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,
between 200 hertz and 15 kilohertz. This makes the beasts'
middle ears resonate, scaring them into swimming away or diving
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