Turning turtles

By Barry Fox Having survived for over 200 million years, some turtle species are now threatened with extinction because they get caught in fishing nets. Martin Lenhardt at the Virginia Institute of Marine Science has worked with turtles accidentally netted in Chesapeake Bay, and found a way to save them (WO 01/17119). Turtles have an air-filled middle ear that resonates at a frequency that changes with age and the depth of water. Lenhardt discovered that if boats emit a 100-kilohertz ultrasound signal, the shells of nearby turtles convert it to a lower frequency,
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