The stranding deaths of about 60 bottlenose dolphins on Texas beaches over the past three weeks has puzzled researchers and is a cause for concern during the calving season, a senior scientist said on Monday.

"This is the calving season so we often have strandings at this time of the year. It's tough to be an air-breather born in the water," said Dr. Daniel F. Cowan, professor of pathology at the University of Texas Medical Branch at Galveston and director of the Texas Marine Mammal Stranding Network.

"But over the last few weeks we have had about 3 to 4 times the usual mortality," he told Reuters.

Most of the carcasses were in an advanced state of decomposition, suggesting that they were carried to Texas beaches from areas further off or up the shore.

Suspected causes include parasites, an outbreak of infectious diseases or red tide, an algal bloom prompted by fertilizers or other excess nutrients.

Most of the dolphins have been too decomposed for a necropsy -- the animal version of an autopsy -- and so volunteers have been burying them on the beaches.

Several of the dolphins which have washed up on shore have been young with umbilical cords still attached.