Low-cost competitors JetBlue Airways and AirTran Airways on Thursday both announced new nonstop flights next spring from Tampa International Airport to San Juan, Puerto Rico. AirTran will begin service April 5 with two daily departures. JetBlue begins May 11 with one flight daily and will add a second the following month. Neither carrier was selling the flights on their websites Thursday afternoon. American Airlines has flown one daily round trip to San Juan for several years.