GALVESTON — Officials are considering an aerial search of coastal waters after finding 21 dead dolphins on local shores. The 21st dolphin was found Tuesday.

Kim Amendola, a spokeswoman for NOAA’s National Marine Fisheries Service, said a survey team might search offshore waters Wednesday.

The dolphins were found in Galveston and Jefferson counties beginning Monday.

The discovery on the Bolivar Peninsula follows an unusually high number of dolphin bodies on local beaches about the same time last year.

Almost 70 dolphins washed up on Galveston County shores between Feb. 27 and March 23, 2007.

Those dolphins were badly decomposed, hampering efforts to test tissue samples. The same is the case with the recent findings.

Scientists are unsure what caused the event in 2007. Tissue tests were either inconclusive or showed no abnormalities, said Blair Mase of NOAA.

Texas has a healthy population of bottlenose dolphins, and it’s common to have deaths in calving season during February and March, Mase said. But finding 20 dolphins on the beach in one day, or 70 in one season, is unusual, she said.

Since 1991, NOAA has classified 41 episodes of stranded marine mammals as unusual mortality events.

Scientists determined causes for some of the events, including diseases, harmful algae blooms that produce toxins and environmental conditions such as El Niño, a periodic change in conditions in the tropical Pacific Ocean that can affect weather worldwide.