NEW YORK (MarketWatch) — A pilots strike at Spirit Airlines entered its third day Monday, forcing stranded passengers to scramble after seats on other airlines already brimming with summer travelers.

The Miramar, Fla.-based budget carrier, which represents less than 1% of the country’s annual passenger-air traffic, competes directly with American Airlines parent AMR Corp. (AMR 8.48, +0.22, +2.66%) , JetBlue Airways (JBLU 6.90, +0.40, +6.15%) and AirTran Airways (AAI 5.92, +0.10, +1.79%) for East Coast travel to the Caribbean and Florida.

“We are accommodating Spirit passengers as best we can, but we have very high load factors over the next few weeks,” said Christopher White, a spokesperson for AirTran.

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Air Tran’s load factor, or the percentage of seats filled on an average flight, hit a record 85% last month, White said, and the numbers are looking high throughout the summer.

Indeed, the entire industry has trimmed its domestic seat capacity over the last two years as higher fuel prices and last year’s recession forced them to abandon less profitable routes and departure times. That’s helped to push fares higher and return carriers to profitability, but it leaves little flexibility when customers face disruptions.

Shares for AMR rose nearly 3% to $8.48 and AirTran shares climbed 2.8% to $5.98. JetBlue Airways jumped nearly 7% to $6.95, helped by a rating upgrade at Hudson Securities, a securities trading firm.

Pilots flying for privately-held Spirit walked off the job on Saturday after negotiations over a new labor contract fell apart. The 450 pilots, represented by the Air Line Pilots Association, have been working without a contract for four years.

All Spirit Airlines flights were cancelled through Tuesday and customers will get credit for the full amount of the flight, as well an extra $100 toward a future flight, the company said. Read more about the strike.

The pilots strike it the first for a U.S. carrier since the spring of 2001, when 1,400 pilots at Delta Air Lines’ regional-service provider Comair picketed for about three months.

It wasn’t clear how long the strike at Spirit could last. As of Dec. 31, Sprit had about $143 million in liquidity, providing the airline with some leeway, according to data provided by Hudson Securities, a securities trading firm.

Spirit Airlines has hubs in Ft. Lauderdale, Fla., and Detroit, serving primarily leisure travel destinations in the Caribbean, Central America, South America, and Florida, as well as Atlantic City, N.J., Myrtle Beach, S.C., and Washington, D.C.

AMR will likely be the biggest beneficiary of the strike because of its hub in Miami, according to Hudson Securities analyst Daniel McKenzie. The legacy carrier serves 32 of the 37 destinations served by Spirit.

To a lesser extent, Sprit also competes with Southwest Airlines (LUV 12.25, +0.18, +1.49%) and Delta Air Lines (DAL 13.96, +0.51, +3.79%) , he said.

Earlier McKenzie upgraded JetBlue to neutral from sell.

“The ultimate [earnings] benefit from the Spirit strike is naturally a function of how long the strike lasts,” he said. “For now, we’re using the opportunity to update estimates for lower fuel prices and slightly better revenue trends at both AMR and JetBlue.”

Christopher Hinton is a reporter for MarketWatch based in New York.

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