all these new seats, but they hope to funnel through Seattle a lot of traffic from Chicago, New York, Denver, and other locations.
The two airlines are eyeing each other warily. The Japanese are said to respect United's huge domestic network and ability to feed traffic to the Orient. Some say this was one reason the bilateral negotiations dragged on so long. United respects JAL's international prowess and reputation for excellent service.
United had hoped to have an edge by offering ''mileage based'' fares that reflect the fact that Seattle is more than 500 miles closer to Tokyo than are San Francisco or Los Angeles, but they were disallowed by the Civil Aeronautics Board in favor of ''common'' fares, that is, fares nearly equal to those for other West Coast cities.
Meanwhile, the big expansion in international air service is causing businessmen in the Pacific Northwest to re-examine their casual approach to promoting tourism.
United Airlines, of course, is working hard to lure more Japanese tourists to the Pacific Northwest, which its Japanese public relations firm calls the ''New'' West Coast, as opposed to the ''old'' West Coast further south.
Hotel and other travel interests in the Northwest have been busy meeting with the Japanese tour industry trying to put together tour packages with the large travel agencies such as the Japan Travel Bureau and Japan Air Lines.
Until recently, the Pacific Northwest never loomed large for the Japanese or foreign tourist, who was more likely to visit San Francisco, Las Vegas, or Disneyland.
In 1981 Seattle hosted only about 50 Japanese tour groups, according to one hotel executive, far fewer than the 300 or more that visited Vancouver, British Columbia, a city about the same size as Seattle.
But Seattle does have a famous volcano nearby, and some travel industry people are talking about a two-day tour on which Japanese tourists would visit such attractions as the Boeing airplane assembly plant in Everett and Mt. St. Helens before driving south to Lake Tahoe or some other scenic attraction before returning home through San Francisco.