Marine officials are worried that they may be facing another significant dolphin die-off in the Gulf of Mexico after 21 of the marine mammals were found washed up on Texas beaches over two days. The carcasses were discovered a year after about 70 dolphins washed up in the same area over a two week period."We are concerned, especially because (of) what happened last year," said Blair Maise, the marine mammal stranding coordinator for the national marine fisheries service."There may be more."Researchers weren't able to determine a cause for last year's significant die-off because the animals' carcasses were too decomposed when they were discovered.The bottlenosed dolphins found on Monday and Tuesday on the Bolivar peninsula near Galveston, Texas were also badly decomposed, but officials are hopeful they may still be able to determine a cause."We're going to take a more proactive role (this time) in trying to do aerial searches for fresher carcasses so we can get better information on the cause," Maise said in a telephone interview.Researchers have collected samples from the dolphin carcasses which will be studied to see if signs of disease can be found.They will also study satellite photos taken over the past three weeks to see if the dolphins could have been killed by a red tide, a harmful algal bloom which is becoming increasingly common in the Gulf."It's been around for hundreds and hundreds of years, the red tide, but what's unusual is the areas where we're starting to see it more and the impact on the marine life," Maise said.These red tides have been responsible for the bulk of the mass marine kill-offs recorded in the Gulf Coast in recent years, she said."Over the years these unusual mortality events have become more and more frequent," Maise said.Some 41 mass kill-offs in US waters have been classified as unusual mortality events since 1991, 23 of which were in the past three years. The latest dolphin incident has not yet been classified.