Most years about 4000 marine mammals beach on US coasts and the causes are often hard to determine. New research in Florida tested the hearing of beached or net-entangled animals that survived, and found that almost 60% of stranding bottlenose dolphins were severely or profoundly deaf.
The animals were tested at rescue facilities around Florida. The results are hardly surprising given that dolphins and whales are dependent on their hearing for navigation and feeding using their echolocation sense. It is possible that animals with damaged hearing might be more likely to strand if they cannot orientate themselves properly.
Dr David Mann of the University of South Florida was lead researcher in the study, "We used the same methods to test the dolphins as is used to measure hearing in human babies. Like human babies, some dolphins are born deaf and can probably survive well with their mothers until weaned, but hunting on their own can be problematic, although some do seem to adapt and manage."
It is important to differentiate between animals who can no longer survive in the wild because of their deafness, and those that can safely return to the wild after a period in rehabilitation.
Causes of deafness in whales and dolphins are similar to those in humans and range from exposure to chronic noise eg shipping lanes, disease, trauma and ageing. Often the finger is pointed at Naval sonar activities when mass strandings of marine mammals occur around the world.
According to the world foremost expert on marine mammal hearing, Darlene Ketten of Woods Hole Oceanographic Institute, fewer than 300 animals have been stranded in association with naval exercises in the last 50 years.
Compared to between 100,000 and 400,000 animals a year being caught up as bycatch in our fisheries less than 6 animals a year are killed due to the impact of navy sonar. A mass stranding of beaked whales occurred in 2000 in the Bahamas during a US Navy sonar exercise and these species are now known to be the only marine mammals affected by sonar.