In the swampy wetlands of southern Mexico, officials and researchers are struggling to explain the deaths of dozens of manatees, the chubby marine mammals once confused with mermaids by ancient mariners. Fishermen who navigate the muddy waters inland from the coast of Tabasco state have discovered since May at least 28 dead West Indian manatees, also known as sea cows, along the Bitzal River and nearby streams. The cause of the deaths remains a m…