Fishing nets are estimated to kill upward of 300,000 cetaceans every year. The problem is the nets are too thin and amorphous for the animals to see with their sonar and so they get caught unintentionally. Now, a beacon used by the British Navy to mark mine fields has been miniaturized to the extent that it can be strung on to fishing nets to warn cetaceans to stay away. The device is a spherical plastic shell which acts like a satellite dish to amplify and reflect sonar waves. It is entirely passive (meaning it doesn't transmit sound, only listens for it), which gives it a big advantage over battery-powered active-sonar equivalents which can be difficult to maintain. When a dolphin's sonar hits it, the signal is sent back so that the animal knows something is ahead in the water and can swim to avoid it.